Sample Investigation of 201216 and 201217 Boreholes in the Frame of

Transcription

Sample Investigation
of 201216 and 201217 Boreholes
in the Frame of
Technical Cooperation Namibia;
“Groundwater Investigation in the Cuvelai-Etosha Basin“
vom 04.05. - 31.07.2009
Berichterstatter:
André Walzer
Auftraggeber:
Bundesministerium für Wirtschaftliche
Zusammenarbeit und Entwicklung
(BMZ)
BMZ Projekt-Nr.:
2006.2073.2
BGR Projekt-Nr.:
05-2326
Datum:
December 2009
Kurzfassung
In dem vorliegenden Bericht wird Sedimentgestein im zentralen Norden von Namibia hydrogeologisch untersucht. Motiviert ist diese Arbeit durch die im Norden Namibias ansässige
ländliche und städtische Bevölkerung, zu deren ausreichender Versorgung mit Trinkwasser
guter Qualität ein wesentlicher Beitrag geleistet werden soll. Im Rahmen eines Projektes des
Deutschen Bundesministeriums für wirtschaftliche Zusammenarbeit und Entwicklung soll
die gesättigte hydraulische Leitfähigkeit einer süßwasserüberlagernden Schicht abgebildet
werden um anhand dessen Aussagen, u¨ ber eine mögliche Verunreinigung durch Salzwasserintrusion treffen zu können.
Geophysikalische Messungen bilden die Grundlage für die Untersuchung dieser Arbeit
und lassen die Existenz eines kontinuierlichen Süßwasseraquifers von ca. 3400 km2 Fläche
vermuten. Die Datenerhebung wurde in Form von drei Spülbohrungen und zwei Kernbohrungen getätigt. Die Proben wurden lithologisch angesprochen und im weiteren Verlauf mehreren Laboranalysen unterzogen. Dabei wurde sowohl die gesättigte hydraulische
Leitfähigkeit des Kernmaterials mit Hilfe einer Triaxzelle bestimmt, als auch der mineralogische Aufbau des Sediments, die Korngrößenverteilung und die Porosität. Zusätzlich
erfolgte die Auswertung eines Pumpversuches unter Ausschluss von Beobachtungsbrunnen.
Dennoch konnte die hydraulische Leitfähigkeit und Transmissivität der süßwasserführenden
Schicht abgebildet werden.
Bei hydrogeologischer Betrachtungsweise der Sedimentabfolge und unter Kombination
der genannten Daten und Analyseergebnisse kann bestätigt werden, dass ein oberer, weitgehend salziger Aquifer durch eine hydraulisch wirksame Trennschicht von einem unteren,
gespannten Süßwasseraquifer separiert wird. Die Existenz der jeweiligen Schichten im
Projektgebiet wurde für sämtliche aufgeführten Bohrlöcher ermittelt. Abschließend lässt
sich feststellen, dass die hydraulisch wirksame Trennschicht durch die Eigenschaften eines
Aquitards im oberen und unteren Bereich gekennzeichnet ist, während sie im Zentrum die
Eigenschaften eines Grundwasserstauers widerspiegelt.
V
Abstract
This report covers a hydrogeological investigation of sedimentary rocks in the central-north
of Namibia. The study has been motivated by the rural and urban population in the north of
Namibia. It is intended to provide drinking water of good quality to the resident population.
In the framework of a project by the German Federal Ministry for Economic Cooperation and
Development, the saturated hydraulic conductivity of a layer is investigated which overlies a
layer containing fresh water. It is intended to determine a possible degradation of the fresh
water by intrusion of saline water.
The investigation of this report has been done on the basis of geophysical measurements
which indicate the existence of a continuous fresh water aquifer with a covered area of about
3400 km2 . Data acquisition has been done by means of three mud rotary drilled boreholes
and two cored boreholes. Laboratory analyses were performed after lithological description
of the core samples. These analyses comprise of an evaluation of the saturated hydraulic
conductivity on basis of triaxial testing, determination of the porosity as well as grainsize
analyses. Additionally, interpretation of pumping test in absence of observation wells provided hydraulic conductivity and transmissivity of the fresh water layer.
Corresponding to all data and results of laboratory analysis, the hydrogeological succession of the sediment can be determined by an upper, mostly saline aquifer and a lower,
confined fresh water aquifer which is separated by an hydraulic non-conductive layer. The
existence of the relevant layers is confirmed within all drilled boreholes. Finally, it can be
concluded that the separating layer is characterised by the attributes of an aquitard at the
upper and lower margins whereas it shows the attributes of an aquiclude within the centre.
VI
Contents
Kurzfassung
V
Abstract
VI
Acknowledgments
VII
Contents
VIII
List of Figures
XI
List of Tables
XIII
Abbreviations
XV
1. Introduction
1.1. Background . . . . . . .
1.2. Framework of the Project
1.3. Objectives . . . . . . . .
1.4. Report Outline . . . . .
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2. Description of the Study Site
2.1. Location & Demarcation . . . . . . . .
2.2. Natural Environment in Near-Surface .
2.2.1. Climate . . . . . . . . . . . . .
2.2.2. Vegetation & Land Use . . . . .
2.2.3. Soils . . . . . . . . . . . . . .
2.3. Topography & Surface Water Drainage .
2.4. Geology . . . . . . . . . . . . . . . . .
2.4.1. Pre-Kalahari Geology . . . . .
2.4.2. Kalahari Geology . . . . . . . .
2.5. Hydrogeology . . . . . . . . . . . . . .
2.5.1. Kalahari Sequence Aquifers . .
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3. Methods & Techniques
3.1. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1. Mud Rotary Drilling . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2. Core Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Laboratory Work & Methods of Calculation . . . . . . . . . . . . . . . . .
3.2.1. Evaluation of k-Value on Basis of Pressure Depending Perfusion in
Triaxial Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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VIII
3.2.2. Saturated Hydraulic Conductivity after Darcy . . . . . . . . . . . .
3.2.3. Grainsize Analysis & Porosity . . . . . . . . . . . . . . . . . . . .
3.2.4. Indirect Evaluation of Hydraulic Conductivity Based on Grainsize
Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.5. Mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.6. Methods for Analysation of Pumping Test Data . . . . . . . . . . .
4. Results & Interpretation
4.1. Previous Results . . . . . . . . . . . . . . . . . . . . .
4.1.1. TEM Investigation . . . . . . . . . . . . . . .
4.1.2. Boreholes Prior to TEM Investigation . . . . .
4.2. Lithological Characterisation of Mud Rotary Boreholes
4.2.1. Borehole 201045 . . . . . . . . . . . . . . . .
4.2.2. Borehole 201046 . . . . . . . . . . . . . . . .
4.2.3. Borehole 201047 . . . . . . . . . . . . . . . .
4.3. Lithological Characterisation of Cored Boreholes . . .
4.3.1. Borehole 201216 . . . . . . . . . . . . . . . .
4.3.2. Borehole 201217 . . . . . . . . . . . . . . . .
4.4. Overview of Drilling and Logging Results . . . . . . .
4.5. Saturated Hydraulic Conductivity . . . . . . . . . . .
4.5.1. Triaxial Cell . . . . . . . . . . . . . . . . . .
4.6. Grainsize Analysis & Porosity . . . . . . . . . . . . .
4.6.1. Grainsize Analysis . . . . . . . . . . . . . . .
4.6.2. Porosity . . . . . . . . . . . . . . . . . . . . .
4.7. Mineralogy . . . . . . . . . . . . . . . . . . . . . . .
4.8. Test pumping . . . . . . . . . . . . . . . . . . . . . .
5. Discussion
5.1. Hydrogeologic Estimation of Layers
5.1.1. Lateral . . . . . . . . . . .
5.1.2. Vertical . . . . . . . . . . .
5.2. Aquitard or Aquiclude? . . . . . . .
5.3. Aquifer Capacity & Outlook . . . .
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Bibliography
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Appendix
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A. Hydraulic Conductivity - Triax
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B. Grainsize Analysis
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C. Geophysical Measurements of Drilling Fluid
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D. Photos
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E. Mineralogical Report
167
IX
List of Figures
1.1. Geographical location of Namibia in relation to Africa . . . . . . . . . . .
1
2.1. Location of the investigation area within the CEB . . . . . . . . . . . . . .
2.2. The annual average rainfall across north-central Namibia (modified after
Mendelsohn et al. (2000) . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Topography of the Cuvelai Etosha basin based on SRTM data . . . . . . . .
2.4. Simplified drainage system of Owambo basin in northern Namibia (remade
after Mendelsohn et al., 2000) . . . . . . . . . . . . . . . . . . . . . . . .
2.5. Water-bearing ”Iishana” in the Eenhana region, Namibia (May 2009) . . . .
2.6. Location of the Owambo Basin (after Miller, 1997) . . . . . . . . . . . . .
2.7. Geological cross-section across the Owambo basin from Rucana to Tsumeb
(Vertical exaggeration 1:30; Hipondoka, 2005) . . . . . . . . . . . . . . . .
2.8. Kunene & Cubango Megafan in relation to core-drilling boreholes (modified
from Google, 2009) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9. Cross-section of the Kalahari Group Sediments in the Owambo Basin along
the 16◦ 41’ longitude; Hipondoka (2005)(modified from Kempf, 2000) . . .
2.10. Aquifersystems of the CEB (abbreviated after Bittner, 2006), arrows indicate
groundwater flow direction . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.1. TEM - Transient Electromagnetic Measurements within the project area . .
3.2. EDI Fluid-Finder (van Wyk, 2009) . . . . . . . . . . . . . . . . . . . . . .
3.3. Tube with core catcher holding core (top-left) & wire line - diamond coring
drill bit (bottom-right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Drilling rig of Major drilling used for the project. Supply of rods at the
bottom left. Mud pit at the bottom. 5 m3 tank of drilling fluid can be seen on
the right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5. Photo & schematic image of breadboard construction ’triaxial cell’ . . . . .
3.6. Example of recorded data during testing of hydraulic conductivity & adapting of best-fit line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4.1.
4.2.
4.3.
4.4.
4.5.
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Estimated lateral extend of deeper fresh water aquifer KOH2 . . . . . . . .
Litholog and geophysical log of borehole 201045 . . . . . . . . . . . . . .
Litholog of core borehole 201216 and comparison to 201045, dots point out
core samples and their relevant depth, red bars refer to core loss . . . . . .
4.6. Litholog of core borehole 201216 and comparison to 201047, dots point out
core samples and their relevant depth, red bars refer to core loss . . . . . .
4.7. Summary of cumulative grainsize curves of investigated core samples . . .
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XI
4.8. Comparison of evaluated hydraulic conductivity values of samples from borehole 201216 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9. DTA-MS analysis for core samples of borehole 201216; DSC-graph (Differential Scanning Calorimetry) . . . . . . . . . . . . . . . . . . . . . . . . .
4.10. SEM - Scanning Electron Microscope comparing undisturbed (l.) and dried
(r.) sample containing smectite, of borehole 201216 . . . . . . . . . . . . .
4.11. Specific capacity curves of step test pumping in 201045, -46, -47 . . . . . .
5.1. Lithology of project boreholes in relation to their location within the assumed area of the aquifer KOH2 . . . . . . . . . . . . . . . . . . . . . . .
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XII
List of Tables
2.1. Stratigraphy of the Owambo basin (abbreviated after Miller, 1997) . . . . .
2.2. Stratigraphy of the Kalahari Sequence (abbreviated after Miller, 1997) . . .
12
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3.1. Overview of field work & data collection for the project ’Groundwater Investigation in the CEB’ . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Overview of laboratory work in technical cooperation with the BGR . . . .
3.3. Depth related core samples for laboratory analyses . . . . . . . . . . . . .
21
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4.1. Evaluation of saturated hydraulic conductivity (Triaxial testing - borehole
201216) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Evaluation of saturated hydraulic conductivity (Triaxial testing - borehole
201217) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Evaluation of saturated hydraulic conductivity (grainsize analysis) . . . . .
4.4. Porosity for samples of borehole 201216 . . . . . . . . . . . . . . . . . . .
4.5. Mineralogical composition of core samples at 201216 . . . . . . . . . . . .
4.6. C-/S- analysis of core samples at 201216 . . . . . . . . . . . . . . . . . . .
4.7. Summary of step drawdown test data for 201045, -46, -47 . . . . . . . . . .
4.8. Aquifer parameters evaluated on basis of test pumping . . . . . . . . . . .
4.9. Recovery analysis after constant rate pumping test for boreholes 201046 and
201047 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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XIII
Abbreviations
a.m.s.l. . . . . . . . . . above mean sea level
BGR . . . . . . . . . . . Bundesanstalt für Geowissenschaften und Rohstoffe - Federal Institute
for Geoscience and Natural Resources
BMZ . . . . . . . . . . . Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung - Federal Ministry for Economic Cooperation and Development
CEB . . . . . . . . . . . Cuvelai-Etosha Basin
CEC . . . . . . . . . . . Cation Exchange Capacity
DIN . . . . . . . . . . . . Deutsches Institut für Normung - German Institute for Standardization
DTA-MS . . . . . . . Differential Thermal Analysis - Mass Spectrometer
DWAF . . . . . . . . . . Department of Water Affairs and Forestry
EC . . . . . . . . . . . . . electric conductivity
EDI . . . . . . . . . . . . Exploration Drilling International
GCS . . . . . . . . . . . Geographical Coordinates
GWL . . . . . . . . . . . Groundwater Level
ICP-OES . . . . . . . Inductively Coupled Plasma Optical Emission Spectrometer
IRS . . . . . . . . . . . . Infrared Spectroscopy
IUPAC . . . . . . . . . International Union of Pure and Applied Chemistry
LOI . . . . . . . . . . . . Loss On Ignition
LWL . . . . . . . . . . . Lowered Water Level - influenced WL by depression cone of well
SEM . . . . . . . . . . . Scanning Electron Microscope
SRTM . . . . . . . . . . Shuttle Radar Topography Mission
SWL . . . . . . . . . . . Static Water Level - uninfluenced GWL
TC . . . . . . . . . . . . . Total Carbon
TDS . . . . . . . . . . . . Total Dissolved Solids
XIV
TEM . . . . . . . . . . . Transient Electromagnetic
TIC . . . . . . . . . . . . Total Inorganic Carbon
TOC . . . . . . . . . . . Total Organic Carbon
TS . . . . . . . . . . . . . Total Sulphur
XRD . . . . . . . . . . . X-ray diffraction
XRF . . . . . . . . . . . X-ray Fluorescence Spectrometer
XV
Chapter 1. Introduction
1. Introduction
1.1. Background
Namibia is an arid to semi-arid country with high
annual mean air temperatures and a low amount of
precipitation. The country is very susceptible to
droughts as the major amount of precipitation falls
only within a few months of the year and its distribution alternates vastly. Additionally, the high potential evaporation rate inflicts high losses of water
which implies that a surface water supply system is
very susceptible.
The Namibian population is estimated with about
2 million inhabitants in which the Cuvelai-Etosha
Fig. 1.1.: Geographical location of Namibia
basin (CEB) is the most densely populated part of the
in relation to Africa
country and one of the fastest growing areas from an economic point of view (Zauter, personal communication). The location of the CEB is demonstrated in figure 2.1 on page 5.
Most of the population in the CEB live in concentrated zones where fresh water is available
due to abundant water points or seasonal floods (Bittner, 2006). Since independence in 1990
a drinking water pipeline system supplies most of the population in the Cuvelai-Etosha Basin
with drinking water from the Kunene River/Calueque dam (Angola) in the north-west (Zauter & Katjimune, 2007). More than 3400 km of pipeline were constructed between 1990
and 2003 with about 2100 water points which are mainly storage tanks at schools and clinics
as well as taps spaced along the pipelines (Bittner, 2006). Whereas the design capacity of
the lined canal feeding the pipeline system was build for 6 [m3 /s] only about 2.2 [m3 /s] are
pumped from the Calque dam in Angola. In other areas of the CEB more than 240 boreholes
have been drilled to provide groundwater for human consumption and livestock (Bittner,
2006). While the groundwater is provided in sufficient quantity the quality varies intensely
within the CEB. Prior investigations have proven that the groundwater is of better quality at
the margins of the CEB while it contains a higher salinity within the center (Schildknecht,
2007).
-1-
1.2 Framework of the Project
Wisely coordinated management of the water resources is necessary for sustainable water
supply. The aim of the Namibian government is to provide access to safe drinking water
for every inhabitant within a radius of 2.5 km around the living area (Bittner, 2006). The
project ’Groundwater Investigation in the Cuvelai-Etosha Basin’ was launched to improve
the insecure condition of fresh water supply and to decrease the dependency on water from
the Angolan Calueque dam.
1.2. Framework of the Project
The project ’Groundwater Investigation in the Cuvelai-Etosha Basin’ has a duration of
6 years and runs its first phase from October 2006 to April 2010. The project is in the
framework of German-Namibian Technical Cooperation, executed between the Republic of
Namibia and the Federal Republic of Germany. The Federal Institute for Geosciences and
Natural Resources (BGR) acts on behalf of the Federal Ministry for Economic Cooperation
and Development (BMZ) . The Namibian counterpart institution is the Department of Water
Affairs and Forestry (DWAF) in the Ministry of Agriculture, Water and Forestry.
For a successful operation of the envisaged objectives in water management, the understanding of the hydrogeological situation in the Cuvelai-Etosha basin is essential. The proposed project should provide the hydrogeological base information and hence contribute to
(Zauter & Katjimune, 2007):
• supply adequate, clean and safe water to the rural population of the Cuvelai-Etosha
Basin;
• improve the health and standard of living of the people through the provision of a more
accessible water distribution network;
• increase agricultural and livestock production, and possible aquaculture;
• ascertain to what extent the available water will be able to support infrastructure development including possible new growth points;
• investigate possibilities of economical use of the saline groundwater;
• become more independent from shared water resources.
-2-
1.3 Objectives
1.3. Objectives
In the first phase of the project an intermediate objective is to determine the groundwater
resources of the Iishana- and Nipele-subbasin in a quantitative and qualitative manner. Investigation boreholes were drilled on this account whereby fresh water was encountered in the
depth of 241 m during drilling of a borehole in the Nipele-subbasin. As part of further examinations of the groundwater system in the Cuvelai-Etosha basin, geophysical investigations
were performed in the relevant subbasins to receive information on the lateral distribution
of the encountered fresh water (Schildknecht, 2008). A regional extend was assumed after
interpretation of the geophysical data which will be referred to as the investigation area respectively project area of this report. Due to the upper reaches of the sediment which are
known to comprise of saline water it was furthermore assumed that fresh water is separated
from saline water by a confining or semi-confining layer which occurs with a regional extend within the project area as well. To check on the correctness of these interpretations,
five boreholes were drilled in two stages. The boreholes are situated at positions of lowest interpretation error in combination with a maximum thickness of the fresh water aquifer
according to the geophysical investigations. The Institute of Groundwater Management at
Dresden University of Technology was appointed to supervise the second stage drilling and
secure undisturbed core material for further laboratory analyses. The recovered cores are
the first cores of sufficient length of the unconsolidated Kalahari sediment in Namibia and
probably of the whole Kalahari basin of central and southern Africa. They offer the first
opportunity to study the sedimentology of a thick section of this succession and to obtain
therefrom a better understanding of its hydrogeological condition. The main objective of
this investigation is thereby to determine the saturated hydraulic conductivity of the core
material and give a conceptual view on the hydrogeological mechanisms of the sediment and
of the separating layer in particular. It is intended to specify the separating layer in terms
of being aquitard or aquiclude. This includes the implementation of other discovered results
within the laboratories of the BGR such as mineralogy and grain size analysis. Lithological description of the five boreholes and geophysical investigation will be elaborated. All
received information of the project area and core material is then combined in a significant
context to describe the hydrogeological condition of the sediment as accurately as possible.
1.4. Report Outline
Chapter 2 provides a general description of the study site and introduces the natural environment of the project area which provides the main parameters and conditions for all further
modeling purposes. Thorough attention is given to the geological and hydrogeological de-
-3-
1.4 Report Outline
scription as the main focus of this investigation is a confined layer and a deep seated fresh
water aquifer.
Chapter 3 presents the methods and techniques used within this work. Data acquisition in
terms of field and laboratory work is described. The methods of data evaluation for hydrogeological parameters and the content of the core material are introduced within this chapter
as this investigation covers interdisciplinary aspects of research.
Chapter 4 presents the received results of all methods and techniques described in this
report. This is the main part of the work and more information is presented as the results are
interpreted and analysed within their subchapters. Critical inspection on possible analysing
errors of the applied methods are seen in context to the received results.
The last chapter (chapter 5) combines the presented and interpreted results in a coherent
context to determine the hydrogeological condition of the project area from a conceptual
point of view.
-4-
Chapter 2. Description of the Study Site
2. Description of the Study Site
2.1. Location & Demarcation
The Cuvelai-Etosha basin (CEB) is located in the central north of Namibia and is one of
the most densely populated parts of the country (figure 2.1). It is part of the greater Owambo
basin, extends over an area of about 97600 km2 which stretches from E 14◦ 10’to 18◦ latitude
and S 17◦ 20’ to 20◦ longitude. Furthermore, the CEB is divided into four sub basins which
were defined by the basin demarcation project. The demarcation project was carried out
in the framework of integrated water resources management of Namibia and had the main
objective to establish equally sized basins throughout the country where relevant matters of
water can be addressed adequately (Bittner, 2006). The basins and sub basins were not only
demarcated by means of hydrological parameters but with regard to physiographic parameters, water supply and consumption, population density and political/administrative regions,
infrastructure, socio-economic and cultural units. All available criteria for the delimitation
was rated with regard to importance, accuracy and relevance (Bittner, 2006).
Fig. 2.1.: Location of the investigation area within the CEB
-5-
2.2 Natural Environment in Near-Surface
The location of the investigation area is highlighted by the pink square in figure 2.1, contains about 8800 km2 and extends over a length of 110 km by a width of 80 km. The cored
boreholes 201216 and 201217 which were drilled as part of the data collection of this investigation are located at (GCS (Geographical Coordinates, WGS-84) [decimal degrees]:
201216 - latitude: -17.597
and
longitude: 16.252
201217 - latitude: -17.396
and
longitude: 16.680
2.2. Natural Environment in Near-Surface
This chapter provides information about the natural environment in near-surface. The
climate, vegetation, land use and soils of the Cuvelai-Etosha basin and the project area in
particular will be elaborated. More information on theses topics can be found in Mendelsohn
et al. (2000) and Struckmeier & Christelis (2001).
2.2.1. Climate
Namibia is an arid country which is according to the common Köppen classification characterised as a dry climate (Plöthner & Bittner, 2001). The climate of the Cuvelai-Etosha
basin in particular is defined as semi-arid with a huge gradient of rainfall across the region
(Mendelsohn et al., 2000). The CEB demonstrates low values of precipitation in the west
with about 250 to 300 mm per year to high values in the east which vary from 550 to 600 mm.
The potential evaporation increases in the same direction with about 2700 to 3000 mm per
year (Plöthner & Bittner, 2001). Most of the precipitation occurs between November and
April while carrying about 96 % of the annual rainfall. Due to high evaporation rates, low
humidity, frequently blowing wind and limited vegetation cover, the effective rainfall of the
area is reduced to about 80 mm per year (Bittner, 2006).
The average temperatures range from 17◦ C in June and July to about 25◦ C within October
and December (Mendelsohn et al., 2000). Maximum temperatures of about 30 to 35◦ C can
be reached from September to December whereas the minimum varies from 7 to 8◦ C in
June. The groundwater temperature is affected also due to the high mean air temperatures.
An in-situ measurement showed values of 23 to 26◦ C (June 2009).
2.2.2. Vegetation & Land Use
Due to the more humid conditions in the north-eastern part of the CEB the major type of
vegetation in these regions is made of woodlands and forest savannas (Struckmeier & Christelis, 2001). Further westwards the woodlands shift more and more into savannas which
-6-
2.2 Natural Environment in Near-Surface
Fig. 2.2.: The annual average rainfall across north-central Namibia (modified after Mendelsohn et al. (2000)
dominate the western part of the CEB. The project area which is situated in the north-central
of the CEB shows a transfer of both types with a preponderate share of savanna type vegetation. Under consideration that the CEB is the most densely populated area of Namibia
with most of the inhabitants living in rural communities a high dependency on agriculture
can be deduced. Therefore, it is detected that about a third of the land belongs to small-scale
farmers, so called communal farmers (Bittner, 2006). Their agriculture is dominated by livestock farming, i.e. cattle, goats, donkeys and poultry while some small-scale irrigation is
also practiced by various communities and farm holdings. Fountain and borehole water of
shallow aquifers is used for irrigation purposes (Struckmeier & Christelis, 2001).
2.2.3. Soils
The soil of the CEB is classified into nine soil types comprising mainly of sand and clay
(Mendelsohn et al., 2000). All soil types differ in their potential for crop cultivation but a
general poor water-holding capacity and a low nutrient but high salt content is noticeable
(Bittner, 2006). The soils of the project area have been highly influenced during the history
of Namibian geology. Continuous water erosion and aeolian denudation processes as well as
alternating sedimentation rates caused the typical Kalahari lithology (Miller, 2009, personal
communication). The rivers Cobango/Okavango and Kunene as well as an ephemeral river
-7-
2.3 Topography & Surface Water Drainage
system brought a mixture of gravel, sand, silt and clay with variable sorting into the project
area (see chapter 2.3).
2.3. Topography & Surface Water Drainage
The Etosha Pan is the lowest point of the CEB with a minimum elevation of about 1000 m
a.m.s.l. (see figure 2.3). The surrounding mountains show elevations of 1400 m a.m.s.l.
on average on the western rim, of about 1900 m at the highest point south-east of Tsumeb,
as well as about 1200 m a.m.s.l. in the north-eastern part of the CEB. So the topography
declines from all directions towards the Etosha Pan, which has consequently no outflow and
is the major discharge area of almost the entire region. It receives water not only from the
Cuvelai catchment area but also from the Otavi Mountain Land and the dolomite arc located
in the south and west of the CEB (Bittner, 2006). Furthermore, it exhibits the lake Etosha
when it was flooded due to heavy rainfalls in the rainy season. Without any surface runoff
the drained water must either evaporate, or percolate into the ground. As the project area
is part of the Cuvelai/Oshana drainage system it is also situated within this intercontinental
basin. Moreover, the project area follows a very shallow morphology that increases from the
south-west with 1044 m to the north-east with 1159 m a.m.s.l..
Fig. 2.3.: Topography of the Cuvelai Etosha basin based on SRTM data
-8-
Fig. 2.4.: Simplified drainage system of Owambo basin in northern Namibia (remade after Mendelsohn et al.,
2000)
Figure 2.4 gives an overview of the three main drainage systems within the Owambo
basin: The Kunene, the Okavango and the Cuvelai/Oshana river system. A high amount of
interconnected river channels is recognisable in the latter which result from flooding of the
so called ”Iishana” and lead to an extensive inland river delta. The Iishana (singular: Oshana) originate because of recurring heavy rainfalls in Angola and northern Namibia during
the summer months when the drained water cuts into the underlying plane Kalahari Sands,
forming raised areas in between (Bittner, 2006). Due to their high appearance in the centralnorth of the CEB, the relevant sub basin is named Iishana. The nature and pattern of water
flows in the Iishana are extremely variable (Mendelsohn et al., 2000). They depend on spatial and seasonal distribution in Angola and the CEB itself. When ’good’ rain falls over the
area, a large flood, named ’Efundja’ comes down bringing large numbers of fish and yet
sometimes danger with it as flash floods threaten peoples homes and lifes. Flooded Iishana
are nonetheless essential for north Namibia not only because the water is used for drinking, pasture farming, irrigations, washing, etc. but as well it ensures fishing grounds and
recharges groundwater supplies. Figure 2.5 shows remains of water-bearing Iishana during
the Namibian autumn. Accumulation of salt as a result of the high evaporation rates are
visible within the Iishana during the dry season. Due to these Iishana, the small elevation
gradient of the project area and the geology (see chapter 2.4), the landscape of the project
-9-
Fig. 2.5.: Water-bearing ”Iishana” in the Eenhana region, Namibia (May 2009)
area shows a gently undulating relief that has a major influence on the entire drainage system. Figure 2.4 points out that the Cuvelai/Oshana river system is recharged in Angola and
its lateral extent stretches from the Kunene catchment in the west to the Okavango catchment
in the east. Data on the mean annual discharge of the catchment is not recorded due to the
highly variable runoff characteristics.
The Kunene River in the west has its source in the Bié highlands of Angola and flows
into the Atlantic on the border of Angola and Namibia. For much of its course, it flows
southwards, as if towards the Etosha Pan but then it turns westwards sharply. The characteristic flow of the upper Kunene River is quite different from the one of the lower Kunene
River. Whereas upstream of the Calueque dam, the river shows a mature profile with a
lower gradient it reveals a steeper gradient further downstream as well as it passes several
rapids. Hipondoka (2005) believes, that these distinct geomorphological expressions have
been rooted in the regional geological development of the subcontinent. While the upper
Kunene which is believed to follow still the old river course and has drained historically
into the Etosha Pan, the lower Kunene could have been either an eroded late carboniferous
glacial valley or built up through graben tectonics. However, it is assumed that the two parts
of this surface drainage have been merged during the Pliocene/Early Pleistocene (Hipondoka, 2005). In addition, the Kunene refers to a catchment area of about 106500 km2 with a
mean annual discharge of ca. 5.5 billion m3 /a (Nakayama, 2003).
The Okavango River has its source at the Benguela Plateau of eastern Angola and terminates in the Okavango Delta in Botswana. As the third largest river in southern Africa, it has
a headwater basin of 121700 km2 and a mean annual discharge of ca. 9.2 billion m3 /a. The
Okavango forms the border between Namibia and Angola for about 400 km (Hipondoka,
2005).
- 10 -
2.4 Geology
Generally, the surface runoff in the CEB is very limited due to an overall sandy substrate
which causes all surface water soak away immediately, or it is captured in one of the numerous clayey and salty pans (Bittner, 2006). Both, the northern Cuvelai drainage as well as the
Etosha Pan which drains also the south-western dolometic arc, are the reason for naming the
basin Cuvelai-Etosha.
2.4. Geology
The study area is situated in the intra-continental Owambo Basin which was formed during
the post-cretaceous tectonic development of southern Africa (Momper, 1982). Then again,
the Owambo Basin owes its origin to the break-up of a super-continent named Rodinia (see
Hipondoka, 2005) and is located on the Congo Craton between 14◦ E to 18◦ E and between the
northern border of Namibia to 19◦ 15’S (Miller, 1997). It extends northwards into southern
Angola and could continue into western Zambia (figure 2.6).
Furthermore, it is floored by mid-Proterozoic crustal
rocks of the mentioned Congo Craton and contains
about 8000 m of sedimentary rocks of the Nosib,
Otavi and Mulden Groups of the late-Proterozoic
Damara Sequence (Miller, 1997). In addition to about
360 m of Karoo rocks the basin is overlain by a
blanket of semi-consolidated to unconsolidated Cretaceous to Recent Kalahari sequence sediments of approximately 600 m. The Name Owambo basin is often interchanged with Etosha basin or Cuvelai-Etosha
basin. However, The Etosha basin is merely a subbasin of the wider Owambo. Throughout history
the CEB was filled with various sediments at very
mixed sedimentation rates. Occasionally, the CEB Fig. 2.6.: Location of the Owambo Basin
(after Miller, 1997)
was drained south east while most of the times it had
no outflow at all (see chapter 2.3). A short introduction to the stratigraphy of the Owambo
basin is given in table 2.1.
The geological information on the Owambo basin was generally achieved from outcrops
along its margins, interpretation of aeromagnetic, seismic and gravity surveys as well as
thinly distributed wells.
- 11 -
2.4 Geology
Table 2.1.: Stratigraphy of the Owambo basin (abbreviated after Miller, 1997)
Era
Sequence
Recent to Tertiary
Cretaceous
Jurassic
Juras.-U. Trias
Lower Permian
Late Proterozoic
Kalahari Sequence
Group
Karoo Sequence
Damara Sequence
Mulden Group
Otavi Group
Nossib Group
Max.
thickness
[m]
750
80
500
137
379
5200
6800
1300
Mid-Proterozoic
2.4.1. Pre-Kalahari Geology
During the Late Precambrian the super-continent Rodinia started to break up. Within
this protracted, multiphase process a variety of sandstones, known as the Nosib Group were
deposited and formed the basis of the Damara Sequence (Miller, 1997; Mendelsohn et al.,
2000). This occurred between roughly 900 and 730 Ma ago (Miller, 1983). As the rifting
continued, a stable platform was evolved on which dolomites and limestones were deposited.
This accumulation of carbonated rocks is named Otavi Group and took place between 730
and 700 Ma ago.
Spreading of Rodinia and with that spreading in the Damara Orogen was followed by
reverse plate motion. This tectonical movement lead to collision of continental fragments
and resulted in development of the super-continent Gondwana. Furthermore, it caused the
dolomites and limestones along the edge of the Owambo basin to be folded and tilted upwards to form a rim to the basin (Mendelsohn et al., 2000). That rim now forms the hills
around Tsumeb, Otavi and Grootfontein. Erosion of the mountain belts around the basin
and with that, accumulation of new sediments of the Mulden Group followed around 650 to
600 Ma ago (Miller, 1997).
The Dwyka glacial period which was an almost worldwide period of glaciation occurred
300 Ma ago. It also affected Namibia, including the Owambo basin. The deposition of
the Karoo sediments started during this time (Mendelsohn et al., 2000). The glaciers cut
deep valleys from the western edge of the Owambo basin across the Kunene Region to the
Atlantic Ocean. Evidence to these glacial valleys is still visible today as the Kunene river
follows one. The Dwyka formation, caused by the glacial period is overlain by the Prince
- 12 -
2.4 Geology
Fig. 2.7.: Geological cross-section across the Owambo basin from Rucana to Tsumeb (Vertical exaggeration
1:30; Hipondoka, 2005)
Albert formation which started when conditions began to change about 280 Ma ago (after
Miller, 1997). Glaciers melted and retreated, the sea level rose as warmer climates prevailed
gradually. Carbonaceous shales, sandstones, siltstones and beds of organic material derived
from plants were deposited in the basin due to the seawater (Hipondoka, 2005). The early
Permian (250 Ma) and the end of the Triassic (200 Ma) were characterised by desert conditions and by aeolian sandstone deposition which resulted in the Etjo formation (Miller,
1997). These latter events were overlapped by a passive rifting of Gondwanaland that started
during the middle Jurassic.
2.4.2. Kalahari Geology
For the past 70 Ma years the Owambo basin has been filling up with sand, silt and clay
that was eroded from higher grounds surrounding the area. Cycles of climate change with
wet and dry periods followed each other (Mendelsohn et al., 2000). Rivers drained into the
basin bringing sediments with them known by deposits called Ombalantu, Beisep, Olukonda
and Andoni formation. Ombalantu represents the base and Andoni the top of the named formations. These four formations form the youngest unit of the basin - the Kalahari Sequence.
The following lithological and stratigraphical descriptions of the Kalahari formations are
based on the work of Miller (1997, 2008c) and mainly consider the sediments and distribution within the Cuvelai-Etosha basin as part of the larger Owambo basin.
- 13 -
2.4 Geology
Fig. 2.8.: Kunene & Cubango Megafan in relation to core-drilling boreholes (modified from Google, 2009)
Generally, the Kalahari Succession close to the Angolan border may be up to 600 m thick
and thins eastwards to the pre-Kalahari basement outcrops along the Okavango river. Figure 2.9 on page 16 displays a cross section of the Kalahari Sediments within the Owambo
basin and the relevant project area. Much of the sediment in the Owambo basin is largely
unconsolidated or only partially consolidated and appears to have been deposited by the
sand-dominated Cubango megafan in the east and the much smaller, mud-dominated Kunene
fan in the west (see figure 2.8). Some cemented sands were logged as sandstones, although
cementing is usually limited. Due to the unconsolidated to generally poorly consolidated
nature of the Kalahari sediments, it has only been possible to core this succession on rare
occasions. In addition to this, drilling companies experience significant difficulties while
retrieving unconsolidated core material, so core loss is the consequence. Because of this
reason boreholes 201216 and 201217 of the relevant project were abandoned at a depth of
266 m respectively 235 m when 390 m respectively 330 m were planned originally.
Exactly when Kalahari deposition began and what constitutes the base of the Kalahari in
the Kalahari basin is not well defined. In Namibia, Botswana and South Africa, the base of
the Kalahari Group is taken as the first unconsolidated or semi-consolidated sediments that
overlie hard basement rocks, commonly of the Karoo Supergroup.
- 14 -
2.4 Geology
Table 2.2.: Stratigraphy of the Kalahari Sequence (abbreviated after Miller, 1997)
Era
Sequence
Formation
Lithology
Recent to
Tertiary
Kalahari
Sequence
Andoni
white sand, light green clayey sand,
green clay
reddish brown, poorly sorted sand
red sand and clay
red semi consolidated clay
Cretaceous
Olukonda
Beisep
Ombalantu
Max.
thickness
[m]
550
152
50
80
Ombalantu Formation - A basal, red, fine grained, semi consolidated but friable formation with variably siclified mudstones but almost entirely consisting of clay. It does not
crop out, has a broad elongate distribution extending from the southeast to the northwest of the basin and reaches a maximum thickness of 80 m. Gypsum and Gypsum
crystals occur in the upper part of the formation. Miller (2008c) evaluates its deposition to be mainly of the accumulation of fine clastics in a shallow, low energetic,
deltaic environment. A restricted continental basin with a significant and sufficient
amount of evaporation was required to lead to the appearance of gypsum.
Beisep Formation - A gravel deposit which is widespread, generally reddish in colour and
represents a period of rapid and extensive input of material from the basin margins.
With a maximum thickness of 50 m it is the thinnest of the Kalahari Formations. It
consists of well rounded sand and clay stone clasts which are set in a matrix of fine to
medium grained, argillaceous, calcareous to dolomitic sandstone.
Olukonda Formation - A friable, poorly consolidated, reddish brown, poorly sorted massive sand and sandstone formation with a limited distribution but a broad elongate sub
outcrop similar to the Ombalantu Formation. It contains a few thin gritty and pebbly
layers and is up to 152 m thick.
Andoni Formation - It occurs throughout the Owambo basin as a cover to all underlying
units and consists of interbedded white medium grained sand, light greenish clayey
sand and green clay. In zones, the predominantly sand varies between 10 and 200 m
and shows an unconsolidated, slightly pyritic or hematitic condition. The top part of
the section contains numerous irregular shaped dolocrete and calcrete nodules which
are embedded in polished, angular to sub rounded grains of quartz which in turn make
up to 90 % of the sand. Sorting improves upwards in the sequence. The appearing
of clay layers within this formation varies in thickness between a few centimeters and
150 m (Ombalantu borehole in Miller (2008c)). They are often silty and/or sandy.
- 15 -
2.5 Hydrogeology
Fig. 2.9.: Cross-section of the Kalahari Group Sediments in the Owambo Basin along the 16◦ 41’ longitude;
Hipondoka (2005)(modified from Kempf, 2000)
2.5. Hydrogeology
After Plöthner & Bittner (2001), all groundwater within the CEB flows towards the Etosha
Pan which is the base level of the groundwater flow system due to the structure of the basin
and because the pan is the deepest point. Three main groundwater flow systems can be
determined within the CEB due to its basic topography (Bittner, 2006).
1. Groundwater that is recharged in the fractured dolomites of the Otavi Mountain Land
at the southern and western rim of the basin. It flows northwards and feeds the aquifer
system of the Karoo and Kalahari sequences (Plöthner & Bittner, 2001). The major
part of this water evaporates rapidly as it discharges through springs along the southern
margin of the Etosha Pan.
2. A deep seated, multi-layered Kalahari Aquifer System which flows from Angola in
southern direction towards the Etosha Pan and the Okavango river. This groundwater
flow system forms the focus of analysis within this investigation.
3. A shallow Kalahari Aquifer in the central part of the CEB which superimposes both
previously described groundwater flow systems (Bittner, 2006). It mainly consists of
saline water and originates from regular floods, respectively from the Efundjas whose
runoff is determined by the ephemeral stream, respectively Iishana.
The schematic map demonstrated in figure 2.10 was the first attempt by Bittner (2006) to
give a rough overview of the aquifer locations within the intra-continental Cuvelai-Etosha
basin. Bittner (2006) defined names for the relevant aquifer systems which are used within
this investigation as well. Detailed descriptions can be found in Bittner (2006) and Plöthner
& Bittner (2001).
- 16 -
2.5 Hydrogeology
Fig. 2.10.: Aquifersystems of the CEB (abbreviated after Bittner, 2006), arrows indicate groundwater flow
direction
Aquifers which have their headwaters in the Otavi Mountain Land are estimated and differentiated on geological units rather than local occurrence (Bittner, 2006). Geological information and aquifer potential are for practical reasons considered to determine one large aquifer
unit even if inhomogeneities and facies changes occur throughout the basin. Experience
and information of similar geological lithology were used to estimate aquifers, aquicludes
or aquitards in combination to determine the Otavi Dolomite aquifer system (DO). DO is a
thick fractured aquifer system which represents the main hardrock aquifers of the southern
and western CEB (Bittner, 2006).
2.5.1. Kalahari Sequence Aquifers
The following descriptions of the five major Kalahari aquifers are based on the work of
Bittner (2006) and were named after the region or locality where they occur or where they
were first described.
- 17 -
2.5 Hydrogeology
Etosha Limestone Aquifer - KEL
The KEL is present at the southern and western margin of the CEB and shows a thickness
of more than 100 m in certain areas. It has an easy accessibility due to a shallow groundwater table, provides good yields of a likewise water quality and is therefor constituted as
an economically important aquifer. It is recharged to a minor extend by the DO aquifers but
isotope studies showed that the major recharge of the KEL contributes to the north-westerly
groundwater flow.
Oshivelo Multi-layered Aquifer - KOV
The KOV was first encountered at Oshivelo from where its extends in a north-western and
eastern direction. While parts are confined, the aquifer is artesian for elevations of lower than
1100 m a.m.s.l. (at Oshivelo & towards Etosha Pan) and provides yields of up to 200 [m3 /h].
It is mainly comprised by gravel sand and its recharge is assumed to be of throughflow from
KEL in major quantity.
Omusati Multi-zoned Aquifer - KOM
The KOM is present in the west of the Etosha Pan and comprises of unconsolidated and
semi-consolidated sediments of the Kalahari sequence. The sediments are mainly sand and
clay but also calcrete/dolocrete and evaporitic deposits. Furthermore, it comprises of mainly
brackish groundwater with fresh water lenses in places. KEL and DO aquifers recharge the
the Omusati-Multilayered aquifer. The salinity of the subsurface sediments is generally very
high causing the groundwater quality to decline. Changes can be very sudden with fresh
water occurring in boreholes of the KEL only a few kilometers away from boreholes with
saline groundwater in the KOM. The KOM is separated from the KOS as it is not recharged
by the Cuvelai drainage system.
Oshana Aquifer - KOS
The Oshana aquifer is a shallow aquifer system with a maximum thickness of 80 m and an
average thickness of 10 to 15 m. The KOS superimposes the confined multi-layered aquifer
system of interest (KOH) within the northern part of the CEB. It is an unconfined and perched
aquifer system that comprises of a relatively thick sequence of Andoni Formation and is
recharged by the regular flooding of the Cuvelai/Oshana drainage system described in chapter 2.3. The water level gradient is very flat and as a consequence of the high evaporation
rate described in chapter 2.2.1 the Iishana leave salts as products which can be hydrated to
portion during flooding. This in turn leads to an increasing salinity of the shallow aquifer.
The degree of salinity within this aquifer relates directly to the amount of precipitation. The
- 18 -
2.5 Hydrogeology
higher the precipitation rate, the greater the amount of groundwater and the lesser the concentration of solutes. Due to the seasonal and constantly shifting depositional environment,
a cross bedding of sandy and clayey layers appears within the aquifer proportions. Correspondingly, local aquifers, aquitards and/or aquicludes are alternating which limits an areal
determination of the hydrogeological properties. The KOS is known to be encountered in
the entire region of the project area in general although some boreholes reveal its absence.
It is therefore stated that the KOS is a discontinuous aquifer system which consists of small
groundwater lenses with variable groundwater quality.
Ohangwena Aquifer - KOH
The drilling and with that the whole data collection of this investigation has been done in
the Ohangwena Aquifer system. The KOH is a multi-layered porous aquifer system which
was encountered east of Ohangwena. It lies in the Iishana and Nipele subbasin and the
groundwater is estimated to flow southwards in the direction of the Etosha Pan. Moreover, it
is assumed that an upper Aquifer KOH1 is separated by an aquiclude or aquitard to a lower
aquifer KOH2 . KOH1 has been intersected between Eenhana and Okongo at depths between
60 and 160 m and represents a major water source within the region (Plöthner & Bittner,
2001). It consists of the light greenish clayey sand of the Andoni Formation and appears
in terms of fresh water close to the Angolan border. Towards the south it becomes brackish
to saline within the distance of a few kilometers and is therefore not developed for drinking
water purposes.
The interesting and deeper seated fresh water aquifer KOH2 was intersected in the same
area as KOH1 . It was encountered in the Nipele-subbasin and is assumed to have a continuous and regional extend. Due to its great depth of 130 to 380 m it is situated partly within
the Olukonda Formation and has not been explored precisely. Like KOH1 , the recharge area
is assumed to be in southern Angola. The water quality is fresh for the east and north of
Eenhana but becomes more saline towards the south-west where it is still regarded as water
of good quality according to the Namibian Drinking Water Classification System (based on
values of electric conductivity). The general salinity of the aquifer, however, is lower than
any salinity measured in the upper aquifer KOH1 (Plöthner & Bittner, 2001). After Schildknecht (2007), the existence of the fresh water aquifer was reconfirmed during drilling of
two boreholes close to Eenhana. Significant decrease of electric conductivity measurements
were observed at greater depths of borehole 37070 which indicates a non-saline environment. Furthermore, Schildknecht (2007) reasoned that the existence of a fresh water aquifer
subjacent to a high saline aquifer implies that a separating layer parts both aquifers and that
the hydrostatic pressure of the KOH2 must be greater than of KOH1 . Otherwise would the
- 19 -
2.5 Hydrogeology
upper saline water percolate into greater depths and mix with the fresh water of KOH2 due
to the higher density of saline water. The greater hydraulic pressure must further on result
from a hydraulic connection to a higher recharge area which lies probably in a north-eastern
direction (Angola). The exact geometric proportions and expansion of the aquifer and the
separating layer have not been determined yet.
After detailed research it was not possible to obtain sensible information of the recharge
area in Angola. It is therefore assumed that these natural actualities resemble the actualities
of the CEB which are described in chapter 2.2.
- 20 -
Chapter 3. Methods & Techniques
3. Methods & Techniques
The methods and techniques of data acquisition are described within this chapter. It is divided into two main subchapters. The first subchapter deals with field work and sampling.
This includes information on five drilled boreholes which were drilled in two stages. The
first stage comprised of 3 mud rotary drilled boreholes while the second stage drilling was
performed to receive undisturbed core material of the Kalahari sediment and the KOH multilayered aquifer system (chapter 2.5) in particular. The second subchapter deals with laboratory work and methods of calculation. All analyses and evaluation methods applied to the
sample material are described thoroughly. These analyses are necessary to understand the
hydrogeological characteristics of the sediment.
3.1. Sampling
The cornerstone of further investigations on the hydrogeological condition in the project
area was laid between March and September 2008. Several TEM (Transient Electromagnetic) measurements were carried out in cooperation with the DWAF as part of the investigation on the groundwater system in the Cuvelai-Etosha Basin. The TEM measurements
provide the basis for all further investigation purposes on the KOH aquifer system, including
this investigation.
Table 3.1.: Overview of field work & data collection for the project ’Groundwater Investigation in the CEB’
Investigation
method
TEM
ID
Date
Amount Comment
March-Sep. ’08
440
71 in
investigation area
Mud
rotary drilling
201045
201046
201047
Feb.-April ’09
3
In-situ
geophysic
Core drilling
201216
201217
May-July ’09
2
Laboratory
work
- 21 -
3.1 Sampling
Fig. 3.1.: TEM - Transient Electromagnetic Measurements within the project area
TEM was developed to determine the electric conductivity of the ground to a depth of
a few hundred meters without the expensive procedure of drilling. It is an active method
of measurement in which a periodical electromagnetic signal is induced into the ground.
Conclusions to the distribution of the electric resistance can be drawn from the measurement
of the subsiding voltage in its relation to time. A vertical profile can be established that
shows the electric conductivity (EC) of the ground. High EC values relate thereby to a high
salinity whereas low EC values relate to a non-saline environment.
Analysis of these measurements indicated non-saline layers at depths greater than 200 m
that were assumed to be a deep fresh water aquifer with a regional occurrence within the
basin. Three boreholes were drilled between February and April 2009 to confirm the existence of the deep fresh water aquifer. The three boreholes were situated in areas where the
existence of the fresh water aquifer is well established by the sounding curves and where
interpretation of the sounding curves exhibits the lowest modeling errors. Metzger Drilling
was contracted for mud rotary drilling as well as geophysical sampling of these three boreholes (201045, 201046, 201047 (see figure 3.1)).
Two core boreholes were drilled between May and July 2009 since the motivation for
further investigations on the hydrogeological characteristics of the project area was laid by
boreholes 201045, -46 and -47. Again, these two core boreholes (201216, 201217) were
positioned close to the sites of best TEM sounding curve interpretation due to the already
- 22 -
3.1 Sampling
gathered information on mud rotary boreholes and to check on the correctness of the TEM
interpretation. Core borehole 201216 was positioned in 12 m distance to 201045 while 100 m
distance are between 201217 and 201047.
3.1.1. Mud Rotary Drilling
Most of the information within chapter 3.1.1 is based on the scientific investigation and
work of Braam van Wyk and his report ’Groundwater Investigation of the Cuvelai-Etosha
Basin, 2009’.
According to the interpretation possibilities of the TEM soundings, it was planned to penetrate and sample the sediment to the depth of 390 m. This depth should intersect subjacent
layers to the assumed aquifer KOH2 which were estimated to comprise of clay to a large extend. Therefore, the three mud rotary boreholes were drilled to a depth of 390 m at 201045,
whilst 201046 and 201047 were drilled to 266 m and 383 m respectively. Borehole 201046
had to be abandoned at 266 m due to problems in the alignment, respectively straightness of
the borehole. Afterwards these boreholes were developed into wells with PVC casings and
screens in the region of the assumed aquifer KOH2 . The screens have an area of 50 mm2
each and were installed at the depth of:
201045: 243.70 - 266.50 m
and
283.60 m - 346.30 m
201046: 236.50 - 259.30 m
201047: 214.50 - 294.30 m
Mud rotary drilling was done throughout the entire drilling process. The drill string consisted of a 311 mm tri-cone roller bit, followed by the EDI ’Fluid-Finder’, a centralizer, seven
or eight drill collars and smaller drill pipes. The drill collars measures 204 mm OD (outer
diameter) and 70 mm ID (inner diameter), and weighed a 1000 kg each, whilst the drill pipes
were 146 mm OD and 136 mm ID, weighing 200 kg per pipe.
The drilling fluid was a synthetic polymer mix made up of CAP 21, EEZI-MIX and clear
water. Circulation of the drilling fluid was done by means of a centrifugal pump where the
rates depended on the drilling depth as the flow rate of the centrifugal pump decreases at
depth.
- 23 -
3.1 Sampling
The EDI Fluid-Finder is a useful tool for sampling
an testing of specific groundwater bearing horizons.
Its dimensions are 0.17 m in diameter, 1.9 m in length
and an open area of 45 cm2 through which water flow
can occur. It is attached between the drill bit and the
centralizer while the reason for applying the FluidFinder is to investigate the sediment in-situ. Hydrogeological parameters can be evaluated by withdrawal of groundwater samples during drilling process and without demounting of the drill rods. The
Fluid-Finder requires mud rotary drilling, direct flush
only which is a disadvantage when accurate geological samples are required. Undisturbed ground samples cannot be taken if drilled that way.
Fig. 3.2.: EDI Fluid-Finder (van Wyk, 2009)
Data recorded on site
The geological logging was done as accurately as possible by taking out mud samples as
a mixture of every five meters. The samples were described immediately on site to minimize
errors caused by colour shifting due to redox reactions. For exploration purposes the direct
flush rotary technique did not turn out to be a useful technique since the time lag in sampling
was to high for accurate logging at great depths (van Wyk, 2009).
The Fluid-Finder was used, as already described above, to take water samples at specific
aquifer horizons with the purpose of receiving information on the hydrogeological condition
of the sediment as well as the general chemistry and stable isotopes of the groundwater.
Except for that, penetration rates [m/min] as well as the conductivity of the drilling fluid
were recorded during drilling process. In addition to the geological information derived
from the drilling, borehole geophysics were executed on each borehole after completion.
The following parameters were logged in situ:
• Gamma [CPS]
• Induced conductivity [mmho] resp. [mS]
• Temperature [◦ C]
The natural gamma is principally radiated among the naturally occurring elements uranium, thorium, their derivates, as well as the potassium isotope 40 K. Generally bound in clay
minerals, the potassium isotope reflects therefore the clay content of the investigated soils or
- 24 -
3.1 Sampling
rocks (Füchtbauer et al., 1967). Hence, in situ measured gamma radiation was plotted in so
called ’gamma-ray logs’. That helped in identifying interesting layers and stretches of the
rock formation as a higher gamma radiation implied a higher clay content.
Test pumping was performed in ways of a step-drawdown-test followed by a 48-hour
constant rate test, both containing a respective recovery period. Four steps were implemented
during step testing with a one hour duration for each step. The constant rate test was done
to at least 95 % recovery. Wyk (2009) believes that with a maximum discharge of 16 m3 /h
the aquifer parameters were described adequately. The data collection gathered during test
pumping includes rest-, as well as pumped water levels, regular discharge measurements and
electric conductivity recordings.
3.1.2. Core Drilling
According to TEM sounding curve interpretation it was intended to core the Kalahari sediment and receive sample material of the upper aquifer KOH1 , the presumed separating layer
and of the possible deeper seated aquifer KOH2 . Data was collected for boreholes 201216
and 201217 in terms of geophysical investigations and core material during a six week field
trip. The core material comprised of Kalahari sediment and was withdrawn from the depth
of 80 m to 390 m at site 201216, respectively 80 m to 320 m at site 201217. Observation of
core drilling and gathering of relevant geological and hydrogeological data started on site
201216 when drilling was already at the advanced depth of 212 m.
Mud rotary drilling was done for the first 80 m below surface. After that coring took place
based on the wire line diamond coring HQ-3 size. This includes a tube with inner diameter
of 61.1 mm inside a rod which shows an inner diameter of 96 mm. Figure 3.3 gives an
example of the used diamond core drilling bits (bottom-right) and shows the core catcher
(top-left) that is preventing the core from sliding out of the tube during operation. Both tube
and rod had a length of 3 meters. The method of counter rotating pipes was used to receive
’undisturbed’ data, respectively cores of highest possible quality. Rods moved clockwise
during operation. Simultaneously, the tube rotated counter clockwise with lesser speed. A
slightly smaller layer inside the tube which was barely moving at all rotated clockwise again
and ’jacketed’ the core carefully. To guarantee smooth rotating all pipes ran on ball bearings.
The drilling fluid was a synthetic polymer mix made up of CAP21 and water. Recurring
core loss and immense water loss was detected during coring. The water loss in particular
was suspected to relate to cavities within the unconsolidated rock. Therefore, DRILLVIS
was added to the drilling fluid from the depth of 263.15 m at site 201216. DRILLVIS is a
drilling viscosifier with a high molecular weight comprised of polyacrylamide/acrylic acid
- 25 -
3.1 Sampling
Fig. 3.3.: Tube with core catcher holding core (top-left) & wire line - diamond coring drill bit (bottom-right)
copolymers. It provides faster hardening and a higher viscosity compared to CAP21. Hence,
decrease of water loss as well as a very viscous drilling fluid was observed from 263.15 m at
borehole 201216. Throughout the drilling of borehole 201217 DRILLVIS was used merely.
Constant circulation of drilling fluid could not be monitored. The drilling fluid was recycled
whenever mud pits where almost full so that at least a minimum of 10000 liters (two 5000
liter tanks) was provided at all times.
During normal operation 3 m where cored in one go. The drilled meters per run were
reduced as a first step due to core loss which was experienced on almost every run from the
depth of 212 m at borehole 201216 until it was abandoned at a depth of 266.15 m. Thereby,
the weight of the core lessened and it was more likely that the unconsolidated rock within
the tube will not slide out. Several problems emerged at the drilling sites 201216 and 201217
which lead to water and/or core loss and made a constant repetition of collecting data impossible. They will be described here shortly.
From the 9th until the 13th of May the drilling machines where out of operation at site
201216. Decay of the drilling fluid causes it to lose its binding properties after approximately
six days so that the risk of wall cake collapse increases (van Wyk, 2009). As casing was
driven down to a depth of 80 m the upper part of the borehole was secured. Small, partly
- 26 -
3.1 Sampling
Fig. 3.4.: Drilling rig of Major drilling used for the project. Supply of rods at the bottom left. Mud pit at the
bottom. 5 m3 tank of drilling fluid can be seen on the right.
wall cake collapses at circa 187 m were observed in the later days of the total operating time
which comprised 20 days at borehole 201216. To avoid this problem on site 201217 the
drilling team split up and worked in two shifts. The second shift drove the casing down after
the first drilled core in the morning. A maximum of 15 meters distance between drilling
depth and casing intended to minimize the risk of a borehole collapse as well as the risk of
water loss due to cavities.
After the depth of the assumed aquifer was reached at approximately 220 m of borehole
201216 difficulties appeared which were caused by the then encountered, extremely soft and
muddy material. Appendix D gives evidence as it shows pictures of soft core material with
a very high water content. One of the drilling fluid purposes is to stabilise the borehole by
hardening and creating a wall cake with the surrounding rock. Furthermore, it prevents the
rods from getting stuck by building up pressure to the surrounding rock masses. As soon
as the drilling stops, the pressure is not upheld because the tube has to be pulled upwards.
The nearby ground was extremely soft and even mobile so it pressed on the rods and hold
them tight as soon as the pressure was absent. Additionally, particles of the unconsolidated
- 27 -
3.1 Sampling
material were sucked into the rods as a vacuum effect occurred whilst lifting the tube. As a
consequence, the tube could not be lowered to basal level on the next run.
The logging of core and mud rotary boreholes will outline that the lithological development in the project area is extremely heterogeneous and often intersected by pebbles or layers
of calc- and dolocretes (chapter 4.4). These hard calc- and dolocretes can get dragged along
during drilling process in the softer sandy environment which can lead to core loss. Calcretes
and/or dolocretes can block the tube and grind the soft environment by rotating along with
the drilling bit.
Major Drilling used water of mud rotary boreholes 201045 and 201047 as the water supply
for coring at 201216 and 201217. The approximate distance between mud rotary borehole
201045 and core borehole 201216 is 12 m. This short distance could be a reason for some
of the observed difficulties as interferences due to pressure fluctuations must be taken into
account. To avoid interferences at the second drill site due to water withdrawal out of 201047,
core borehole 201217 was relocated at a distance of about 100 m to 201047.
Personal observation of the drilling progress stopped on the 11th of June at a depth of
202 m on site 201217. Information was received constantly in the following weeks, with
reports on recurring core loss and casing that got stuck. The decision was made to abandon
borehole 201217 at a depth of 235 m due to the already described difficulties and resulting
delays.
Data Recorded during Drilling & Logging of Cores
A pre-log of the core material was executed immediately after core withdrawal. This
included a provisional estimation of the main rock type, determination of rock colour as well
as the notes on the condition of the sediment.
Temperature [◦ C], pH value and the electric conductivity (EC) [µS/cm] were recorded
of the drilling fluid during working process. A constant repetition of collecting data was
impossible due to the described problems and irregular drilling rate but recording of data
was done in a sensible framework and whenever possible. It was attempted to receive mud
samples whenever core loss was detected. The attempt was abandoned due to the lesser
rotating velocity and slower penetration rate in core drill procedure and the resulting poor
yield of particles washed out by the drilling fluid.
The logging of cores started in the end of June and was done within the grounds of the
Geological Survey of Namibia by Dr. Roy Miller, an expert in Namibian Geology. The entire
core material had to be transported to Windhoek which was done after drilling had finished
- 28 -
3.1 Sampling
on each site. Thereby, a few weeks time lie between core withdrawal and logging. The core
material was protected from evaporation and solar radiation by means of shade and plastic
tubes during all times of storage and transport. Protection measurements were performed
immediately after withdrawal of undisturbed samples from the ground to guarantee cores of
highest possible quality.
- 29 -
3.2 Laboratory Work & Methods of Calculation
3.2. Laboratory Work & Methods of Calculation
To receive significant results in numerical modeling and/or establishing a conceptual model
it is essential to describe the complex processes of nature as best as possible. The modeling
parameters, initial and boundary conditions must either be known or estimated. A plan for
several laboratory test was established in July 2009 in which it is intended to characterise the
hydrogeological condition of the CEB as accurate as possible. Table 3.2 presents all ideas
to analyse the samples of core boreholes 201216 & 201217. The laboratories could not deliver data to all analyses in time. Therefore, it should be noted that in account of the time
consuming laboratory work the other analysis results have to be interpreted at later stages.
Representative samples of the entire core material were chosen to characterise the hydrogeologic condition of the area as best as possible. These samples were send to the BGR for
analysing methods presented in table 3.3. The dispatched selection was based on information
derived from:
• Geological logging of cores such as:
– Rock
– Colour
– Grain size
– Amount of calcretes & dolocretes
• Photos of cores and taken during drilling process
• Geophysical logging of hole 201045, -46, -47 such as:
– Natural Gamma radiation [CPS - counts per second] - referring to a possible clay
content
– EC values [mS] - referring to the salinity
• Personal observation
– Preliminary rock type determination
– Qualitative condition of the cores
The total of 19 samples were send via two consignments whereas 9 samples were taken
from 201216 plus 10 samples taken from 201217. In order to allow different laboratory
analyses, it was indispensable to divide each sample into smaller parts.
- 30 -
Table 3.2.: Overview of laboratory work in technical cooperation with the BGR
Analyser
Eluate & centrifugal outcome
Geophysical measurements
Pressure depending perfusion in triaxial cell
He - porosity
Mineralogy of clay
Grainsize analysis
Mineralogy of sands
Micropaleontologic measurements
Morphometry
Carbon - Oxygen isotopes analysis
Carbon 14 dating
Progress
In process
In process
Results received (Rr)
Rr
Rr
Rr
Rr
In process
In process
In process
In process
3.2.1. Evaluation of k-Value on Basis of Pressure Depending
Perfusion in Triaxial Cell
Origination
The hydraulic conductivity of the ground is highly influenced by pore volume and pore
size distribution. It is related to the structure and texture of the ground to a large extend,
and determines the flow behaviour of the groundwater (Reimann, 2004). Moreover, it is
an anisotropic value, i.e. it varies within the three directions of space. Under laboratory
conditions it is usually tested for one dimension, mostly vertical which was done within this
investigation also.
After DIN, 18130 (Deutsches Institut für Normung - German Institute for Standardization), a convenient method for specifying the hydraulic conductivity (k-value [m/s]) can be
performed by using a triaxial cell. To draw conclusions from triaxial testing it is necessary
to describe the rate of flow through a porous medium in a framework that implies the condition of continuity as well as conditions of Darcy’s law. Basically, this requires laminar flow,
steady state and analysis in a macroscopic range. An image of the breadboard construction
can be seen in figure 3.5 on page 33 and will be described within this chapter.
Groundwater flows through an aquifer are driven by the difference in water pressure (or
head) over the aquifer (Brassington, 2007). While the imbalance in groundwater levels is
called head loss (∆h) and is usually expressed in meters, the slope of the water table is
named the hydraulic gradient (∆h/L) which is the dimensionless ratio of head to distance.
- 31 -
Table 3.3.: Depth related core samples for laboratory analyses (Column layer def.: ’s’ - salty aquifer or overburden; ’b’ - brackish aquifer or overburden; ’c’ - confining layer; ’f’ - fresh water aquifer, layer
expressions are preliminary assumptions that might differ with final determinations)
Sample-ID
201216-19
201216-22
201216-28
201216-45
201216-52
201216-54
201216-72
201216-73
201216-81
201216
Depth [m]
126.90 - 127.25
136.69 - 137.17
153.80 - 154.22
201.26 - 201.76
218.45 - 218.87
219.50 - 219.92
248.50 - 249
249 - 249.45
265.65 - 266.13
Layer
c
c
c
c
c
c
f
f
f
Sample-ID
201217-9
201217-18
201217-25
201217-38
201217-43
201217-55
201217-70
201217-73
201217-84
201217-98
201217
Depth [m]
Layer
89.90 - 90.30
b
99.50 - 100.01
b
105.05 - 105.60
c
117.99 - 118.50
c
126.43 - 126.87
c
151.62 - 152.18
c
195.60 - 196.10
c
199.60 - 200.08
f
213.20 - 213.70
f
228.50 - 228.99
f
The Darcy equation relates the groundwater flow rate Q [m/s] to the hydraulic gradient ∆h/L
and the cross-sectional area A [m2 ](see equation (3.1) on page 34).
Breadboard Construction & Realisation
In preparation for triaxial testing it was convenient to undertake several steps. The cored
diameter of 61 mm had to be reduced to 50 mm due to the measurements of the triaxial cell.
Chapter 3.1.2 outlined that all samples demonstrated a very soft texture. To avoid damages or
destruction each sample was stripped off to the desired diameter by hand. More preparations
were done by means of chemical analysis of the core solution. Afterwards, its essential
ingredients were combined in a drafted aqueous solution to express the actualities of the
natural environment as best as possible. To avoid misunderstandings this drafted solution is
meant when speaking about ’the solution’ in the course of this report. The solution showed
a pH value of 9.8 while it had a concentration of 12 mg/l NaHCO3 and 159 mg/l Na2 CO3 .
The adding of sodium hydrogen carbonate and sodium carbonate was done to prevent the
samples, respectively clay minerals of the samples from swelling. If samples would swell
out during triaxial testing the hydrogeological properties of the unconsolidated rock matrix
would change. This in turn would lead to errors of the flow rate and cause changes of the
hydraulic conductivity. A hydraulic connection throughout the whole system is imperative
to establish any flow rate. Consequently, the core sample as well as filter stones had to
be in a saturated condition. Filter stones were added to the construction for well balanced
distribution of the solution. It was intended to eliminate selective flows within the samples.
- 32 -
Fig. 3.5.: Photo & schematic image of breadboard construction ’triaxial cell’
The saturation of samples was managed by adding lead time on the testing procedure whereas
the filter stones were saturated before installation.
The whole equipment consisted of two pumps applying the needed pressure, the degasified
solution, a metal bonded glassen cage that could withstand pressures up to 10 bar, filter stones
on either end of the sample, a rubber sleeve that jacketed the core sample and rubber pipes
for connections.
Having undertaken all preliminary steps, the undisturbed sample was jacketed by the rubber sleeve and filter stones were added on either end. Now the whole devise was inserted
carefully into the cell. It was important to not smear the material as pores might close and
flow characteristics would be changed. Afterwards, the glassen cage was added and filled
with water. It becomes apparent that the rubber sleeve separated core and solution from surrounding water. To meet approximate states of pressure in depths of core withdrawal the first
pump was used to apply ambient pressures that ranged between 5 and 8 bar.
After set up, the testing of pressure depending perfusion could start eventually. The second pump was filled with solution to establish a constant rate of flow which was needed to
generate a constant head loss (∆h). This was carried out by applying a constant pressure
(or head) on the ’inflow side’ of the sample. Small changes on the head due to barometric
conditions or the constant outflow of the solution were corrected automatically. By means
of this investigation the implemented heads ranged from 0.2 to 6 bar. To guarantee laminar
flow, the applied pressure was not greater than 6 bar. Lesser hydraulic gradients were not
generated due to the extremely small permeability revealed within this report (see chapter
- 33 -
4.5). Hence, hydraulic gradients which would equal the natural conditions were not applied
during testing as originally planned.
The applied hydraulic gradient resulted in a laminar flow from bottom to top which was
digitally recorded at a constant interval. Besides, the flow through a sample in a bottom-up
process enables possible air entrapments to exhaust. For validation reasons ∆h was modified as often as possible during multiple executions of the test (see table 4.1 on page 66
and 4.2 on page 68), though some samples were just tested once as the procedure is very
time-consuming. In any case, the applied pressures of the first pump had to be greater than
pressures of the pump generating the hydraulic gradient at all times. Thereby securing that
water flow occurs through the porous material and not between sample and rubber sleeve.
Testing times varied for each analysis because flow char
3.2.2. Saturated Hydraulic Conductivity after Darcy
As already mentioned, the saturated hydraulic conductivity was evaluated based on steady
state and laminar flow. Therefore, the flow rate was visualised for each sample. The outflow
of solution by the pressure pump was plotted against its chronological sequence. An example
is given in figure 3.6 (All graphs can be seen in appendix A). With regard to a preliminary
lead time, a linear line of best-fit was used to determine an average flow rate as accurately as
possible, thereby representing the required constant conditions and rate of flow.
Considering sample proportions and terms of steady state, the saturated hydraulic conductivity was evaluated after Darcy:
K=
V
t
·L
A · ∆h
[ ms ]
(3.1)
K - Saturated hydraulic conductivity [m/s]
V - Volume of discharge respectively solution [m3 ]
t - Time of discharge [s]
L - Core sample length [m]
A - Cross-sectional area [m2 ]
∆h - Difference in water levels - head loss [m − water column] at 21◦ C
Conversion of pressure units can be estimated roughly with:
1 bar = 100 kPa ≈ 10 m water column.
- 34 -
Fig. 3.6.: Example of recorded data during testing of hydraulic conductivity & adapting of best-fit line
The exact equation that was used to converse from kPa to [m − water column] and is
shown in equation (3.2)
kPa
[m − H2 O] =
(3.2)
g · ρ · 0.001
with
ρ = 998.1
kg
m3
and
g = 9.81
m
.
s2
When hydraulic conductivity is measured either in laboratory analyses or in the field it
is important to ensure that the temperature of the test water equals the usual groundwater
temperature in the investigated aquifer. Otherwise, a correction factor has to be applied to
the results (Brassington, 2007). As the average in-situ temperature of the groundwater was
measured with 24◦ C the relevant conversion factor of cT = 1.09 was used (Reimann, 2004).
K 24◦ C = K · cT = k · 1.09
[ ms ]
(3.3)
As the whole analysing equipment, including core samples and solution, were stored in
the same room and protected from direct solar radiation at all times, their temperature was
determined to equal the air temperature within the laboratory. Random samples of solution
and core temperature were taken occasionally, confirming this estimation. The average air
- 35 -
temperature was amounted with 21◦ C, hence all calculations of the hydraulic conductivity
were done on this basis and transferred to conditions of 24◦ C afterwards by applying the
conversion factor cT .
3.2.3. Grainsize Analysis & Porosity
Grainsize Analysis
A basic method in hydrogeology is the presentation of results based on the grain size
distribution. Defined rock names and important statistical parameters can be derived from
the grain size frequency distribution and the cumulative grainsize curve (Füchtbauer et al.,
1967). The grainsize analysis defined the grain composition of core samples from sedimentary rocks in the project area. This was done by shaking the sedimentary samples through a
series of sieves with decreasing mesh openings. This procedure is reasonable for grainsize
diameters of more than 0.063 mm (Reimann, 2004). All sieves residues were weighed separately while their sum must comply to the their total weight. Then, the weight is transformed
into the cumulative ’percent finer by weight’ which can be plotted on semilogarithmic paper
(see figure 4.7 in chapter 4.6.1, page 72).
Normally, the grainsize distribution of the fines (clay & silt, with grain sizes ≤ 0.063 mm)
is determined by a hydrometer test which is based on the fundamentals of Stokes’ law. This
describes the connection between specific gravity of grains, settling velocity and grain size.
Upon consultation of several BGR scientists it was decided not to perform a hydrometer
test as this would change the natural occurring aggregates of the sediment and would provide results which do not represent the natural actualities. This decision implies that further
division into grain sizes smaller than 0.063 mm was not performed. If it becomes apparent
during the course of this project that further examination of the fines is necessary, hydrometic
analysation can be done at a later stage.
3.2.4. Indirect Evaluation of Hydraulic Conductivity Based on
Grainsize Analysis
Three methods are presented within this chapter to estimate the hydraulic conductivity
based on the effective grain size (dW ) which represents the mixture of grain sizes of an
investigated sediment.
- 36 -
Hazen
In need for finding an effective grain size Hazen (1893) performed several experiments
and came to the following conclusion (Hölting, 1996):
2
K = C · d10
[ ms ] , if dW ∽ d10
(3.4)
0.7 + 0.03T
86.4
d60
U=
d10
C=
C - Empiric coefficient, depending on the water temperature, the lithological structure of
the rock and its uniformity coefficient U
U - Uniformity coefficient
d10 - Grain size corresponding to the 10% line on the grainsize curve [mm]
d60 - Grain size corresponding to the 60% line on the grainsize curve [mm]
T - Water temperature in ◦ C
Beyer
Beyer (1964) found a functional connection between dW and d10 based on graphical evaluation of the effective grain size on well rounded, quaternary and tertiary sands and gravels in
northern Germany (Hölting, 1996). After numerous comparisons of the hydraulic conductivity determined in laboratory analyses and pumping tests he was able to evaluate a hydraulic
conductivity that depended on d10 , which again showed a dependency on the uniformity coefficient U. He presented his results in graphs and tables that can be found in various books
and technical literature about hydrogeology (amongst others on pages 69 - 70 in Langguth &
Voigt (1980)). These tables were used to estimate the hydraulic conductivity in the case of
known percentages for d10 and d60 . Therefore, d10 and d60 were read off the relevant grain
size curves (Appendix B).
- 37 -
Kozeny & K¨
ohler
Kozeny & Köhler modified the original equation of Hazen with regard to porosity and
roughness parameters of the investigated rocks:
τ
ε3 2
2
K = C · dW
= 405 · 10−4
d
r
1+ε W
ε=
ε - Void ratio
[ ms ]
(3.5)
n
1−n
n - Porosity
τ - Ratio of the kinematic viscosity at 10◦ C to its matching value at aquifer temperature
dW - Effective grain size
r - Roughness parameter - Varies from 1 (smooth sphere) to 5.5 (sharp-edged grains) (after
Hütte, 1951)
The effective grain size was evaluated after Köhler:
∑ d1i ∆Gi
1
=
dW
∑ ∆Gi
(3.6)
1 1
1
1
= ( + )
di 2 du dl
(3.7)
Gi - Percentage of grain size classification i
du - Grain diameter at the upper limit
dl - Grain diameter at the lower limit
i - Index of a grain size classification which corresponds to the mesh diameters of the sieves
Porosity
Density and porosity are important parameters which describe the micro textural conditions of the ground. The porosity and the pore size distribution influence the grounds ability
on hydraulic, thermal and aerial conductivity as well as on sorption processes and its ability
to distort (Klinkenberg, 2008). The total pore volume was not investigated completely. In
fact, the porosity was determined within the dried state and up to a pore diameter of 100
to 150 µm, thus excluding macropores partly. Pores are classified after IUPAC (1985) into
- 38 -
micropores (< 0.002 µm), mesopores (0.002 − 0.05 µm) and macropores (> 0.05 µm). The
porosity was calculated for all samples by:
n=
(ρs − ρ) · 100
ρs
(3.8)
n - Porosity [Vol. − %]
ρs - Grain density [g/cm3 ]
ρ - Bulk density (excluding macropores) [g/cm3 ]
The grain density was evaluated by helium pycnometry. Helium can even enter the smallest voids or pores and gives therefore a grain density of very high accuracy. Within this
procedure, the volume of the solid phase was determined for all samples by the expansion of
helium in its relation to the assigned pressure. With knowledge of their respective weight, ρs
and can be identified for each sample. The bulk density in turn, was determined in an equivR
alent procedure. But instead of helium, DryFlowas
used which is a quasi fluid medium. It
is a powder comprised of spherules with varying diameters (Klinkenberg, 2008). The finest
spherules (diameter of 50 µm) fit thereby in the gores of larger ones, which in turn fit into
gores of still larger spherules and so on. Thereby, pores which are smaller than 50 µm are
not filled with medium. If the same sample is used, this implies differences in density results
for both procedures (helium and Dryflo). The according value is smaller for Dryflo. If all
pores larger than 50 µm were filled by Dryflo the resulting density could be evaluated up to
a pore diameter of 50 µm. In reality, Dryflo cannot reach all pores. The resulting density is
rather evaluated up to a pore diameter of 100 to 150 µm. As the density evaluated by Dryflo
is set as bulk density while the grain density is defined by the helium procedure, the porosity
can then be calculated by equation (3.8).
3.2.5. Mineralogy
The mineralogic examination contributes to the hydrogeological investigation of this work
as it provides information on the composition and contents of the investigated ground. This
can reconfirm the determination of hydrogeological layers. Several analysis methods were
performed to determine the mineralogic composition of the CEBs’ sediment. Visual, thermo
analytical, chemical, and other tests were applied with the assistance of several BGR scientists to describe the minerals in a quantitative and qualitative way. 10 g of each core sample
was dried at 60◦ C and ground by a mortar mill as a necessary preparation. The following
explanations on mineralogic analyses are based on the work of Klinkenberg (2008).
- 39 -
XRD
A qualitative examination of the mineralogical composition was done using XRD (X-ray
diffraction). The basic idea of X-ray analysis is that each crystalline substance has its own
characteristic atomic structure which diffracts X-rays in a characteristic manner (Füchtbauer
et al., 1967). Cu − Kα - radiation was generated employing a diffractometer. The samples
had a diameter of 28 mm and were investigated from 2◦ to 80◦ 2Θ with a step size of 0.02◦
2Θ.
DTA-MS
Thermo analytical investigations were performed using the method of DTA-MS (Differential Thermal Analysis - Mass Spectrometer). 100 mg of powdered sample material was
equilibrated at 53% relative humidity and heated from 25 to 1000◦ C with a heating rate of
10 K/min. Its use is to identify minerals as well as their stability which are conditioned
partly by their degree of crystallinity.
XRF
Chemical composition of the powdered samples was determined by using a XRF (X-ray
Fluorescence Spectrometer) . Therefore, the samples were mixed with a flux material and
melted into tablets. These tablets were analysed by wavelength dispersive X-ray fluorescence
spectrometry (WD-XRF). LOI (Loss On Ignition) was measured as 1000 mg were heated to
1030◦ C for 10 min. The samples were not dried to constant weight at 105◦ C prior to LOI
investigation and due to the fact that they were exposed to 1030◦ C, the mass reduction during
LOI does not correspond to the content of organic matter. It was rather used as a verification
value to determine the chemical components.
Carbonate & Sulphur
Another method to examine the carbon content, both organic and inorganic was performed
by measuring the TC (Total Carbon) with a LECO CS-444-Analysator. Moreover, the TS
(Total Sulphur) content was investigated simultaneously. The device was used to heat 170 180 mg of the sample material within an oxygen atmosphere up to 1800 - 2000◦ C. Carbon as
well as sulphur were emitted in the form of CO2 and SO2 which was recorded by an infrared
detector. To distinguish between TIC (Total Inorganic Carbon) and TOC (Total Organic
Carbon) , the carbonates had been removed in a second procedure prior to analysation. The
samples were treated several times with HCl (hydrochloric acid) for as long as gas evolution
could be observed. Then, TIC is the difference of TC and TOC.
- 40 -
CEC
CEC (Cation Exchange Capacity) was measured using a modified Cu-Triethylenetetramine method after Meier & Kahr (1999). Methods to determine CEC involve the complete exchange of the naturally-occurring cations by a cationic species, such as ammonium,
K, Na, methylene blue and Cu(II) ethylendiamine complex. To obtain an accurate estimation of CEC, it is necessary to exchange the ions completely. Therefore, it must either be
a high surplus of exchanged cations or the relevant cations need to have a high affinity for
the clay mineral (Meier & Kahr, 1999). Compared to other methods, Cu(II) complexes with
triethylenetetramine and tetraethylenepentamine allow an easy and rapid CEC determination. Firstly, because they form stable complexes with high affinity for clay, and secondly,
it is possible to record the CEC in accurate manner by photometric analysis. The samples
were mixed with 50 ml of de-ionized water and 10 ml of 0.01 mol Cu-triethylenetetramine
solution. Afterwards, the samples were shaked for 2 hours, followed by centrifugation. The
supernatant solution was removed carefully (Meier & Kahr, 1999), and the exchangeable
cations were recorded with a ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer).
IRS
A further method for qualitative identification of minerals was done by applying the nondestructive mineral analysation of IRS (Infrared Spectroscopy). According to Füchtbauer
et al. (1967), the main use of IRS is the possibility of identifying carbonates and feldspars
for sedimentary rocks. The rate of 1 mg sample per 200 mg KBr was applied to measure
IR (KBr pellet technique). Again, a thermo spectrometer was used to measure the issued
bandwidths of IR. A good evaluation of minerals is possible due to using his procedure on
undisturbed as well as on dried samples.
ESEM
SEM (Scanning Electron Microscope) enabled the optical characterisation of core material
. SEM was operated in two modes, in low vacuum mode (0.6 mbar) and ESEM (Environmental SEM - up to 10 mbar). The advantage of ESEM-mode is that investigations of moist
samples is possible by which the risk of changes within the samples structure due to collapsing minerals is excluded (Klinkenberg, 2008).
- 41 -
3.2.6. Methods for Analysation of Pumping Test Data
The assumed aquifer KOH2 was investigated by means of pumping tests to identify hydrogeological parameters such as hydraulic conductivity and transmissivity. Pumping tests
could not be executed in the cored boreholes. The idea of a well construction in boreholes
201216 & 201217 was abandoned due to the in chapter 3.1.2 described difficulties and problems. Further, it was not possible to execute any pumping tests while coring because the
operating drill manager did not want to increase the risk of a wall cake collapse as both
boreholes showed a high instability besides, the EDI-Fluidfinder cannot be used during the
core drilling procedure (see chapter 3.1.1). Some pumping tests were performed in the direct
environment of the cored boreholes i.e. within the wells of boreholes 201045 and 201047.
An additional test was executed at borehole 201046. The approach to interpret the recorded
data was to analyse both, the step test data and the constant rate test data for transmissivity
(van Wyk, 2009). Interpretation of the step-drawdown-test has been done by using Cooper
& Jacobs’ method for unsteady state flow in confined aquifers. For verification reasons, the
transmissivity was evaluated additionally by means of the method of steady state flow in
confined aquifers after Thiem & Dupuit. Both methods will be described here shortly. All
calculations were done on data provided by van Wyk (2009).
As presented in chapter 3.1.1, the recorded data included rest and pumped water levels,
EC and discharge measurements. Discharge rates were taken both by using a 20 l bucket and
a stop-watch as well as flow meter readings.
Assumptions which underlie both methods are (Kruseman & de Ridder, 1970):
• The aquifer has a seemingly infinity areal extent.
• The aquifer is homogeneous, isotropic and of uniform thickness over an area influenced by the pumping test.
• Prior to pumping, the piezometric surface and/or phreatic surface are (nearly) horizontal over the area influenced by the pumping test while during pumping groundwater is
only flowing horizontally.
• The pumped well penetrates the entire aquifer and thus receives water from the entire
thickness of the aquifer.
• The aquifer is confined.
Further assumptions are needed for the method of Cooper & Jacob:
• The aquifer is pumped with a variable discharge rate.
- 42 -
• Flow to the well is in unsteady state.
Whereas the method of Thiem & Dupuit needs the following assumptions:
• The aquifer is pumped at a constant discharge rate.
• The flow to the well is in steady state.
Kruseman & de Ridder (1970) write also that some of the assumptions can seldom be satisfied in nature. Slight deviations are therefore not prohibitive to the methods of application.
Cooper & Jacobs’ Method
Cooper & Jacobs’ method is a modified version of the Jacobs’ method which in turn is
based on the Theis’ formula (Kruseman & de Ridder, 1970). For the excact derivation of
the function refer to Kruseman & de Ridder (1970). An important step of this method is,
however, the plot the measured drawdown s on the ordinate against the logarithm of time t
on the abscissa. The resulting values form a straight line with a positive slope. This positive
slope is essential as the values of t0 , ∆(s/Q) and t have to be read off this line if equation
(3.9) and (3.11) shall be used. The basic expression of Jacobs’ formula can be changed with
regard to the variable discharge during step pumping. This implies that the drawdown s has
to be replaced by the specific drawdown s/Q which is the drawdown per unit discharge.
Furthermore, the time t has to be replaced by t which is the weighted logarithmic time of a
log cycle (Kruseman & de Ridder, 1970). As a result, the transmissivity is calculated by:
T = k·D =
2.30 1
4π ∆(s/Q)
(3.9)
T - Transmissivity [m2 /s]
k - Saturated hydraulic conductivity [m/s]
D - Thickness of the water-bearing layer [m]
∆(s/Q) - Difference in the specific drawdown per log cycle of t [s/m2 ]
Thiem & Dupuits’ Method
On the other hand, the well function of Thiem & Dupuit is derivated from Darcys’ law and
the water flow through a cylindric barrel surface in a porous rock (Walther et al., 2004). For
further information on the derivation of the method, please refer to Kruseman & de Ridder
- 43 -
(1970), Langguth & Voigt (1980) and Walther et al. (2004). The final equation for confined
aquifers which was used within the context of this report is expressed by:
Q = 2π · k · D
h2 − h1
lnr2 − lnr1
and
with
h1 = h2 − s
(3.10)
r2 > r1
h1 - Height from bottom of the Aquifer to the drawdown water level [m]
h2 - Height from bottom of the Aquifer to the piezometric surface before pumping [m]
r1 - Effective radius of pumping well [m]
r2 - Distance between pumping well and its point of influence, resp. the radius of the cone
of depression [m]
The storativity coefficient was calculated with the known values for kD, and t0 which is
the time t = t0 at s/Q = 0.
S=
2.25 · kD · t0
r2
(3.11)
The radius of the depression cone (r2 ) is not defined precisely as it depends on various
influences such as boundary conditions, inhomogeneities, gradient conditions, recharge of
groundwater, etc. (Walther et al., 2004). r2 was calculated by the common used and an often
cited empiric approaches of Sichardt and Kusakin in equation (3.12) resp. (3.13).
√
r2 = 3000 · s · k
p
r2 = 575 · s · k · h2
(3.12)
(3.13)
These empiric estimations for r2 may vary. Still, Langguth & Voigt (1980) prove that even
modifications of about 100 % on values of r2 just have small influences on equation (3.10).
Equations (3.12) and (3.13) provide sufficient values of r2 .
- 44 -
Chapter 4. Results & Interpretation
4. Results & Interpretation
The introduction reveals that a main objective of the project is to secure the water supply of the project area. The fresh water aquifer KOH2 as a future groundwater resource is
a matter of great importance for local and national development. This investigation contributes to the entire project by characterisation of the overlying layer which is assumed to
be either a confining and/or semi-confining layer. More information is given by this work
as the separating layer was not investigated solitary but in combination with aquifers KOH1
and KOH2 . Therefore, the following chapter gives detailed descriptions and interpretations
of the received results. Attention will be turned towards all data recordings, methods and
calculations described in chapter 3.
4.1. Previous Results
The most interesting hydrogeological section of the examined sediment is of course the
deeper seated fresh water aquifer KOH2 . The reason for a more detailed investigation of
its lateral extend was given during the drilling of borehole 37070 in Eenyama. The village
Eenyama is situated about 13 km south-east of Eenhana and is highlighted by the cluster of
TEM (Transient Electromagnetic) measurement points in figure 4.1. During the drilling of
37070 two water samples were taken at the depth of 151 m and 241 m. While saline water
was encountered at 151 m the low value of electric conductivity (EC) indicated the existence
of the fresh water aquifer KOH2 at the depth of 241 m with 745 mg/l TDS (Total Dissolved
Solids) .
4.1.1. TEM Investigation
Several TEM measurements which are described in chapter 3.1 were performed based on
the measured EC in borehole 37070. 71 TEM (440 in total) measurements in the project area
provided data of electric conductivity up to the depths of a few hundred meters. Interpretation
has been done whereas high values of EC refer to a saline environment and low values to a
possible fresh water aquifer. The first TEM measurements were executed in the village
Eenyama at borehole 37070 as it is the origin for the entire investigation campaign of KOH2 .
- 45 -
4.1 Previous Results
Fig. 4.1.: Estimated lateral extend of deeper fresh water aquifer KOH2
The first estimation on the lateral extend of KOH2 has been done by Harald Zauter (project
manager) by means of interpretation of all provided TEM results (figure 4.1). As there have
not been further investigations along the margins of the estimated aquifer the lateral extend
which is demonstrated in figure 4.1 is the best possible evaluation till this day. It was used
as the basis for all considerations.
4.1.2. Boreholes Prior to TEM Investigation
Various boreholes have been drilled within the project area but only eight are deep enough
to penetrate the assumed aquifer KOH2 . The location of all eight boreholes is presented
in figure 4.1, of which boreholes 37070, 34355 and 34470 were drilled prior to the TEM
measurements and therefore prior to the current groundwater investigation project. Of these
three boreholes, 37070 and 34335 lie within the area of KOH2 whereas 34470 is situated
outside. All three were drilled with the mud rotary technique and to a depth of 260 m for
34470 and 37070 whereas 34335 was drilled to a depth of 240 m. Lithologs of the main rock
types are presented in Appendix F.
The lithologs of boreholes 34470, 37070 and 34335 show similar results. Sequently layers
of sand and clayey sand are alternating throughout the entire borehole. The logging of cored
boreholes 201216 and 201217 will outline that the major difference between sand and clayey
sand is hard to distinguish for the upper 240 m of the sediment. These regions reveal an
- 46 -
4.2 Lithological Characterisation of Mud Rotary Boreholes
overall unsorted and inhomogen nature. For instance, it is possible that the main rock type of
clayey sand in borehole 34470 is logged as sand in borehole 34335. A remarkable unison is
still visible as all boreholes are intersected by a clay layer which is varying only in thickness
and depth. Boreholes 37070 and 34335 reveal the clay layer at the depth of about 230 m,
whereas it is found at the depth of 160 m for 34470 respectively. Subjacent layers of clayey
sand are shown in all boreholes which are underlain by sand in 34470 and 37070 from the
depth of 240 m onwards. From the hydrogeological and top to bottom point of view it is
concluded that all boreholes show a section of better hydraulic conductivity, respectively a
possible aquifer, followed by a possible aquitard and/or aquiclude which is again followed
by a possible aquifer.
4.2. Lithological Characterisation of Mud Rotary
Boreholes
The mud rotary boreholes 201245, 201046 and 201047 are the first boreholes established
on behalf of the current project. They cover the first stage drilling to investigate the hydrogeological characteristics of the project area. The lithological description of these boreholes
was done by Braam van Wyk and Roy Miller on composite samples of every five meters. It
should be noted that van Wyk logged the mud samples directly on site while Millers descriptions were made a few weeks later, within the grounds of the Geological Survey of Namibia.
The presented results are the supplemented composition of information based on their elaborations. The obtained depth is 390 m for 201045, whilst 201046 and 201047 were drilled
to 266 m and 383 m respectively (van Wyk, 2009).
4.2.1. Borehole 201045
Lithological overview and in-situ measured geophysical values can be seen in figure 4.2.
The lithology encountered, consisted of fine, poorly sorted, clayey sands which become more
cemented at depths greater than 250 m. The upper 10 to 40 m of the matrix reveal a stickiness
even though the sediment does not appear to have any clay content. This phenomenon does
occur for various boreholes drilled in the CEB and while it is clearly visible immediately
after drilling it is hard to detect after the sample has dried out (van Wyk, 2009).
0 − 200 m: Unconsolidated, fine to medium grained sand with an abundant content of silt
and/or clay. Grainsize distribution shows that medium grained sand dominates this
section whilst coarse grained sand increases from depths of 150 m. Generally, poorly
sorted sediment is observed with colours ranging from white/grey, light green to purple
- 47 -
in the deeper regions. Sedated increase of gamma values are observed with greater
depths. This indicates the existence of clay even if the percentage of clay which is
highlighted by the pink bar in figure 4.2 does not show an increase within the clay
fraction. Parts of the sand fraction might in truth be cemented clay particles due to
the results of the grainsize analyses on core material from borehole 201216 which
are demonstrated in chapter 4.6.1. EC measurements signalize a saline environment.
According to the stickiness between 10 and 40 m the upper part of the unit is regarded
as an aquitard while it is considered to be a saline or brackish aquifer between 40 and
80 m. From 80 to 202 m the continuously increasing gamma log indicates that this
part is an aquitard with decreasing hydraulic conductivity.
200 − 230 m: Sandy clay, containing fine grained sand of fair sorting. Eventual appearance
of calcretes. The overall percentage of grains smaller than 63 µm shows increased
values compared to the section above. The gamma log shows two peaks which indicate
an increase of the clay fraction as well. First occurrence of light red colour. The
grainsize distribution refers to a main rock type of fine sand. On this account it should
be noted that the grainsize distribution of all boreholes has been done by dry sieving
only. Hence, the share of the fines (clay, silt) differs in actual values compared to the
percentages which are presented in the following figures (see also chapter 4.6.1). This
section is considered to be an aquitard.
230 − 270 m: Well rounded, fine sand with better sorting and of white colour. Cemented
sand and calcrete nodules up to 240 m. The lower section is comprised by slightly
limy but unconsolidated sand. A strong decrease of the electric conductivity is visible.
Gamma values decrease as well but less excessive. 228 m might be the upper edge of
fresh water aquifer KOH2 . Only one mud sample could be analysed between 250 and
300 m.
270 − 380 m: Mainly, medium grained sand of poor sorting. Remarkable change of colour
from white to red and the degree of cementation/consolidation increases from this
level downwards. The unit becomes solid from 300 m. Calcrete occurs throughout the
section. Low values of gamma and EC are comparable to section above up to about
350 m. Both show increased values from 350 m onwards. Probably, aquifer KOH2
ends at this depth. Discrepancies are visible between the lithological and geophysical
log. They occur due to the drilling technique which generates a time lag for samples
delivered from great depths.
- 48 -
Fig. 4.2.: Litholog and geophysical log of borehole 201045, Grain size distribution: pink = clay, grey = silt,
yellow = fine sand, orange = medium sand, brown = coarse sand
- 49 -
380 − 390 m: Red clay, also signalised by higher average values of the gamma log. A
single mud sample of this depth has been investigated mineralogically. It shows a
major presence of smectite (about 20 %) and minor presence of illite and muscovite.
The overall content of clay is about 35 %. The content of carbon refers to calcite whilst
organic matter is absent. The smectite occurs in Na-form which is rare. In summary, all
parameters suggest that the smectite is derived authigenic and hence from the alteration
of mafic minerals (see also chapter 4.7, mineralogy).
Fluid Finder
Two water sampling and aquifer testing procedures were performed using the EDI Fluid
Finder. Investigations at 201045 were done in sections 278 to 280 m and 339 to 342 m. Both
testings provided excellent water, Group A after the Namibian Water Quality Classification
System which is based on EC values only. van Wyk (2009) provides a range of extrapolated
transmissivity values for a possible aquifer thickness with 110 to 240 [m2 /d]. Due to the
fact that the exact lengths of the tested sections are unknown, precise values of hydraulic
conductivity cannot be derived. The estimated range is given by 1.3E-5 to 2.8E-5 [m/s].
4.2.2. Borehole 201046
Lithological overview and in-situ measured geophysical values can be seen in figure 4.3.
The encountered lithology is comparable to borehole 201045 although 201046 contains more
coarse grained sand and calcretes. Again, the stickiness is visible for the upper 10 to 75 m of
the material which is considered to be hydraulic non-conductive. The sequence of colours is
similar to 201045 (van Wyk, 2009). The upper reaches start off with a light grey/white that
turns into green at about 90 m which is typical for the Andoni formation. From about 170 m
onwards the lithology turns red which is characteristic for the Olukonda formation.
0 − 90 m: Poorly sorted, silty sand which is logged as fine sand. Grainsize distribtion reveals that this section is dominated by medium grained sand of white/grey colour. The
whole section contains grains with diameters between silt and coarse grained sand.
The section has a sticky appearance between 10 and 75 m and calcrete nodules occur
from 30 m onwards. Therefore, it is estimated that the aquitard starts at a depth of
75 m. Moreover, gamma log and penetration rate show a peak at 80 m which is indicative for a higher clay content and contributes to the appearance of an aquitard. EC
values of this section signalise a saline environment.
90 − 164 m: Poorly sorted, fine sand of light green colour. Again, the distribution of grains
shows that medium sized grains dominate this section. Gamma values increase slowly
- 50 -
but constantly from 134 m downwards, thereby indicating the increase of the clay content. Moreover, the penetration rate starts to alternate which signalises a hard-soft-hard
environment. It is likely that this formation cemented to a large degree by calcretes,
with sand showing a generally high content of lime. In addition to this, the sand is
cemented by clay in places.
164 − 190 m: Poorly sorted, fine sand of purple red colour. It is likely that the Olukonda
formation is reached in this section. According to van Wyk (2009), it has to be taken
note of that a considerable lag occurred in the delivery of the samples due to insufficient flush velocities. This causes mixing of samples to a high degree. An increased
content of clay was observed due to grainsize distribution and gamma log. This section
is considered to be an aquitard.
190 − 230 m: Poorly sorted sand with a high lime content. Penetration rates of more than
10 [min/m] were recorded. A slow reaction was visible which signalises dolocrete
after applying 10 % HCl on samples. High gama values appear in this section, again
refering to a high clay content. This section is considered to be an aquitard.
230 − 258 m: Poorly sorted, unconsolidated white to light grey sand. Gamma values as
well as electric conductivity values decrease from the start of this section. Therefore,
this unit is considered to be the fresh water aquifer KOH2 .
258 − 266 m: Miller logged this section as fine sand of red colour. Braam van Wyk on the
other hand describes this section as pure clay that matches the gamma values of this
section to a higher account. The induced conductivity log shows significant changes
compared to the section above and demonstrates an environment of low salinity. Due
to the premature termination of drilling described in chapter 3.1.1, no information can
be provided of the deeper units. It is not clear if these clay layers mark the bottom of
the KOH2 aquifer already but it is assumed that these clays are merely an intermediate
layer.
- 51 -
Fig. 4.3.: Litholog and geophysical log of borehole 201046, Grain size distribution: pink = clay, grey = silt,
yellow = fine sand, orange = medium sand, brown = coarse sand
- 52 -
Fluid Finder
Due to problems with the alignment, respectively straightness of the borehole which are
described in chapter 3.1.1 this borehole had to be abandoned at a depth of 266 m. Hence,
only one unsuccessful test was executed with the EDI Fluid Finder (van Wyk, 2009). The
test was performed in a depth from 128 to 130 m. Aquifer parameters were not provided
as no water samples could be taken. Considering the geophysical investigations it is not
clear if this section is seen either to be a saline aquifer of very low hydraulic conductivity
or an aquitard already (see also chapter 4.4). It was definitely detected by the performed EC
measurements that this section demonstrates a saline environment.
4.2.3. Borehole 201047
Lithological overview and in-situ measured geophysical values are presented in figure 4.4.
The lithology is generally dominated by fine to medium grained sand. Again, the ’sticky
matrix’ is visible between the depth of 0 and 80 m. Two remarkable differences are observed
as this borehole contains more soft lime as well as it shows a higher amount of silt in general.
0 − 80 m: Poorly sorted, white to light brown silty sand. Sand is unconsolidated and
consists mostly of fine to medium sized grains with a few coarse grains embedded.
Gamma log shows low values which indicate that clay is absent. Due to the high silt
content this section is still considered to be an non-aquifer, respectively aquitard.
80 − 110 m: Poorly sorted, white to light brown fine to medium grained sand. Stickiness
of matrix is absent from 63 m downwards. A few sparsely occurring, hard patches of
sand which are cemented by calcretes. This section was already known to be aquifer
KOH1 prior to this project. At this location, KOH1 is comprised by fresh water which
was already encountered at borehole 200651 in close distance.
110 − 200 m: Poorly sorted, white to light brown, clayey sand that turns red from 140 m.
Again, this is typical for the Olukonda formation. A remarkable and rapid change
of gamma and electric conductivity values can be seen at both the upper and lower
edge of the section. The high values of gamma indicate a high clay content even if
logging of mud samples just reveal small amounts of clay. It is possible that the clay
fraction was taken into solution by the drilling fluid which would result in washed out
clay particles which in turn explains the minor amount of clay in the mud samples.
Due to the high variation of the penetration rate, this section is considered to be of
hard and soft layers alternating sequently. Summing up, it is reasoned that this section
comprises aquicludes and aquitards with a varying but low hydraulic conductivity.
- 53 -
200 − 300 m: Poorly sorted, red to brown, medium grained sand with some coarse grained
sized particles embedded. Higher appearance of calcrete nodules and lime. Penetration
rate shows a higher progress per meter compared to section above. From the depth of
240 m onwards a mixture of red and grey, unconsolidated sand is visible within the
same horizontal level. The entire section reveals low values of gamma and electric
conductivity (EC ranges between 43 - 132 [mS/m]) which indicates that the fresh water
aquifer KOH2 is intersected at this depth.
300 − 383 m: generally poorly sorted, red brown, sandy clay showing some patches of
fine sand with better sorting. Increased values of EC signalise an environment of high
salinity while gamma log matches the high clay content shown in the litholog. This
section is considered to be an aquitard of very low hydraulic conductivity, respectively
an aquiclude at greater depths.
Fluid Finder
Two water sampling and aquifer testing procedures were performed using the EDI Fluid
Finder. Investigations at 201047 were done in sections from 265 to 267 m and from 329 to
331 m. Both samples showed EC values which classify them as ’Group A’, excellent water.
Van Wyk provides a transmissivity value with 5 [m2 /d] for the upper testing which is situated
in the section of the aquifer KOH2 . Again, a precise derivation of the hydraulic conductivity
is not possible but an estimated value can be given with 6.43E-7 [m/s].
- 54 -
Fig. 4.4.: Litholog and geophysical log of borehole 201047
- 55 -
4.3 Lithological Characterisation of Cored Boreholes
4.3. Lithological Characterisation of Cored Boreholes
The recovered cores are the first cores of any length of the unconsolidated Kalahari Group
in Namibia and probably in the whole Kalahari Basin of Central and Southern Africa (Miller,
2009, personal communication). It is the first opportunity to study the sedimentology of this
succession and obtain a better understanding of its evolution.
The lithological logging of core material is based on the work of R. Miller and has been
carried out on the grounds of the Geological Survey of Namibia. Pre-logging of cores and
additional information was collected by the author prior to the official logging of core samples (see chapter 3.1.2). The provided results of boreholes 201216 and 201217 are therefore
a combination of both.
Major Drilling Group Inc. could deliver 142.62 m for 201216 and 136.99 m for 201217 of
the originally desired 310 m of core material from borehole 201216 and 260 m from borehole 201217 respectively. The described difficulties in chapter 3.1.2 caused core loss of 16 %
at 201216 and 9 % at borehole 201217. In addition to this, instabilities of both core boreholes led to the decision to abandon the boreholes at depths of 266 m for 201216 and 235 m
for 201217 respectively. Exact core description can be seen in Appendix F and it has to be
pointed out that the lithologs presented in figures 4.5 and 4.6 show a summarised version of
the very detaild logs provided by Miller. Temperature [◦ C], pH value and the electric conductivity (EC) [µS/cm] were recorded of the drilling fluid during coring and are presented
in appendix C. They do not reveal a depth related behaviour and are highly influenced by the
synthetic drilling viscosifier DRILLVIS and CAP21.
4.3.1. Borehole 201216
This borehole is situated 12.20 m away from borehole 201045 and Miller used information
from the log of the latter to fill in the core log where there was no core recovery. A general
difference between the logging of cores and mud rotary samples is visible in figure 4.5. As
coring was done to deliver undisturbed samples of the sediment it can be assumed that it
provides results of higher precision compared to log of mud samples. The logging of core
borehole 201216 shows that it consists almost entirely of highly unusual sediment which is
dominated by unbedded clayey sand with about 10 to 30 % of clay. The main grain size of
the sediment is defined by fine to medium grained sand of poor sorting. The drilling fluid
showed the average pH-value of 9.06 and the average temperature of 27◦ C.
0 − 80 m: No core withdrawn of borehole 201216 within this section. Lithologgical unit
of sand has been used to describe the main rock type of the section due to elaborations
of Miller and Wyk (compare log of borehole 201045 on page 47).
- 56 -
80 − 200 m: Poorly sorted, light grey to light green, clayey sand. The dominating grain
size is fine sand but a minor and variable as well as fairly uniformly scattered coarseto very coarse-grained sand component, as well as few granule-sized (2 - 4 mm) grains
are occurring in patches throughout the entire section. Scattered calcretes are also
visible and a hard zone of dolocrete nods is found at 174 m with a thickness of 33 cm.
The colour turns to light red purple from this depth downwards. A few thin zones of
sandy clay with a thickness of 6 to 30 cm appear which contain a fine grained sand
component that makes up between 30 and 50 % of the clay. The section shows a
high unsorted condition. Samples of this section are mainly comprised by quartz and
feldspar and a minor component of muscovite and iilite. The lithological description
of this section and the continuously increasing gamma log of borehole 201045 indicate
that this part is an aquitard with decreasing hydraulic conductivity at increasing depth.
200 − 220 m: Generally, good sorted fine sand of light green colour which is intersected
by patches of purple red colour which in turn are comprised by finer sand and a clay
content with values of up to 30 %. Drilling fluid matches the purple red colour for parts
of the section. Patches of white calcrete nods are observed in places. This section of
clayey sand demonstrates a higher cohesion of the sediment compared to the section
above. It is detected that the grain size decreases while the sorting increases. Core
samples of this section show in addition to quartz and feldspar a variable content of
smectite.
220 − 230 m: Fairly well sorted, light green and purple red, sandy clay. An even higher
cohesion of the sediment is observed. A remarkable 11 cm thick, almost solid calcrete
layer from 227.40 to 227.51 m. Due to the high core loss that started at about 210 m
no samples could be investigated in the laboratories of BGR. This and the last section
are considered to be aquitards of very low permeability which is backed up by results
of investigations in borehole 201045.
- 57 -
Fig. 4.5.: Litholog of core borehole 201216 and comparison to 201045, dots point out core samples and their
relevant depth, red bars refer to core loss
- 58 -
230 − 250 m: A high core loss of 66 % was observed within this section, but retrieved core
is well sorted, light grey to green clayey sand which is cemented up to 239 m. Abrupt
change from 239 m to dark red and grey, unconsolidated sand without changing of
grain size. Occasionally occuring calcretes from 247 m. Between 239 and 243 m, thin
(few centimeters) sandy clay layers intersect the generally and continuously decreasing
amount of clay and increasing amount of fine to medium grained sand which is visible
with preceding depth. The largest occurring sandy clay layer is visible from 240.70 to
241 m which has an estimated percentage of clay with about 60 % and shows a high
cohesive condition. Due to mineralogic investigations done on samples taken from this
section, the relevant hydrogeological layer is considered to be a ’fuzzy layer’ which
shows all signs of an aquitard with low hydraulic conductivity in the upper reaches
while the hydraulic conductivity increases continuously with depth. An abrupt change
to KOH2 is not visible. This will be discussed more detailed in chapter 4.4.
250 − 266 m: Well sorted, fine sand of red or brown colour in general. Brown sand shows
less clay than red or grey sands and reveals a very unconsolidated condition. Drilling
fluid matches brown colour of sand. This section is considered to be the fresh water
aquifer KOH2 .
4.3.2. Borehole 201217
This borehole is situated close to the border of Namibia and Angola with a distance of
about 100 m to the mud rotary borehole 201047. Again, Miller used the information of the
latter to fill in the core log in sections where core loss appeared. The logging of core material
from 201217 shows that it consists, like borehole 201216, of the same highly unusual sediment which is dominated by unbedded clayey sand almost throughout the entire borehole.
The main grain size of the sediment is defined by fine to medium grained sand of poor sorting but unlike borehole 201216 a higher content of silt is visible. The drilling fluid showed
the average pH-value of 8.98 and the average temperature of 23◦ C.
0 − 80 m: No core withdrawn from borehole 201217 in this section. Lithologgical unit of
sand has been used to describe the main rock type of the section due to elaborations of
Miller and Wyk (compare log of borehole 201047 on page 53).
80 − 110 m: Poorly sorted, light green, clayey sand which shows the dominating grain
size of fine sand. About 50 % core loss within this section. Core is characterised by
consolidated sand which is intersected by layers of white silcretes up to a thickness of
15 cm and to the depth of 90 m. Calcrete layers appear as well. From the depth of 90 m
the sand becomes less consolidated and very wet. The last five meters of this section
- 59 -
are occasionally intersected by ferrous indurated sandstone. The section is considered
to contain fresh water due to the already given descriptions at logging of 201047 and
due to the low values of electric conductivity measured within the drilling fluid.
110 − 144 m: Poorly sorted, alternating zones of green, clayey sand and sandy clay. A
variable but abundant silt content is observed throughout the section. White calcretes,
dolocretes as well as silcretes interlaminate the entire section and become more abundant towards the deeper reaches. A thick dolocrete layer of 43 cm is observed at 140 m.
The colour of the drilling fluid changes from grey which was observed in the last section to green-brown.
144 − 170 m: This section consists of sandy clay that changes abruptly to a purple grey
which is the typical colour of the Olukonda formation. Drilling fluid matches the core
description. A low amount of calcretes and dolocretes is visible but has definitely decreased compared to section above. Only a little core loss is observed in this section
which is due to the finer material and the detected cohesion of the sediment. This
section is considered to be an aquitard and furthermore, the region of the lowest permeability encountered within this borehole.
170 − 200 m: Well sorted, green grey, clayey and fine sand which is consolidated in the
upper 5 meters. A few purple grey streaks of sandy clay are still visible but appear
only in patches. A high amount of clay and silt is observed between 185 and 195 m.
Again, this region shows more purple grey intersections of sandy clay which match
the gamma values recorded in the relevant depth of borehole 201047.
200 − 235 m: Well sorted, grey, clayey sand for the upper five meters which turns into dark
grey sand with threads of clay still present. From 210 m downwards clay is almost
none existent anymore. Sand of this section is comprised by fine to medium grained
particles and the colour changes to red brown while the sand still reveals a fair sorting
but is completely unconsolidated. The core of the entire section is very soft and wet
which results in a high brittleness. This section is considered to be the clay-poor and
sand-rich fresh water aquifer KOH2 . This statement is confirmed by results from test
pumping and triaxial testing of samples withdrawn from this section.
- 60 -
Fig. 4.6.: Litholog of core borehole 201216 and comparison to 201047, dots point out core samples and their
relevant depth, red bars refer to core loss
- 61 -
4.4 Overview of Drilling and Logging Results
4.4. Overview of Drilling and Logging Results
From a hydrogeological point of view, the TEM soundings, all lithological and geophysical information presented, and especially the composition of the core material demonstrate,
that the sediment can be divided into five hydrogeological sections. The first, or upper section
is generally considered to be an aquitard due to the clayey sand which dominates the sediment. These upper reaches are defined by van Wyk (2009) as ’sticky matrix’ which are not
clearly identified by gamma values. Gamma values do not always identify the appearance of
clay as gamma responds mainly to potassium (K 40 ) containing clays but the potassium isotope is not a necessary component of clay minerals (compare chapter 3.1.1). However, it is
known, that section I can contain seasonal and very shallow (10 - 15 m), perched groundwater resources of saline and/or fresh water. They are recharged by heavy rain falls during the
rainy season. As a conclusion section I is determined to be part of the KOS aquifer system
described in chapter 2.5.1.
The second section is generally considered to be an aquifer. Lithology of the cored boreholes and geophysics measured in the mud rotary boreholes as well as earlier drilled boreholes prove that it was encountered at all drilling sites. Therefore, a regional extend is detected. The electric conductivity measurements performed in boreholes 201045 and 201047
show that the water of this aquifer shows a high content of solutes at 201045 which implies
a saline groundwater condition, while it shows a low content of solutes at 201047, implying
a fresh water resource. This is the typical aquifer condition for the region and has been encountered during drilling of various boreholes prior to this investigation. Fresh water along
the margins of the CEB becomes more and more saline towards the centre. As this relates to
the typical condition of the known aquifer KOH1 , section II is determined to be part of it.
All lithologs and geophysics show that section II is underlain by a layer of high clay and/or
silt content. Just as section II, this section (section III) is determined to have a regional occurrence. Investigations of boreholes 201045, 201046 and 201216 show a sedated gradation
from section II into III. The clay content increases slowly but constantly from about 80 m
which can be seen in the litholog of 201216 as well as in the measured gamma values of
201045 and 201046. Therefore, section III is considered to be an aquitard with decreasing
hydraulic conductivity at increasing depth. The aquitard reveals the highest amount of clay at
the depth from 200 to 230 m. It is still not clear if this region should be defined as aquiclude
because neither litholog nor the performed geophysical investigations permit a qualified declaration. Results of triaxial testing in chapter 4.5 and mineralogy in chapter 4.7 will give
further information on this topic. The clay content decreases below 230 m which is assumed
to result in a higher hydraulic conductivity. Investigations on boreholes 201047 and 201217
reveal a more abrupt change of grain sizes, colours and measured geophysics at the change
- 62 -
of sections II and III. This indicates a stratigraphic break which could have been influenced
by fluvial sedimentation. Another indication for the lack of sedimentation is given due to
the thickness and depth of this section. Section III, respectively the aquitard is encountered
between 110 and 200 m. Unlike investigations at previously described boreholes (201045,
-46, 201216), these boreholes reveal their clay maximum closer to the surface and in depths
between 144 and 170 m. The clay maximum is accompanied by the strongest appearance
of purple colour. The purple colour can relate to a ferric (Fe3+ ) sediment with small pore
diameters. Water can seep into ferric sediments containing bigger pores, thus reducing the
iron to Fe2+ what results in a loss of the purple colour. As streaks of purple are visible
throughout section III, this indicates that patches of higher water permeability are alternating with impervious patches. Summing up, it can be concluded that section III separates
the aquifer KOH1 from underlying layers and is therefore the confining layer of fresh water
aquifer KOH2 .
Section IV is only partly explored in the cored boreholes but investigations of mud rotary
boreholes include its entire length. It ranges between 200 and 290 m at boreholes 201047
and 201216, while between 230 and 350 m at boreholes 201045 and 201216 respectively.
The abundant amount of good sorted sand grains, and the decreasing amount of clay which
is completely absent at greater depths of section IV signalise to have intersected the aquifer
KOH2 . Corresponding to the TEM results, an environment of low electric conductivity was
encountered in all boreholes within this section. Hence, the regional extend of the aquifer is
assumptive. An exception is spotted upon regarding borehole 34470. The results of natural
gamma and electric conductivity measured up to the depth of 240 m in borehole 34470 conclude that it does not intersect the fresh water aquifer. Gamma values do not show significant
changes throughout the borehole and the electric conductivity presents values which relate
to a high salinity.
The bottom end of the fresh water unit and beginning of the section V was only encountered in borehole 201045 at 370 m and at 300 m for borehole 201047 respectively. All other
boreholes either were abandoned beforehand or were not planned to reach such depths. High
gamma values and a high clay content are observed upon investigating section V of borehole 201046. It is not defined certainly if this represents the end of the fresh water unit
or is merely a smaller, intersecting layer. It is guessed, however, that the clay refers to an
intermediate layer.
An overall unsorted nature of the upper sediment was observed during investigations. This
statement refers to the sorting of section I to III. Due to being an inland river delta with a very
shallow morphology river beds shifted regularly during the sedimentary history of the CEB.
- 63 -
In addition to this, water levels changed frequently due to droughts, flash floods and meanflow conditions which alternated periodically in height. The flash floods in particular which
are floods of high intensity and short duration, resulted in an irregular erosion of sediment.
Eroded material was deposited rapidly at dwindling of discharge. This rapid deposition
in combination with an insufficient transport distance are typical for an unsorted nature in
the sediment. Bioturbation is another reason for the poor sorting of the sediment. During
the detailed examination of the core material some offset colours were observed in form of
purple streaks which were already described in section III. These streaks seem to follow old
root systems and burrow passages generated by benthic flora and fauna. Some few of these
passages are calcretised due to precipitations of calcium carbonate (CaCO3 ) which forms
often along root systems in a calcareous environment. The poor sorting is accompanied by
an overall inhomogen condition of the sediment. Almost all sections show grain sizes from
clay to coarse grained sand (compare core log, Appendix F). For instance, granule-sized
grains were observed in many clayey patches respectively layers. Moreover, variations of
grain shapes are observed as well. Well rounded grains signalising a long transport distance
are in close contact with sharp edged grains transported just over a short distance. The overall
unsorted nature improves from the depth of about 200 m at borehole 201216, respectively
144 m at 201217 and the sediment is in a well sorted condition. Especially in regions where
fine sand occurs to a high account. Therefore, it appears that the finer the grain size of the
sand fraction becomes the better the sorting of this section becomes. The layer of the finest
sands is as well the layer with the maximum amount of clay and/or silt. Again, all boreholes
reveal such a layer but with varying thickness.
Core loss was a general problem during the drilling procedure. In addition to the given
explanations of core loss in chapter 3.1.2, a sedimentary dependency is visible when regarding core loss in its relation to depth. Layers of non-cohesive material, i.e aquifers KOH1 and
KOH2 show a higher rate of core loss compared to the intersecting layer which is comprised
by cohesive material. A picture of the used drill bit and core catcher is presented on page
26. This particular core equipment is used normally for indurated rocks. As the investigated
sediment consists of unconsolidated material in the reaches of KOH1 and KOH2 it is likely
that a different core catcher would have caused less core loss.
All withdrawn material, i.e. mud samples and cores, were taken out of the Andoni and
partly out of the Olukonda formation. The withdrawn material consists of highly unusual
sediment that in regions can neither be defined as Andoni nor Olukonda formation (Miller,
2009, personal communication). For instance, the aquifer unit in core borehole 201217
seems to be underlain by more clayey, vary coloured sediment before typical Olukonda is
reached. Upon consultation of R. Miller it is considered to give the sediment between Andoni
- 64 -
4.5 Saturated Hydraulic Conductivity
and Olukonda formation a new name, either as a formation in its own right or as a member in
the Andoni Formation. The name Ohangwena Formation or Member seems suitable. After
Miller, the following stratigraphy is suggested:
201216 : 0 - 260 m Andoni formation, 260 - 266 m new formation, 266 - 390 m Olukonda
Formation
201217 : 0 - 212 m Andoni formation, 212 - 297 m new formation, 297 - 383 m Olukonda
Formation
4.5. Saturated Hydraulic Conductivity
4.5.1. Triaxial Cell
The results of triaxial testing are presented within this chapter. Core samples out of the
following hydrogeologic layers were investigated to receive further knowledge on hydrogeological parameters and flow characteristics of the groundwater in the project area. Aquifer
KOH1 , the intersecting layer and the underlying and confined aquifer KOH2 . Tables 4.1 &
4.2 show values of the saturated hydraulic conductivity based on evaluations of Darcys’ law.
It should be pointed out that the lithological determination in these tables differs compared
to the lithological description given in chapter 4.3. Table 4.1 & 4.2 relate to a more detailed lithology whereas results presented in chapter 4.3 relate to a summarized lithological
description.
Hydraulic conductivity values range between 7.45 · 10−6 [m/s] and 3.51 · 10−12 [m/s] at
borehole 201217 and from 8.94 · 10−8 [m/s] to 7.10 · 10−13 [m/s] at borehole 201216. According to DIN 18130, all received values relate to a poor or very poor water permeability.
Core samples 201216 and 201217 demonstrate a similar vertical development. Higher kvalues in the upper regions are decreasing downwards and show their minimum at 201 m for
201216 and 151 m for 201217 respectively. The hydraulic conductivity of underlying samples reveals an increasing development at increasing depth. To draw further conclusions from
the examined sediment, it is essential to refer these values to the evaluated hydrogeological
layers and rock formations they have been taken from.
As the coring started on borehole 201216 with a depth of 80 m it was not possible to
investigate samples from layers which are certainly defined as KOH1 . Samples 201216-19 &
201216-22 were taken on the assumption of being withdrawn from the fuzzy layer (see also
chapter 4.4). This means, that it was not clear if they were taken either from KOH1 , the upper,
- 65 -
Table 4.1.: Evaluation of saturated hydraulic conductivity (Triaxial testing - borehole 201216)
ID
Depth
201216X
[m]
-19
127.14 - 127.25
-22
136.80 - 136.90
-28
153.95 - 154.04
-45
201.53 - 201.63
-52
218.45 - 218.56
-54
219.52 - 219.60
-72
248.50 - 248.60
-73
249.02 - 249.13
-81
266.02 - 266.13
Testing
hyd.
Rock
time conductivity formation
after
[m−H2 O] [min]
[m/s]
Miller
7
1225
7.11E-10
clayey
20
1580
1.54E-10
sand
50
1418
3.36E-10
355
2.47E-11
clayey
20
950
4.26E-12
sand
7000
4.49E-12
20
2830
1.35E-11
clayey
60
1380
6.36E-12
sand
20
4000
clayey
n.a.
20
3500
sand
20
8050
2.16E-12
clayey
60
330
7.10E-13
sand
20
7062
6.79E-12
silty
60
2640
4.98E-12
clay
20
6050
5.97E-10
clayey
60
319
2.50E-8
sand
5
107
9.77E-10
sand
10
90
2.14E-9
20
1590
7.75E-9
2
105
8.35E-9
sand
5
130
8.94E-8
∆h
Layer
aquitard
aquitard
aquitard
aquiclude
aquiclude
aquiclude
aquitard
aquitard
aquifer
brackish aquifer, or already from the aquitard. When seen in context to the entire borehole,
sample 201216-19 shows a high value of hydraulic conductivity. The sedated but constant
increase of clay, described in chapter 4.4 reflects the results of the hydraulic conductivity
values based on triaxial testing. Sample 201216-19 is comprised by a clay content that refers
to 10 - 16 CPS natural gamma (measured in 201045). Its k-values are located in the region
of 10−10 [m/s]. Followed by the subjacent lying material of sample 201216-22 with values
of 23 to 39 CPS that results in a hydraulic conductivity in the range of 10−11 to 10−12 [m/s].
The results of sample 201216-28 are similar whereas the hydraulic conductivity becomes
still lower for samples below the depth of 201216-28.
Samples 201216-45, -52, -54 provide the lowest hydraulic conductivities evaluated. Again,
this reflects the high clay content determined by litholog and geophysical investigations
which shows proportions of 30 to 40 % in this region. With 7.1 · 10−13 [m/s], the least measured k-value relates to the depth of 218.50 m. Sample 201216-45 is determined as hydraulic
- 66 -
non-conductive due to the fact that the flow through sample was not established during triaxial testing. This corresponds to the high clay content detected by litholog and natural gamma
radiation because around the depth of 201 m the maximum of natural gamma with more than
61 CPS was measured in borehole 201045.
Results of samples that were taken below 230 m show values comparable to sample 20121619. It was difficult to assign a hydrogeological layer for samples 201216-72 and 201216-73
as they show signals of both aquiclude/aquitard and aquifer. Core log and grain size analysis both indicate that samples 201216-72 and 201216-73 are dominated by fine and medium
grained sand. They are located at the depth of 248 to 249 m where the lithological description
changes from clayey sand to sand, although clay is still visible in the latter. The measured
EC values of 201045 at an equal depth refer to an environment of low salinity. This indicates that samples -72 and -73 are located in the fresh water aquifer KOH2 . On the contrary,
the stickiness of these samples which was observed during core withdrawal, and the evaluated hydraulic conductivity values relate to a very poor water permeability. It seems as if
samples -72 and -73 are located in a transitory zone. The very inhomogen condition of the
sediment matches with the just presented considerations as it means that grains of all sizes
are in close contact, thereby rejecting the development of uniform layers. The boundary
lines between layers become fuzzy which implies that the geohydrological parameters show
a condition of smooth transition between layers. Therefore, the investigated samples are seen
to be withdrawn out of the fuzzy layer on the threshold from aquitard to aquifer of very poor
permeability.
Sample 201216-81 shows the highest value of hydraulic conductivity. Even if logged as
sand, the grainsize distribution reveals a comparatively average share in the fractions of silt
and clay (see figure 4.7). Therefore, it is located in the fuzzy layer between aquiclude and
aquifer KOH2 . Core material was not received below this depth as the sediment was very
soft, brittle and unconsolidated. Considering the geophysical information of the relevant
depth in borehole 201045 it is assumed that this sample refers to the bottom end of the fuzzy
layer and can be seen to be part of the deeper aquifer for modeling purposes.
With regard to the depth related developing observed for cores of 201216, likewise outcomes were found after testing of cores from borehole 201217. Samples taken from the
upper and lower regions of the succession show k-values which range from 1 · 10−6 [m/s]
to 1 · 10−8 [m/s]. Samples from the separating layer result in values of 1 · 10−11 [m/s] and
1 · 10−12 [m/s] which reflects the clay content determined by natural gamma and core log.
Furthermore, these samples (201217-38, -43, -55, -70) resemble the results of the aquitard
section determined for 201216. Again, the lowest evaluated hydraulic conductivity can be
found at the maximum of clay content and hence for sample 201217-55. Corresponding to
- 67 -
Table 4.2.: Evaluation of saturated hydraulic conductivity (Triaxial testing - borehole 201217)
ID
Depth
201217X
[m]
-9
90.02 - 90.13
-18
99.68 - 99.80
-25
105.19 - 105.30
-38
118.26 - 118.38
-43
126.43 - 126.55
-55
151.76 - 151.88
-70
195.91 - 196.03
-84
213.56 - 213.70
-98
228.99 - 229.09
Testing
hyd.
Rock
time conductivity formation
after
[m−H2 O] [min]
[m/s]
Miller
clayey
4
160
2.21E-7
sand
clayey
1.9
540
6.27E-8
sand
clayey
1.13
540
1.06E-6
sand
sandy
10
5440
9.07E-12
clay
clayey
20
8873
2.94E-11
sand
20
1120
4.6E-12
clayey
20
12830
3.51E-12
sand
sandy
20
11365
3.29E-11
clay
0.19
152
4.57E-6
sand
0.24
350
7.45E-6
∆h
0.25
380
6.85E-6
sand
Layer
aquifer
aquifer
aquifer
aquitard
aquitard
aquiclude
aquitard
aquifer
aquifer
the inhomogeneity of the sediment (see chapter 4.4) and that all investigations were done on
spot samples it is assumed that the layer of highest clay content consists of hydraulic non
conductive sections for borehole 201217 as well. It is noticeable that the hydraulic conductivity shows generally higher values in the upper and lower reaches compared to borehole
201216. According to DIN 18130 these values are still described by ’poor permeability’.
Higher values in these regions can be explained by the hydrogeological layers these samples
have been taken from. Samples 201217-9, -18, -25 were taken from the depth of the upper
aquifer KOH1 whereas samples -84 and -98 were withdrawn from the deeper fresh water
aquifer KOH2 . The fuzziness of borehole 201216 is not observed upon triaxial testing of
201217. This contributes to the stratigraphic break described in chapter 4.4. Considering
the core log as well as geophysics of 201047, the more abrupt change in gamma, EC-values,
rock colour and grain size are reflected on the results of triaxial testing.
Usually, a hydraulic conductivity of less than 1 · 10−6 [m/s] represents a rock formation
which is dominated by silt or clay as the major component. The results received by triaxial
testing are quite uncommon as all examined samples are dominated by sand, either fine or
- 68 -
medium grained (compare with core log, appendix F & grain size distribution, figure 4.7).
However, the expected behaviour of the hydraulic conductivity in its relation do depth is
identifiably as regions of aquifers are showing higher values compared to the confining layer.
Due to the very low and therefore unexpected results of samples from borehole 201216,
the usefulness of the procedure, i.e. triaxial testing and evaluation after Darcy was called
into question. Is the use of a constant-head for determining the K-value still worthwhile in
zones of 10−12 [m/s] and 10−13 [m/s], or is it compulsive to use falling-head permeameters
for testing as well as a different flow law? Schildknecht & Schneider (1987) wrote on this
account that experimental work done on cohesive sediments produces extremely contradictory statements. Further investigation indicated that divergences applying Darcys’ law in
laboratory analyses result from inadequate, metrological consideration of side effects and of
insufficient establishment of conditions which reflect the natural actualities of the situation.
Schildknecht & Schneider (1987) also say that the validity of Darcys’ law cannot be deduced
by experimental investigation. Nevertheless, there is no reason for using a different flow law.
To sum up, it is concluded that the existence of a non-linear flow in cohesive sediments is
not exclusionary.
To regard divergences due to side effects some samples were tested under the condition of
different pressure heads. It was detected that a few samples show an increase of hydraulic
conductivity with increasing pressure. This contradicts with the applied method of evaluating
the hydraulic conductivity after Darcy. An explanation was found after examination of the
filter stones used on either end of the sample. While extraordinary features were not visible
applying small heads, pores of the filter stones showed fine particles (clay and/or silt) if high
pressures were used. That might indicate a turbulent flow which changed the hydrogeologic
parameters of the samples due to transportation of very fine material within the sediment. It
was not possible to perform the testing with a hydraulic gradient close to real conditions as
most of the samples then should have been tested for several months. The lower, confined
Aquifer (KOH2 ) shows a piezometric surface of 13.6 m below surface at 201216 while it is
37.7 m below surface at borehole 201217 (compare chapter 4.8). On the other hand, the upper aquifer reveals an average phreatic surface between 50 and 80 m below surface (Zauter,
2009, personal communication). Roughly estimated, these groundwater levels would result
in a head loss ∆h of 50 m water column. Considering the thickness of the confining layer,
respectively the distance that has to be covered by water between these two aquifers, the
implemented pressure on the sample would result in approximately 0.005 bar. That implies
1061 d of testing if transferred to the existent laboratory conditions, a sample length of 9 cm
and assuming a hydraulic conductivity of 10−10 [m/s]. Moreover, small head gradients and
long testings generate errors due to noise-effects which result in an impure representation
- 69 -
of the constant calculation parameters. Hence, the minimum generated pressure head on
samples of 201216 resulted in 2 [m − water column], resp. 0.19 [m − water column] for
201217.
Divergences in k-values of ’very poor permeability’ are varying in a very small and therefore still reasonable zone which can be explained by the following. The outflow of solution
during testing of samples which lead to results between 10−11 [m/s] and 10−13 [m/s] was
extremely sparse (compare Appendix A. Consequently, small variations in determining the
average outflow lead to greater relative errors in the calculation of the hydraulic conductivity, while the absolute error was insignificant. The very minor outflow of solution caused
further problems. Some graphs in Appendix A demonstrate a supposed back flow of solution. That, of course, is impossible as there was no reversed pressure gradient. A daily
reoccurrence of this ’back flow behaviour’ was observed upon examination. After checking on air temperature and barometric pressure, it became clear, that these diurnal variations
respectively their effects on the testing procedure could not be eliminated completely. The
so-called back flow can be interpreted on account of the operation sequence of the pressure
pumps. Continuous verification of the hydraulic head was required to guarantee a constant
hydraulic gradient. Both pumps hold therefore an internal pressure sensor which is responsible for all modifications provided by an automatic mechanism of the pressure pumps. As
fluids react on thermal and barometric alterations by expansion or contraction, the pressure
sensor is affected as well and causes the relevant changes. Again, these effects are negligible
in zones of K > 1 · 10−10 [m/s] while this declaration cannot be made for samples with lower
k-values. Miscalculations due to this problem were avoided by ignoring error containing
measurements on determining the average outflow.
Errors in terms of a reduced hydraulic conductivity due to swelling of the clay minerals
were minimised by using a solution that matches the real pore solution in its major contents
and are therefore negligible.
In summary, most of the encounterd problems result from the very poor permeability of
the tested samples. In zones of K < 1 · 10−10 [m/s] carry problems such as rounding errors
a greater weight compared to zones of K > 1 · 10−10 [m/s]. Nonetheless, satisfactory results
were received. It is discovered that two layers of better hydraulic conductivity are separated
by a layer of extremely poor to no permeability. From the hydrogeological point of view, the
results of the lower and upper core samples from borehole 201216 seem to be referring to a
transition zone between aquifer layers and confining layer as they vary only in small range.
Nevertheless, it is assumed that the sediment in the area around borehole 201216 shows kvalues in the range of K > 1 · 10−6 [m/s] as well but that these sections were not intersected
during coring. Aquifer KOH1 is situated above 80 m at 201216 while the deepest sample
- 70 -
4.6 Grainsize Analysis & Porosity
taken refers to the region of transition from aquitard to KOH2 . Borehole 201217 reveals
results which make a more precise definition of hydrogeological layers possible. Based on
the investigations of triaxial testing, the intersecting layer between aquifer KOH1 and KOH2
is seen as an aquitard on the upper and lower ends while it is determined to be hydraulic
non-conductive in the depths of the highest clay content (see litholog).
4.6. Grainsize Analysis & Porosity
Results of grainsize analysis and porosity are presented within this chapter. They provide statistical parameters and additional information which contributes to characterise the
hydrogeological condition of the investigated area. Hydraulic conductivity values are evaluated based on the grainsize distribution to compare the results to values derived from triaxial
testing (compare chapter 4.5).
4.6.1. Grainsize Analysis
Figure 4.7 displays a summary of the cumulative grain size curves of the investigated samples. It should be noted that only results of samples from borehole 201216 can be presented
as no data has been received of borehole 201217 and of samples 201216-73, -81 which were
observed during second stage testing. Single graphs can be seen in Appendix B.
Every cumulative grainsize curve is described by three general elements (Langguth &
Voigt, 1980).
1. The preponderant grainsize
2. The slope of the curve which can be determined by the uniformity coefficient U =
d60
d10
3. The shape of the curve, either S- or concave shaped
All investigated samples demonstrate a well developed S-shape. Unconsolidated sediments with a low uniformity coefficient and a preponderant grainsize of fine or medium
grained sand are indicated by this shape. In addition, a good permeability is not uncommon
compared to sediments comprised by silt or clay. All samples have their dominant grain size
in the region of fine to medium grained sand and a low uniformity coefficient which ranges
between 1.98 and 3.12. The use of estimating the hydraulic conductivity after Hazen, Beyer
and Köhler provides therefore good results in a sandy environment (see table 4.3). The grainsize distribution reveals that almost all samples are normally distributed, with some showing
a skewness.
- 71 -
Fig. 4.7.: Summary of cumulative grainsize curves of investigated core samples
The depth related location of all samples can be seen in figure 4.8. Samples 201216-19,
-22, -28 and -45 show a wider array of grain diameters as they reveal a few aggregates bigger
than 1.12 mm. These samples show the lowest share in the fraction of silt and clay. The
fraction of clay and silt increases for deeper located samples whereas the fraction of coarse
grained sand decreases. Less than 1 % of grains are classified as coarse sand if regarding samples 201216-52, -54 and -72. With 10%, the maximum of clay is found in sample
201216-52 thereby reflecting lithological investigations between 200 and 230 m of borehole
201216. It was detected that the occurrence of finer material increases with depth up to
about 200 - 230 m from where this behaviour is reversed. A particular feature is visible for
sample -72. While the general amount of sand increases, the grain diameter shows its maximum increase in grain sizes of less than 0.355 mm. This implies a finer sand fraction for
sample 201216-72 compared to the upper samples. Samples 201216-19, -22, -28, -45 and
-72 were logged as clayey sand. Sample -72 shows a higher share in the fine sand fraction
whereas the other samples are comprised by a higher share in the medium to coarse grained
fraction. Furthermore, the cumulative grainsize curve of sample 201216-72 appears to have
the steepest slope between 0.112 mm and 0.2 mm, what again points to the high amount of
fine sand. Compared with this, samples 201216-19, -22, -28 display the most broadly based
distribution of grains that matches the inhomogenity described in chapter 4.4.
All results of estimating the hydraulic conductivity after Hazen et al. (see table 4.3 on
page 74) provide values in the region of ’permeable’ (after DIN 18130). The lowest values
were found after calculations based on the method of Köhler, whereas Hazen generally pro-
- 72 -
Fig. 4.8.: Comparison of evaluated hydraulic conductivity values of samples from borehole 201216, -X refers
to sample ID
vides the highest k-values. The minimum difference appears between calculations of Beyer
and Hazen. It is suspected that Köhlers’ method gives best estimations as it is influenced by
the most sediment parameters measured. Though, all values must be seen as a rough estimation because huge differences are recognisable if compared to the hydraulic conductivity
estimated on basis of triaxial testing which is seen to provide results of higher precision
(see figure 4.8). The appearance of a ’smectite gel’ which will be explained in chapter 4.7
provides a possible explanation for these difference. A low amount of hydrated smectite
minerals in a gel-like condition increase the influence of the smectite minerals. They show
equal characteristics compared to sediments containing a higher amount of clay. This refers
to the low hydraulic conductivities of samples 201216-45, -52 and -54 derived by triaxial
testing. Grainsize analyses are performed in a dried condition which causes the ’gel’ respectively smectite solution to evaporate, leaving smectite residuals. These residuals have
a lower influence and lead to higher values of hydraulic conductivity which were evaluated
based on grainsize analyses. Still, figure 4.8 shows that all applied methods reveal the same
vertical sequence of k-values. Higher values of hydraulic conductivity in the upper and lower
reaches are intersected by lower values in the centre.
It is important to note that it was expected to receive a higher content of clay and/or silt
from the grainsize analysis because almost the entire core material withdrawn from 201216
- 73 -
Table 4.3.: Evaluation of saturated hydraulic conductivity (grainsize analysis)
Sample-ID
saturated hydraulic conductivity - k-value
Depth
U = dd60
Hazen
10
Beyer
Köhler
201216-19
201216-22
201216-28
201216-45
201216-52
201216-54
201216-72
[m]
127.09
136.91
154.04
201.63
218.70
219.83
248.67
[m/s]
1.2E-4
7.9E-5
9.5E-5
7.5E-5
2.5E-5
3.4E-5
5.3E-5
[m/s]
3.04E-5
1.44E-5
2.91E-5
8.86E-6
2.81E-6
6.73E-6
3.28E-5
2.38
2.88
3.12
2.97
2.89
2.62
1.98
[m/s]
2.14E-4
1.52E-4
1.91E-4
1.28E-4
6.53E-5
9.42E-5
1.44E-4
was logged as clayey sand. Keeping this in mind and due to the background presumption
on investigating two aquifers separated by a confining layer, all samples demonstrate a low
share in the fraction of smaller than 63 µm.
With regard to the information derived from mineral composition in chapter 4.7 and especially the mineral composition of the smectite containing samples (201216-45, -52, -54),
it becomes obvious that the share of the fines (< 63 µm) is not displayed correctly by the
grainsize distribution. Correspondingly, huge differences are identified if the k-value based
on grainsize analysis is compared to the k-value based on triaxial testing. According to the
fact that all grainsize sections were determined by dry sieving, the low contents of clay and
silt indicate that the clay minerals are arranged as stable aggregates in addition to the occurrence of the smectite gel. These aggregates which show a grainsize of fine to medium
grained sand are in truth clogged clay particles. Therefore, it is necessary to destroy these
aggregates prior to the grainsize investigation by which the fine clay could be extracted. This
would result in a share of the ’fines’ that reflects at least the smectite content determined in
chapter 4.7. According to research assistants of the BGR, all aggregates were left in their
primitive state to guarantee an appearance of natural actuality.
4.6.2. Porosity
As can be seen in table 4.4 show all samples a porosity in the range of 20 to 35%. The
grain density does not indicate a depth relating behaviour. The bulk density on the other
hand shows increasing values the finer the sediment becomes. Its maximum is found in
a depth of 218 m where the fines have their maximum also. Accordingly, the evaluated
porosity follows an equal developing that expresses itself in a higher porosity for samples
dominated by sand (201216-19, -22, -28 and -72). Lower porosity values are observed for
- 74 -
Table 4.4.: Porosity for samples of borehole 201216, up to 150 µm, rounded values
Grain density
Bulk density
Porosity
[g/cm3 ]
[g/cm3 ]
%
-19
2.6
1.8
30.2
-22
2.6
1.9
25.7
Samples 201216 - X
-28
-45
-52
2.6
2.5
2.6
1.9
1.9
2.0
29.0
23.5
20.7
-54
2.5
1.9
24.3
-72
2.7
1.7
35.9
samples containing a higher amount of clay (201216-45, -52, -54). This would be unusual
if investigating the entire porosity of an unconsolidated sediment but not if the porosity is
evaluated for pores of less than 150 µm. As described in chapter 3.2.4, DryFlo was used to
evaluate the bulk density. DryFlo cannot embed itself in pores < 50 µm. If a sediment is
investigated that consists of much, small pores just few DryFlo spherules can embed themself
into the sediment. The measured volume of the solid phase increases correspondingly which
again increases the bulk density and therefore the porosity.
To conclude from the evaluated porosity on a groundwater flow is only suitable to a limited
extend. In this respect it is important to note that the porosity is determined in the dried state,
excluding macropores (pores > 150 µm). This causes the ’smectite gel’, respectively the
solution containing hydrated smectite colloids to evaporate (see also chapter 4.7) in addition
to a collapse of the swelled clay minerals. This in turn creates some porosity not being
present in the natural, probably water saturated state. Moreover, it is extremely complicate
to determine the density of clayey sediments containing smectite. This includes samples
201216-45, -52 and -54. The swelling capacity of these sediments is highly dependent on
the fluid conditions. Variation in moisture percentage and cationic occupancy result in a
change of swelling. According to this, all density measurements were based on the referred
state of 105 ◦ C.
Summing up, it can be said that all investigated samples show typical results under the presented circumstances. A depth related development which reflects the lithology, geophysics
and triaxial testing is visible for both, the grainsize analysis as well as the porosity.
- 75 -
4.7 Mineralogy
4.7. Mineralogy
First and foremost the mineral examination was carried out to characterise the mineralogic
composition of the confining layer as accurate as possible. Knowledge on the quality and
quantity of minerals and clay minerals in particular can provide profound information on
the groundwater flow. Various procedures undertaken are for fine grained (clayey) minerals
and material mixtures but may be employed with the same degree of success for coarse
grained sediments. The methods of investigation to characterise the mineralogy of the CEBs’
sediment were applied on all core samples and provided the following results. A summary
of all analytical methods is demonstrated in table 4.5 while all relevant graphs are presented
in Appendix E. Interpretations were elaborated in cooperation, and are based on the data
provided by S. Kaufhold (BGR). Again, it should be noted that so far, no data was received of
the laboratories on samples of core borehole 201217 and the additionally observed samples
201216-73 and -81.
As can be seen in core log of 201216 and which is confirmed by table 4.5, all samples are
mainly composed of quartz. The qualitative identification of quartz and all other minerals
was done by X-ray diffractometry, infraredspectroscopy and differential thermal analysis.
All samples reveal the for quartz characteristic diffraction at X-ray irradiation, which lies
˚ This statement is reconfirmed after inspection of the IR-graphs.
between 3.34 and 4.26 A.
The typical double peak for quartz at 780 and 800 cm−1 is shown by all investigated samples.
More prove on the high quartz content can be found in logs of earlier drilled boreholes in
the Cuvelai-Etosha basin. Regarding the high amount of SiO2 in all samples which is in fact
representative for all silicates, it is detected that all samples comprise of more than 80 %
quartz.
Feldspar is another important component of all samples in addition to quartz. The feldspar
component could only be assumed after infraredspectroscopy as its typical infrared bands
show low peaks. A subsequent XRD provided an explicit detection of the feldspar component in quality.
˚ When regarding the IR and
All samples show a broad XRD intensity between 10 and 14 A.
comparing of the undisturbed samples with their dried equivalent (dried at 150◦ C for 24 h),
a characteristic AlMgOH-stretching vibration is visible at about 3630 cm−1 . This, and the
XRD intensities are both indicative for smectite and/or ilite. Some samples show a diffuse
XRD intensity in the relevant region. In this respect it is important to note that if methods
of investigation do not determine and distinguish the sample components certainly, it was
decided not to present this information in table 4.5. Therefore, it is possible that traces of
minerals were found by XRD even if they are not marked in table 4.5. Smectite, however,
- 76 -
4.7 Mineralogy
Table 4.5.: Mineralogic composition of core samples at 201216 (* appearance, + major appearance, - minor
appearance, ? unsure)
Samples 201216 - X
Qualitative analysis on mineralogical composition (XRD)
-19
-22
-28
-45
-52
-54
-72
+
+
+
+
+
+
+
±
±
±
?
±
?
?
?
?
?
-
Quartz
Smectite
Feldspar
Calcite
Ankerite
Analcime
Clinoptilolite
Muscovite/illite
-19
*
*
*
*
Quartz
Smectite
Illite
Clinoptilolite
Feldspar
Carbonate
SIO2
T IO2
AL2 O3
Fe2 O3
MgO
CaO
Na2 O
K2 O
P2 O5
LOI
Sum
-28
*
*
*
*
*
-45
*
*
*
Differential Thermal Analysis - Mass Spectrometry (DTA-MS)
-19
-22
-28
-45
-52
-54
-72
*
*
*
*
*
*
*
*
*
*
*
*
Quartz
Carbonate
Organic material
Pyrite
T value (VIS)
-22
*
*
*
*
Infraredspectroscopy (IRS)
-52
-54
-72
*
*
*
*
*
*
*
*
*
*
*
*
*
meq
[ 100g
]
%
%
%
%
%
%
%
%
%
%
%
-19
3.8
94.3
0.1
1.9
0.6
0.3
0.1
0.4
0.6
0
1.6
99.9
-22
15.8
84.9
0.2
5.2
1.7
1.1
0
0.9
1.2
0
4.5
99.7
-28
7.5
Cation Exchange Capacity (CEC)
-45
-52
-54
-72
21.2 20.6 20.4 3.5
Chemical composition of main elements (XRF)
89.3 74.2 76.0 74.1 81.9
0.2
0.4
0.4
0.3
0.2
4.2
9.9
9.1
9.3
3.7
1.0
2.9
2.4
2.9
0.7
0.5
1.3
1.1
1.3
2.0
0.1
0.6
1.1
1.1
3.3
0.6
1.3
1.1
1.3
0.3
1.5
2.8
2.8
2.7
2.1
0
0.1
0.1
0.2
0
2.5
6.2
5.7
6.5
5.6
99.9 99.7 99.8 99.7 99.8
- 77 -
4.7 Mineralogy
was definitely recognised in samples 201216-45, -52, -54. The lack respectively very minor
content of carbonates in relation to the high CEC values measured for these samples indicate
the presence of smectite additionally.
The occurring carbonates were again determined by means of XRD and IR. A rise of the
relative absorbance at the characteristic bands of carbonates (1420 - 1440 cm−1 ) is shown in
the IR graphs of samples 201216-52, -54, -72 (Appendix E). After linking this information
to the reflexion horizon at XRD testing, a possible carbonate classification can be made.
Calcite might occur in all three samples while only sample 201216-72 shows small values
of ankerite. The carbonate content could be determined by the help of the LECO-CS-444
analyser. Table 4.6 shows that the overall found carbon just exists in minor amounts. The
highest per cent by weight is found in 201216-72, the only sample that contains ankerite.
1.2 % of inorganic carbon refer to about 10 % of ankerite due to the fact that pure ankerite
consists of 11.6 % carbon (Matthes, 1993). Conversely, this means that Cinorg. < 0.2 relates
to a carbonate content of less than 2 %. The measured carbonates of sample 201216-45
could not be classified exactly. An additional conclusion can be drawn from the low values
of carbon as this implies that the drilling fluid, which is highly carbonic did not percolate
into the core material.
Samples 201216-19, -22, -45 show the existence of zeolites. X-ray diffractometry is the
best method to identify zeolites at present due to the good detection sensitivity (Kaufhold,
2009, personal communication). Analcime and clinoptilolite could be specified using XRD.
An identification of zeolites by means of IR could not certainly be made. This indicates that
the zeolites occur in minor concentrations in the relative samples.
Muscovite was certainly identified in samples 201216-28, -45 by using the XRD. Sample 201216-22 on the other hand reveals a diffuse intensity in the relevant region of 10 A˚
which could definitely identified to refer to muscovite after enrichment of the relevant grain
fractions.
Table 4.6.: C-/S- analysis of core samples at 201216
Organic C %
Inorganic C %
Total C
%
Total S
%
-19
0
0
0
0
-22
0
0
0
0
Samples 201216 - X
-28
-45
-52
0
0
0
0
0.1
0.2
0
0.1
0.2
0
0
0
-54
0
0.1
0.1
0
-72
0
1.2
1.2
0
- 78 -
4.7 Mineralogy
Fig. 4.9.: DTA-MS analysis for core samples of borehole 201216; DSC-graph (Differential Scanning Calorimetry)
Upon inspection of the graph displaying the DTA-MS curve the following features could
be determined (see figure 4.9). At about 100◦ C all samples show the energetic response
characteristic for water. Another energetic response can be seen at about 500◦ C. The Hydroxyl group (OH-group) of the montmorillonites which belong to the smectite minerals is
dehydroxylised at this temperature. Due to the fact that the clay minerals of the investigated
samples are composed of smectites it is concluded that the higher the clay content, the higher
the energetic response at 500◦ C. Samples 201216-45, -52, -54 show the highest peaks at this
temperature. Moreover, these samples show an energetic response at about 700◦ C. Due to
mass-spectrometic investigation of carbon dioxide these peaks could be identified to refer
to the thermic dissociation of carbonates. Furthermore, are peaks at 578◦ C visible for all
samples. At this temperature the so called ’quartz inversion’ takes place, whereas α-quartz
turns into β -quartz. All curves reveal a spreading above the temperature of 700◦ C which can
be explained by the recristallisation of minerals. Only sample 201216-72 shows a peak at
about 800◦ C. Mass spectrometry of SO2 revealed that this energetic response occurred due
to sulphate which in turn was generated by the two-step oxidation of pyrite.
SEM investigations prove that the smectites are embedded partly in the pores of the quartz
grains (please note that all following explanations refer to smectite containing samples only).
While smectite aggregates are visible on the left of figure 4.10, the right side just shows
smecite threads crossing the quartz grains which is the typical result of an evaporated soft
- 79 -
4.7 Mineralogy
Fig. 4.10.: SEM - Scanning Electron Microscope comparing undisturbed (l.) and dried (r.) sample containing
smectite, of borehole 201216
smectite gel (Kaufhold, 2009, personal communication). The possible existence of a smectite
gel results from the following reasons. According to Kaufhold, a porosity of minimum
20 % is classified as highly porous, if evaluated only for pores of less than 150 µm and
within the dried state (compare chapter 3.2.4 & chapter 4.6.2). During the dried state are all
smectite minerals in a desiccated condition. Upon adding of water the smectites swell out and
develop a smectite-water solution of high viscosity which shows a gel-like behaviour. The
appearance of smectites within a solution implies a good mobility of the smectite containing
clay minerals which can therefore embed themselves within the gores of quartz grains. It
was observed that the smectite content is minor if seen in relation to the entire volume of
the sediment. As the smectite minerals are hydrated partly it can be assumed that a minor
amount of smectite causes a very low hydraulic conductivity as they fill the pore volume of
the sediment to a large extent.
Samples 201216-45, -52 and -54 were taken from the depth of 201 m to 220 m. They
reveal the highest amount of clay which is confirmed by the definite determination of smectite using the XRD and the peaks at about 500◦ C in the DTA-MS curve which refers to the
montmorillonites of the smectite minerals. Another cue for the high clay content is given due
the highest values of alumina (Al2 O3 ) which is a typical component of aluminum silicates,
in turn a typical component of kaolinites. The high clay content of these samples contributes
to the conclusion drawn from lithology and hydraulic conductivity within this depth which
states that the highest clay content of the sediment at drill site 201216 is found in the depth
from 200 to 230 m. Considering the good mobility of the smectites revealed within this
chapter, this layer is likely to be an aquiclude.
- 80 -
4.8 Test pumping
Upon examination almost all samples reveal an unsorted and inhomogen nature. In accordance with the statement of Miller who described this phenomenon during logging of
cores it is also agreed that the sorting improves with increasing depth (see also chapter 4.3).
Mineralogic examination of borehole 201216 shows that the grainsize fraction of the sediment is distinguished by a major amount of sand and a minor amount of clay. Furthermore,
there is no gradation in between which means that the silt contingent is very rare. This results in a bimodal distribution and implies that the grains which were logged as silt are in
actual fact agglomerations of clay. Single pieces of volcanic ash and valves of freshwater
diatoms together with authigenic minerals which are clinoptilolite and analcim were found
during silt fraction analysis. The presence of these very silica-rich authigenic minerals which
can be interpreted to have formed from siliceous particles like diatom valves and volcanic
ashes as well as some other minerals, suggest that also the smectite has formed in situ (Fenner, 2010, personal communication). As smectite is the dominating clay mineral observed
for this sediment and in particular for the clay-rich interval between the two aquifers, it is
concluded that at least some smectite minerals are generated authigenetic. This diagenesis
could explain the bimodal distribution of clay and sand. Furthermore, it could explain the
unsorted and inhomogen nature of the sediment as the diagenesis is not restricted to layers
but highly influenced by the chemical composition of a watery solution percolating through
the sediment.
To sum up, all samples are mainly composed of quartz and feldspar. In addition, minor
components as zeolites and carbonates could be identified. All samples reveal a share in
the clay fraction. Samples 201216-45, -52 and -54 which are withdrawn from the depth of
the confining layer show the highest clay content in terms of smectite and/or illite. Sample
201216-72 which is withdrawn from the fuzzy layer contains more carbonates and some
pyrite.
4.8. Test pumping
Pumping tests were analysed to identify hydrogeological parameters such as hydraulic
conductivity and transmissivity of the deeper seated aquifer KOH2 . Basically, pumping tests
serve two objectives. Firstly, they may be performed in order to determine the hydraulic
characteristics of aquifers or water bearing layers and secondly, they may provide information about the yield and drawdown of the well (Kruseman & de Ridder, 1970). Chapter 3.1.1
shows that three mud rotary boreholes were drilled to a depth of 390 m at 201045, whilst
201046 and 201047 were drilled to 266 m and 383 m respectively. Afterwards, they were
developed into wells with screens in the area of KOH2 . Pump test within the cored boreholes
were not possible as both boreholes were extremely unstable and abandoned at 266 m in the
- 81 -
4.8 Test pumping
Table 4.7.: Summary of step drawdown test data for 201045, -46, -47; measured values provided by van Wyk
(2009) 201045: static water level - SWL = 13.6 m, 201046: SWL = 21.55 m, 201047: SWL =
37.71 m; lowered water level - LWL
Step
I
II
III
IV
t
[min]
60
120
180
240
201045
Q
LWL
[m3 /h] [m]
4.2
14.4
8.1
15.2
11.9
15.8
16
16.4
s
[m]
0.8
1.6
2.2
2.8
201046
Q
LWL
[m3 /h] [m]
6.2
25.25
15.3 29.11
16
29.4
s
[m]
3.7
7.56
7.85
201047
Q
LWL
[m3 /h] [m]
3.2
39.1
7.8
40.1
11.9 41.07
16
41.66
s
[m]
1.39
2.39
3.36
3.95
case of 201216 while 201217 was abandoned at 235 m. Due to the fact that borehole 201045
was drilled in a distance of 12.2 m to cored borehole 201216, respectively 100 m distance
between 201047 and 201217 a direct comparison is feasible. The presented results are based
on pump test investigations of the three mud rotary boreholes and give information about the
parameters of the fresh water aquifer as well as estimations on the yield in terms of specific
capacity curves of the investigated wells.
A first measurement of the water table revealed that the investigated aquifer is under considerable hydraulic pressure. It is assumed that the aquifer is located between 200 m and
348 m in depth due to information on geophysics, lithology and hydraulic parameters which
is presented by this report (compare chapter 5). The static water level (SWL) was already
encountered at 13.6 m below surface in borehole 201045 while at 21.55 m for 201046 and
37.71 m for 201217 respectively.
Step test data was analysed to determine the specific capacity of the well. It is defined
as the relationship between pumping rates and drawdown (Langguth & Voigt, 1980). A
summary of the step drawdown test data is given in table 4.7 while the specific capacity
curve for all wells can be seen in figure 4.11. It should be noted that these curves were
plotted in a single graph to give an overview but cannot be compared directly as they all
refer to a variable thickness of the water bearing layer.
Figure 4.11 gives inforamtion about the yield of the wells. It is observed that all values
follow a continuous curve shape which indicates that no flow boundaries have been encountered during pumping. The depression cone of the pumping wells has therefore not extended
to the aquifer boundaries as this would result in major bends of the specific capacity curves.
With regard to the thickness of the water bearing layer the water level behaviour is typical
for large scale aquifers as the drawdown is minor in comparison to the discharge rate. The
thickness at borehole 201045 was evaluated with 98 m whereas 28 m for 201046 and 96 m
- 82 -
4.8 Test pumping
Fig. 4.11.: Specific capacity curves of step test pumping in 201045, -46, -47
for 201047 respectively. Storage capacity of the aquifer was not evaluated as this requires
observation wells within the area of the depression cone.
Interpolated values of the drawdown can be read off the specific capacity curves. Points in
figure 4.11 show actual measured values whereas connective lines display interpolated ones.
They show that if pumping the borehole at 10 m3 /h, a drawdown of 1.9 m is caused at borehole 201045 while the same discharge results in a drawdown of 2.9 m at 201047 respectively
5.4 m at 201046. These interpolated values provide sufficient results to estimate the drawdown in the planning period of further withdrawals. This becomes obvious if interpolated
values are compared to actual measured lower water levels LWL of the constant rate test.
After pumping with a constant discharge at all boreholes for 48 h the measured water levels
showed just small variations versus their evaluated equivalent.
201045: at discharge of 15.5 m3 /h
measured s : 2.60 m
estimated s : 2.74 m
measured s : 4.38 m
- 83 -
4.8 Test pumping
measured s : 3.86 m
In linear flow conditions and with constant groundwater withdrawal the capacity curve
would show a straight line due to the fact that it is determined by the drawdown and the
discharge rate. Basically, the specific capacity is affected by two elements, the well loss
and the aquifer loss (Brassington, 2007). The latter is defined by the aquifer parameters,
i.e. the hydraulic conductivity. The well loss on the other hand is caused by the hydraulic
gradient which is generated by withdrawal of water from a well. A radial convergent flow
field is generated which can be described by the following. The cross sectional area of
’through-flow’ reduces radially towards the pumped borehole. Therefore, it requires more
energy to move the same volume of water through the same volume of aquifer material in a
radial flow field compared to a linear flow field at similar pumping rates (van Wyk, 2009).
The additional energy required manifests as well loss. Hence, the standard capacity curves
which are normally appearing in technical literature show a higher and unsteady increase of
drawdown at steady increase of discharge.
Table 4.7 and figure 4.11 both show that the actual measured relationship between pumping rates and drawdown is based on a reverse non-linear dependency. The higher the discharge the lesser the increase of the drawdown. This is the typical indication for a confined
aquifer under considerable pressure. In confined aquifers the well loss can be counterbalanced by static hydraulic pressures (van Wyk, 2009). Being in this aquifer condition, large
hydraulic pressures exceed the flow resistance so that more groundwater flows into the well
as actually predefined by linear flow and the relevant well loss. This results in the non-linear
behaviour seen in figure 4.11.
Due to the just described phenomenon it was not possible to determine the transmissivity,
storativity coefficient and the hydraulic conductivity by means of standard procedures for
multi-step pump tests such as Cooper & Jacob. In chapter 3.2.6 was explained that a straight
line with a positive slope is necessary to determine the transmissivity and the storativity
coefficient of the investigated aquifer by equations (3.9) and (3.11) on page 43. While the
method of Cooper & Jacob was applied, the relevant values formed a line with a negative
slope which made further calculations impossible. The method had to be modified to the
Neumans’ method which is a less common method that allows evaluations based on the
demonstrated non-linear behaviour.
A transmissivity chart is created where the measured values of discharge Q [m3 /s] are
plotted (abscissa) against the ratio of discharge to drawdown Q/s [m2 /s] (ordinate). Again,
- 84 -
4.8 Test pumping
Table 4.8.: Aquifer parameters evaluated on basis of test pumping
Step test
Borehole
201045
201046
201047
D
T
2
[m] [m /d]
98
28
96
106.9
34.6
50.6
Constant rate test
Sichardt
Kusakin
k
[m/s]
[m2 /d]
k
[m/s]
[m2 /d]
k
[m/s]
1.26E-5
1.43E-5
6.51E-6
121.93
46.7
87.92
1.44E-5
1.93E-5
1.06E-5
153.26
55.64
107.00
1.81E-5
2.30E-5
1.29E-5
T
T
these values form a straight line with a positive slope. The slope is called the loss/gain
factor (van Wyk, 2009). The point of intersection between a virtual expanded line and the
ordinate gives the so called ’predicted transmissivity’ which is the transmissivity for linear
flow conditions. The loss/gain factor refers to the non-linear component. Both values can
then be used to simulate drawdowns at different discharge rates.
Results of the transmissivity and hydraulic conductivity evaluated on basis of the multistep, as well as the constant rate test can be seen in table 4.8. The three investigated boreholes
extend over an area of about 559 km2 . Despite the large distance to each other it is detected
that the calculated values show similar order of magnitudes in the region of ’permeable’ (after DIN 18130). Hence, the regional extend can be reconfirmed for the investigated aquifer
KOH2 . The constant rate test has been analysed by means of Thiem & Dupuits’ method and
shows slightly higher estimated values compared to the analysation of the multi-step pump
test after Neuman. Slight variations are observed for the two k-values based on the constant
rate analysation due to the fact that the only contrast in their equations is given by the difference in estimating the radius of the depression cone r2 . The method after Sichardt shows
slightly lower values to estimate the radius of the depression cone compared to evaluations
based on the depression cone after Kusakin. As the hydraulic conductivity is calculated by
the function of Thiem & Dupuit, r2 is the only variable within equation (3.10) on page 44
because all other factors are fixed values of the aquifer such as discharge rate, piezometric height and thickness of the saturated layer (aquifer parameters). Hence, the radius of
depression cone has only a minor influence on the well function of Thiem & Dupuit. To
sum up, sufficient results were received of the aquifer parameters which indicate a sediment
respectively aquifer of average water permeability.
A recovery analysis was performed for boreholes 201046 and 201047 after the constant
rate pumping test. The results can be seen in table 4.9. After 48 hours of constant pumping
both boreholes showed a lowered water level (LWL) which results in a drawdown of 4.45 m
for borehole 201046 and 3.71 m for 201047 respectively. After a recovery period of 4 hours
- 85 -
4.8 Test pumping
Table 4.9.: Recovery analysis after constant rate pumping test for boreholes 201046 and 201047
SWL Pump duration
[m]
[h]
201046 21.55
48
201047 37.71
48
Pump rate
[m3 /h]
8.9
15.9
LWL Recovery time
[m]
[h]
26.00
4
41.42
4
RWL Recovery
[m]
[%]
21.58
99.86
37.73
99.94
both boreholes show a recovery water level (RWL) which differs to the static water level
(SWL) in only 3, respectively 2 cm. This refers to a recovery rate of more than 99.8 % within
4 hours which is fast. Considering all presented results whithin this chapter it is concluded
that the deeper seated fresh water aquifer KOH2 occurs with a regional extend and that it is
a groundwater resource of large capacity.
The evaluations were elaborated as accurate as possible but it must be noted that the geometrical dimensions of the aquifer such as thickness of the water bearing layer, depth of the
capping bed and depths of subjacent layers are subjective estimations. Van Wyk (2009) provides values of the hydraulic conductivity based on pump test interpretation as well. Differences to his work result from upgrading the geometric dimensions due to elaborated results of
the cored boreholes 201216 and 201217 which are presented in this report. For example, the
thickness of the water bearing layer D at borehole 201045 and 201216 was modified based
on the evaluated hydraulic conductivity of triaxial testing. Table 4.1 on page 66 shows that
the measured hydraulic conductivity of the deepest samples demonstrate a k-value of ’poor
permeability’. Chapter 4.4 and chapter 4.5 show that these samples refer to a fuzzy layer
which describes the structural change from confining layer to aquifer. Prior to laboratory
analyses, the thickness of the water bearing layer (D) was estimated with 120 m wheres it
was changed to 98 m after considering all presented results such as k-values (triaxial testing),
natural gamma, EC-values, etc..
- 86 -
Chapter 5. Discussion
5. Discussion
The investigation of hydrogeological processes of a multi layered aquifer system addresses
several scientific disciplines. This chapter discusses and integrates the results presented in
chapter 4 of the investigated multi layered Ohangwena aquifer system (KOH) in a coherent
context. The combined results will be compared to a reasonable degree to evaluate the vertical and horizontal extend of the hydrogeological layers. Furthermore, it is intended to give
a conceptual view on the hydrogeological mechanisms of the separating layer between the
two aquifers KOH1 and KOH2 .
5.1. Hydrogeologic Estimation of Layers
For future groundwater withdrawals and numerical modeling purposes it is essential to
have a thorough idea on the thickness of the hydrogeological layers in a sediment. Hydrogeological parameters and mineralogic data can give additional information which contributes
to the estimation of layer boundaries. The sediment of the project area can be divided into
five different hydrogeological layers as was already done in chapter 4.4. The sections of
particular interest can be referred to as section II, III and IV . With the additional information on hydrogeological parameters and mineralogy given in chapter 4, section II relates
to the upper Ohangwena aquifer KOH1 , whereas section III relates to the separating layer
which besides, is the confining layer of the deeper seated, fresh water aquifer KOH2 . The
Ohangwena aquifer KOH2 is determined to be section IV .
5.1.1. Lateral
The existence for a regional extend can be assumed for all sections. The KOH1 aquifer
is commonly known by Namibian scientists and has been intersected by many prior drilling
procedures (compare also Struckmeier & Christelis (2001), Bittner (2006) and Hipondoka
(2005)). Laboratory analyses on the sediment material as well as the lithology of all investigated boreholes and in situ measured geophysics contribute to the regional extend.
Prior to this investigation, section III has only been assumed. This investigation covers
therefore the first detailed geologic and hydrogeologic investigation of the separating layer.
- 87 -
5.1 Hydrogeologic Estimation of Layers
Its existence is reinforced by the geophysical investigations of boreholes 201045, 201046
and 201047 which can be interpreted to relate to a high clay content. The high clay content
has been confirmed due to mineralogic examination of core samples and comprises mainly
of smectite (see chapter 4.7). Laboratory evaluations of the hydraulic conductivity on basis
of triaxial testing have proven the existence of a confining layer. Therefore, it is concluded,
that section III underlies aquifer KOH1 and is a separating layer of high clay content which
was encountered in all boreholes presented in chapter 4. Considering the average distance of
40 km between these boreholes and that they all show similar characteristics of geophysics,
lithology while additionally, 201216 and 201217 show a similarity in the value pattern of
hydraulic conductivity, it is stated that section III extends over the region of the project area
as well.
The existence of section IV respectively the aquifer KOH2 is also indicated due to the
geophysical investigations on the sediment. The lithology as well as the low natural gamma
and electric conductivity values indicate the fresh water aquifer. Results of triaxial testing
contribute to the existence of KOH2 as they confirm a poor water permeability of the sediment (10−8 - 10−6 [m/s] after DIN 18130). Pumping tests which are described in chapter
4.8 gave proof to the existence of the aquifer. In addition to these investigations, the TEM
soundings contribute to delineate the regional extend of the fresh water aquifer. Another cue
for a continuous aquifer is given by the absolute groundwater level of the encountered fresh
water in boreholes 201045, -46 and -47. The height of these boreholes was evaluated based
on SRTM data which has a vertical accuracy of 5.6 m for Africa (Farr et al., 2007). Borehole 201045 is located at 1116 m a.m.s.l. whereas 201046 at 1123 m and 201047 at 1137 m.
In addition to the values of the encountered groundwater table given in chapter 4.8, the absolute height of the phreatic surface above mean sea level results in 1102.4 m for 201045
while borehole 201046 reveals an absolute groundwater level of 1101.45 m and 1099.29 m
for 201047 respectively. With regard to the absolute height error, it can still be stated that the
groundwater levels resemble closely to each other. As these boreholes mount up to an area
of 560 km2 , section IV is estimated to have a regional extend within the project area. From a
lateral point of view the location of the aquifer can only be estimated by means of transient
electromagnetic (TEM) measurements (see also chapter 3.1 and chapter 4.1.1). According
to a first evaluation of the TEM measurements, the area covered by the aquifer mounts up
to 3440 km2 and can be seen in figure 5.1. For numerical modeling purposes it is advised
to refer to the estimation of the demonstrated aquifer area as it is the best evaluation till this
day. It should be noted that figure 5.1 is just a draft and not true to scale. It shows the covered
area of KOH2 and to relates the lithologs of the investigated boreholes to their geographical
location. The demonstrated thickness of the aquifer does not relate to a sound calculation.
- 88 -
- 89 Fig. 5.1.: Lithology of project boreholes in relation to their location within the assumed area of the aquifer KOH2
5.1.2. Vertical
The vertical extend of section II, III and IV can be determined partly due to the performed
investigations at the drill sites of 201216/201045, 201046 and 201217/201047. Section II
has been intersected several times but scientific investigations on its thickness have not been
carried out so far. An areal determination on the height of groundwater table or the upper
boundary of section II can therefore not be given. It is, however, possible to evaluate its
lower boundary due to examination of core material, in situ measured natural gamma values
and electric conductivity (EC). The abrupt change of natural gamma and EC at the depth of
110 m at borehole 201047 indicates a section change which is confirmed by lithology (see
chapter 4.3.2) as well as evaluation of hydraulic conductivity of the core material. k-values
based on triaxial testing range from 10−6 to 10−8 [m/s] above 110 m whereas below, values
are found that range from 10−11 to 10−12 [m/s]. At boreholes 201216 and 201045 the section
change is evaluated to be at a depth of 80 m. Continuous increasing of natural gamma as well
as hydraulic conductivity values confirm the change from aquifer to aquitard. The results of
triaxial testing show values of 10−10 [m/s] and smaller below 80 m. More proof is given due
to the lithology which consists an increasing amount of clay from this depth. The increasing
clay content was also confirmed by the mineralogical investigation described in chapter 4.7.
The boundary between section II and III at borehole 201046 is situated at the depth of 75 m
which has been estimated based on in situ measured gamma values which increase from the
depth of 75 m.
The upper boundary of section III has already been evaluated in the last paragraph and
refers to the depth of 80 m for 201216/201045, 75 m at 201046 and 110 m at 201217/201047
respectively. The lower boundary of section III is well pronounced at borehole 201047 due
to the abrupt change of geophysics at the depth of 200 m. Lithological description of the
core material of 201217 and results of triaxial testing in particular contribute to this determination. The hydraulic conductivity reveals values of 10−11 [m/s] at 195 m whereas the next
deeper situated sample which is in the depth of 213 m shows a k-value of 10−6 [m/s]. The
lower boundary is in a condition of continuous transition at drilling site 201045 respectively
201216. Gamma values and the clay content are decreasing continuously from the depth of
230 m. The same characteristics are visible for the hydraulic conductivity. The only distinct
change is visible upon observing the measured electric conductivity values. Lower values are
shown from the depth of 235 m. This indicates a change to an environment of low salinity
but it does not, however, determine the beginning of the deeper seated fresh water aquifer. In
this case, the EC values can only be used to identify a saline and/or fresh water environment
because they do not relate to the amount of clay or a water permeability of the sediment.
According to the given considerations it can be deduced that the boundary becomes fuzzy.
- 90 -
5.2 Aquitard or Aquiclude?
With regard to the results of triaxial testing it is advised to estimate the change from section
III to IV in the depth of 250 m for modeling purposes as the highest hydraulic conductivity
values were found below this level. Similar characteristics are found at borehole 201046.
The gamma decreases downwards constantly from about 210 m whereas EC values change
abruptly from 220 m. As the hydraulic conductivity was not evaluated for the sediment of
borehole 201046 it is assumed that the condition of the sediment follows the characteristics
of the sediment at 201045. The lower boundary of section III is thereby estimated to be
found at the depth of 230 m.
Again, the boundary between section III and IV has been evaluated in the last paragraph. It is located at depths of 250 m for 201216/201045, 230 m for 201046 and 200 m
for 201217/201047 respectively. Chapter 3.1 explains that the geographical position of boreholes drilled during this project has been chosen due to the interpretation of the TEM sounding curves. It was intended to drill at locations where the thickness of the water bearing
layer is assumed to be maximised. All relevant project boreholes are situated at drilling sites
with an assumed aquifer thickness of more than 100 m. Boreholes 201045 and -47 have been
drilled over the entire thickness of the water bearing layer. The other relevant boreholes were
abandoned beforehand due to problems described in chapter 3.1 and thus the entire thickness
of section IV can only be determined at drilling sites 201216/201045 and 201217/201047.
The basis for an estimation on the bottom end of the aquifer are given by the in-situ measured geophysics and the lithology of the sediment. Both boreholes show abrupt increase
of gamma and EC values at a certain depth. This is accompanied by an increase of clay
content. Clay is accounted for the end of water permeability and consequently the end of
section IV . Accordingly, the boundary level is 348 m for borehole 201045 and 296 m for
borehole 201047. This implies a thickness of the fresh water aquifer of 98 m respectively
96 m. The thickness of the water bearing layer at other locations of the project area can
only be assumed. As the drill sites were chosen according to the interpretation of the TEM
sounding curves with the maximum thickness of the fresh water layer, it is assumed that
the water bearing layer appears with the thickness of about 100 m in the central part of the
project area and shows variations down to 10 to 15 m in marginal areas (Zauter, 2009, personal communication). In the south west of the project area a thickness of less than 10 m is
assumed.
5.2. Aquitard or Aquiclude?
An essential part of this investigation is to determine the confining layer respectively section III as an aquitard or aquiclude to give a conceptual view on the hydrogeological mechanisms and generate the basis for numerical modeling purposes. The existence of the aquitard
- 91 -
5.2 Aquitard or Aquiclude?
or aquiclude has been proven by laboratory evaluation of the hydraulic conductivity on basis
of triaxial testing. These results show that the confining layer is determined to be an aquitard
in the upper and lower reaches whereas it changes into an aquiclude within the center. This is
reconfirmed by the high clay content which increases towards the center where furthermore,
it shows the maximum of clay. A similar vertical alignment can be determined for the water
permeability (see also chapter 4.5). It is evaluated that the sediment at the two core boreholes
consists of:
201216 - 080 to 200 m
0000200 to 230 m
0000230 to 250 m
aquitard of poor to very poor permeability
aquiclude
aquitard of very poor to poor permeability
201217 - 110 to 144 m
0000144 to 170 m
0000170 to 200 m
aquitard of poor to very poor permeability
aquiclude
aquitard of very poor to poor permeability
These estimation can be confirmed by the mineralogic results. The appearance of smectite
minerals in a gel-like condition which was explained in chapter 4.7 implies a good mobility
of the smectite containing clay fraction. Therefore, the clay minerals can embed themselves
in the gores between larger grains which can seal off the lower fresh water aquifer KOH2
from the upper aquifer KOH1 . At this stage and upon consideration of all presented results it
can be concluded that the separating layer is hydraulic non-conductive within the center and
for a resting state. If a continuous withdrawal of groundwater is performed in the pumping
wells, a groundwater flow is induced. With regard in the long-term this mechanical disturbance of the resting state could result in a transportation of the mobile clay minerals towards
the pumping well. This occurs if the groundwater flow exceeds the critical flow velocity
which is needed to disembed the smectite minerals from the gores of sand grains. Subsequently, the separating layer could develop an increased hydraulic conductivity due to the
absence of the ’smectite gel’.
A possibility for salt water intrusion at the deeper fresh water aquifer can result due to
the drawdown of the piezometric height of the confined aquifer KOH2 . If the piezometric
height drops below the phreatic surface of the upper aquifer KOH1 , the existent hydraulic
gradient would be reversed so that the possible groundwater flow is directed from KOH1 to
KOH2 . The groundwater table of the upper aquifer is estimated based on the lithology and
geophysics for borehole 201045 with 55 m and for 201047 with 80 m below surface. With
the measured water tables of KOH2 given in chapter 4.8, the drawdown on the piezometric
height of KOH2 would have to be greater than 45 m to reverse the hydraulic gradient. Corresponding to the evaluated hydraulic conductivity values, the estimated radii of the depression
- 92 -
5.3 Aquifer Capacity & Outlook
cone and the thickness of the relevant layers, the well function of Thiem & Dupuit (equation
(3.10), page 44) was used to estimate the required pumping rate. Hence, a drawdown of 45 m
implies a groundwater withdrawal of more than 100 [m3 /h]. Considering the thickness of the
separating layer and a corresponding pressure head of only a few meters this eventuation is
extremely unlikely.
With a reversed hydraulic gradient and in the case of absence of the hydrated smectite
minerals it is important to investigate if the sediment region containing the clay maximum is
still hydraulic non-conductive. Mehnert & Jennings (1985) show that the hydraulic conductivity of aquitards containing swelling clays increases with a higher salinity of the water. In
the event of a salt water intrusion in the lower regions of the separating layer, i.e. the region
of the fresh water aquitard the water permeability might be enhanced.
5.3. Aquifer Capacity & Outlook
At the present moment it is not possible to give a significant estimation on the yield of
the confined fresh water aquifer. It is assumed that KOH2 is a groundwater source of high
volume due to the measured piezometric height, its good recovery rate after groundwater
withdrawal (chapter 4.8) and its considerable extend within the project area. While this
investigation covers the first evaluation of the hydrogeological parameters of the sediment it
is advised to perform further investigations, such as long term pumping tests in combination
with observation wells to gain information on the storage capacity, recovery ability and yield
of the aquifer. Upon consultation with the project manager a one year pumping test has been
initiated to collect the relevant data. In this connection it is suggested to take regular water
samples in order to analyse the content of smectite colloids. An increase of smectite upon
continuous pumping could show if there are hydrated smectite minerals of good mobility
and if they are transported from the confining layer towards the well.
So far, the direction of the groundwater flow has been estimated based on the CEBs’
topography. A precise measurement of the phreatic surface in relation to the mean sea level
for all boreholes could determine the flow direction as well as a global flow system for
modeling purposes. A pressure gauge would furthermore identify annual changes of the
groundwater table. Another identification of the flow direction could be done by the injection
of tracers.
More boundary conditions should be investigated such as sources and sinks of the aquifer.
The groundwater recharge area is estimated to be situated in Angola and it is recommended to
record the precipitation of this area as well as the lithology of its sediment. For a significant
- 93 -
5.3 Aquifer Capacity & Outlook
result in numerical modeling it is suggested to determine the lateral and vertical boundaries
of the aquifer. As proof of its existence has been given by drilling of boreholes in the northeast and centre, the interpretation of the TEM sounding curves needs to be confirmed for the
southern and western part of the project area. It should attach great importance to examine
the continuity and thinning of the aquifer towards its margins. It is questionable why the
aquifer should end in the regions declared by TEM measurements. An identification of a
non-permeable barrier would also define boundary conditions for numerical modeling.
- 94 -
Bibliography
B ITTNER , A RNOLD. 2006 (November). Desk Study Report: Cuvelai-Etosha Groundwater
Investigation. Tech. rept. Bittner Water Consult CC, P.O. Box 86386, Windhoek, Namibia.
BGR internal report.
B RASSINGTON , R ICK. 2007. Field Hydrogeology. Third edn. The Atrium, Southern Gate,
Chichester, West Sussex PO19 8SQ, England: John Wiley & Sons. The Geological Field
Guide Series.
DIN. 1998. DIN 18130-1; Bestimmung des Wasserdurchlässigkeitsbeiwertes; Laborversuche. Tech. rept. Deutsches Institut für Normung e.V.
FARR , T OM , ROSEN , PAUL , C ARO , E DWARD , C RIPPEN , ROBERT, D UREN , R ILEY, H ENS LEY, S COTT, KOBRICK , M ICHAEL , PALLER , M IMI , RODRIGUEZ , E RNESTO , ROTH ,
L ADISLAV, S EAL , DAVID , S HAFFER , S COTT, & S HIMADA , J OANNE. 2007. The Shuttle
Radar Topography Mission Report. Tech. rept. American Geophysical Union.
¨
¨
F UCHTBAUER
, H ANS , M ULLER
, G ERMAN , & VON E NGELHARDT, W OLF. 1967. Sedimentary Petrology; Methods in Sedimentary Petrology. Stuttgart: E. Schweizerbart’sche
Verlagsbuchhandlung (Nägele u. Obermiller).
F ENNER , J ULIANE. 2010 (February). Micropaleontologist (Diatoms), BGR.
G OOGLE , E ARTH. 2009 (August). Screen print of Border Namibia - Angola. Google Earth.
H IPONDOKA , M ARTIN H.T. 2005. The Development and Evolution of Etosha Pan,
Namibia. Ph.D. thesis, Julius-Maximilians-Universität Würzburg, http://www.opusbayern.de/uni-wuerzburg/frontdoor.php?source opus=1435&la=de.
¨
H OLTING
, B ERNWARD. 1996. Hydrogeologie; Einführung in die Allgemeine und Angewandte Hydrogeologie. Fifth edn. P.O.Box 30 03 66, D-70443 Stuttgart: Ferdinand Enke
Verlag.
¨
H UTTE
, A KADEMISCHER V EREIN. 1951. Des Ingenieurs Taschenbuch. 27 edn. Vol. Three.
Berlin: Wilhelm Ernst & Sohn.
95
BIBLIOGRAPHY
BIBLIOGRAPHY
IUPAC, S ING , K.S.W., E VERETT, D.H., H AUL , R.A.W., L.M OSCOU , P IEROTTI ,
R.A., J.ROUQUEROL , & S IEMIENIEWSKA , T. 1985. Reporting Physisorption Data
for Gas/Solid Systems with Special Reference to the Determination of Surface Area and
Porosity. In: Pure & Applied Chemnistry, vol. 57, no.4. International Union of Pure and
Applied Chemnistry.
K AUFHOLD , S TEPHAN. 2009 (December). Mineralogist, BGR.
¨
K EMPF, J URGEN
. 2000. Klimageomorphologische Studien in Zentral-Namibia: Ein
¨
Beitrag zur Morpho-, Pedo- und Okogenese.
Ph.D. thesis, Julius-Maximilians-Universität
Würzburg, http://www.opus-bayern.de/uni-wuerzburg/volltexte/2003/532/.
K LINKENBERG , M ARTINA. 2008. Einfluss des Mikrogefüges auf ausgewählte petrophysikalische Eigenschaften von Tongesteinen und Bentoniten. Ph.D. thesis, GeorgAugust-Universität Göttingen, http://webdoc.sub.gwdg.de/diss/2008/klinkenberg/.
K RUSEMAN , G.P., & DE R IDDER , N.A. 1970. Analysis and Evaluation of Pumping Test
Data. In: Bulletin, vol. 11. International Institute for Land Reclamation and Improvement.
L ANGGUTH , H ORST ROBERT, & VOIGT, RUDOLF. 1980. Hydrogeologische Methoden.
Berlin, Heidelberg, New York: Springer Verlag.
M ATTHES , S IEGFRIED. 1993. Mineralogie. 4 edn. Berlin, Heidelberg, New York: SpringerVerlag.
M EHNERT, E DWARD , & J ENNINGS , A ARON A. 1985. The Effect of Salinity-Dependent
Hydraulic Conductivity on Saltwater Intrusion Episodes. Journal of Hydrology, 80(3 - 4),
283 – 297.
¨
M EIER , L ORENZ P., & K AHR , G UNTER
. 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with Triethylenetetramine and Tretraethylenepentamine. Clays & Minerals, 47, 386 – 388.
M ENDELSOHN , J OHN , EL O BEID , S ELMA , & ROBERTS , C AROLE. 2000. A profile of northcentral Namibia. Windhoek: Gamsberg Macmillan Publishers. Environmental profiles
project; Directorate of Environmental Affairs, Ministry of Environment and Tourism;.
M ILLER , ROY M C G. 1983. The Pan-African Damara Orogen of South West Africa/Namibia.
Johannesburg: Special publication / Geological Society of Sotuh Africa. Pages 431–515.
M ILLER , ROY M C G. 1997. The Owambo Basin of Northern Namibia. Chap. 11, pages 237–
268 of: S ELLY, R.C. (ed), African Basins. Sedimentary Basins of the World. Amsterdam:
Elsevier Science B.V.
- 96 -
BIBLIOGRAPHY
BIBLIOGRAPHY
M ILLER , ROY M C G. 2008c. The Geology of Namibia. Vol. 3. Windhoek: Geological
Survey Namibia. Ministry of Mines and Energy.
M ILLER , ROY M C G. 2009 (June). Expert in Namibian geology, Geological Survey of
Namibia.
M OMPER , JAMES A RTHUR. 1982. The Etosha Basin re-examined. Oil & Gas Journal, 5,
262–287.
NAKAYAMA , M IKIYASU (ed). 2003. International Waters in Southern Africa. United Nations University Press.
¨
P L OTHNER
, D IETER , & B ITTNER , A RNOLD. 2001. Groundwater in Namibia. Department
of Water Affairs. Chap. Cuvelai-Etosha Basin.
R EIMANN , T HOMAS. 2004 (November). Laborpraktikum: Einführung in die Ermittlung
und Interpretation physikalischer und chemischer Parameter des Untergrundes. Internal
document: TU Dresden; Institut für Grundwasserwirtschaft.
S CHILDKNECHT, F RIEDRICH. 2007 (August). Grundwasserkundung im Norden Namibias;
Vorläufigen Planung von Transientelektromagnetischen Messungen (TEM) zur Grundwassererkundung im Cuvelaibecken. BGR internal document.
S CHILDKNECHT, F RIEDRICH. 2008 (April). Geophysical groundwater survey in the
Cuvelai-Etosha basin; Proposal for confirmation borehole sites for the eastern part of
the basin. BGR internal document.
¨
S CHILDKNECHT, F RIEDRICH , & S CHNEIDER , W ILFRIED. 1987. Uber
die Gültigkeit des
Darcy-Gesetzes in bindigen Sedimenten bei kleinen hydraulischen Gradienten. In: Geologisches Jahrbuch. Reihe C, no. Heft 48. Bundesanstalt für Geowissenschaften und
Rohstoffe & Geologische Landesämter der Bundesrepublik Deutschland.
S TRUCKMEIER , D IETER , & C HRISTELIS , G REG (eds). 2001. Groundwater in Namibia.
Department of Water Affairs.
W YK , B RAAM. 2009 (May). Groundwater Investigation of the Cuvelai-Etosha Basin.
Tech. rept. Bundesanstalt für Geowissenschaften und Rohstoffe. BGR internal document.
VAN
¨
WALTHER , W OLFGANG , P ATSCH
, M ATTHIAS , & KONRAD , C HRISTIAN. 2004. Dynamik
des unterirdischen Wassers. lecture note. Technische Universität Dresden.
Z AUTER , H ARALD. 2009. Project manager of - Groundwater investigation in the CuvelaiEtosha Basin, from April 2007 - April 2010.
- 97 -
BIBLIOGRAPHY
BIBLIOGRAPHY
Z AUTER , H ARALD , & K ATJIMUNE , M ATHEWS. 2007. Fact Sheet for Press Release concerning Groundwater Investigation in the Cuvelai-Etosha Basin.
- 98 -
Appendix
99
A. Hydraulic Conductivity - Triax
Borehole 201216
100
Fig. A.1.: Evaluation of hydraulic conductivity on basis of triaxial testing for sample 201216-19, ∆h = 7 m H2 O - water column, depth: 127.14 - 127.25 m
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
Borehole 201217
120
121
Fig. A.21.: Evaluation of hydraulic conductivity on basis of triaxial testing for sample 201217-18, ∆h = 1.9 m H2 O - water column, depth: 99.68 - 99.80 m
122
123
124
125
126
127
128
129
130
131
B. Grainsize Analysis
Borehole 201216
132
Fig. B.1.: Grainsize Analysis of sample 201216-19, borehole 201216, depth: 127.14 - 127.25 m
133
134
135
136
C. Geophysical Measurements of
Drilling Fluid
137
Fig. C.1.: Geophysical measurements of the drilling fluid during core drilling, values highly influenced by
synthetic drilling viscosifier (DRILLVIS & CAP21) which concentration could not be recorded
138
D. Photos
Borehole 201216
139
Fig. D.1.: Borehole 201216, corebox 1, depth: 80 - 87 m
140
141
142
143
144
145
146
Borehole 201217
147
148
149
150
151
152
153
154
Samples of borehole 201216
155
Fig. D.43.: Sample 201216-19, 126.90 - 127.25 m
Fig. D.44.: Sample 201216-22, 136.69 - 137.17 m
156
Fig. D.45.: Sample 201216-28, 153.80 - 154.22 m
Fig. D.46.: Sample 201216-45, 201.26 - 201.76 m
157
Fig. D.47.: Sample 201216-52, 218.45 - 218.87 m
Fig. D.48.: Sample 201216-54, 219.50 - 219.92 m
158
Fig. D.49.: Sample 201216-72, 248.50 - 249 m
Fig. D.50.: Sample 201216-73, 249 - 249.45 m
159
Fig. D.51.: Sample 201216-81, 265.65 - 266.13 m
160
Samples of borehole 201217
161
Fig. D.52.: Sample 201217-9, 89.90 - 90.30 m
Fig. D.53.: Sample 201217-18, 99.50 - 100.01 m
162
Fig. D.54.: Sample 201217-25, 105.05 - 105.60 m
Fig. D.55.: Sample 201217-38, 117.99 - 118.50 m
163
Fig. D.56.: Sample 201217-43, 126.43 - 126.87 m
Fig. D.57.: Sample 201217-55, 151.62 - 152.18 m
164
Fig. D.58.: Sample 201217-70, 195.60 - 196.10 m
Fig. D.59.: Sample 201217-73, 199.60 - 200.08 m
165
Fig. D.60.: Sample 201217-84, 213.20 - 213.70 m
Fig. D.61.: Sample 201217-98, 228.50 - 228.99 m
166
E. Mineralogical Report
167
F. Lithologs
180

Sample Investigation of 201216 and 201217 Boreholes in the Frame of

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