PHILLIPSKOP YIELD TEST ANALYSIS

Transcription

PHILLIPSKOP YIELD TEST ANALYSIS
Phillipskop Yield Test
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36 Edna Street
Hillcrest
Kimberley
8301
20 June 2016
PHILLIPSKOP YIELD TEST ANALYSIS
1. Introduction
The owner of the farm Phillipskop, near Stanford in the Western Cape Province, plans a tourist
development on his property. The expected water demand of the proposed development is 18
3
m /d, which will be sourced from a borehole on the property.
During June 2016 the owner conducted a yield test on the proposed production borehole. This
test was supervised by Mr Martin de Klerk of Cape Geophysics, who has also sited the borehole.
The undersigned was approached to analyse the yield test data and calculate a sustainable yield
for the borehole. This analysis was completed with data supplied by the client. Although the
author has exercised due care in reviewing the supplied information and compared it with
expected values, the accuracy of the results and conclusions from the review are entirely reliant
on the accuracy and completeness of the available data. Therefore the author does not accept
responsibility for any errors or omissions in the supplied information and does not accept any
consequential liability arising from commercial decisions or actions resulting from them.
The following information was supplied for the borehole:
Main water strike
Rest water level
Artesian flow
Air lift yield
= 84 metres below ground level (mbgl)
= 0.00 mbgl (artesian)
< 0.1 L/s
= 3.0 L/s
2. Yield Testing
The borehole was yield tested for 48 hours at a constant discharge rate (constant discharge test CDT) of 0.53 L/s. The final drawdown at the end of the test was 3.48 m. Full water level recovery
was reached after 48 hours. No step drawdown test (SDT) was performed on the borehole. The
yield test data were analysed by means of various methods such the FC, Theis and CooperJacob methods and a 24 hr sustainable pumping schedule for the borehole determined, as well
as the optimum pump depth and maximum allowable pump drawdown level. An available
drawdown of 17 m was used as indicated by the FC-Method although the distance from the rest
water level to the main water strike is 84 m. This means the analysis is very conservative and the
actual long term sustainable yield of the borehole is likely higher.
The results of the yield test analyses are summarised in
Table 1. A recharge value of 6.25 mm/a was used. No inflection point was reached during the
test and therefore this method yielded irrational values. As a result it was excluded from the
mean value.
Table 1 indicates that the average sustainable yield, as determined by the FC-method, of this
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borehole is 0.85 L/s at continuous pumping, or 73.4 m /d. The FC-Method further calculates a T2
-4
value ranging between 7.2 and 10.6 m /d whilst the S-value ranges between 3.9 x 10 and 4.23 x
-3
10 . Both the fractal dimension and log derivative values (2.18 and 0.1 respectively) indicate a
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good fracture network with radial flow. Therefore, there is no preferred direction of groundwater
flow and the aquifer behaves similar to a primary aquifer.
Table 1: Results of FC-Method Yield Test Analyses
The Recovery method was also used to determine the long term sustainable yield. In this case
the Recovery method will yield a conservative sustainable yield due to the limited drawdown
during the yield test. The recovery analysis is indicated in Table 2 which indicates a long term
3
sustainable yield of 0.27 L/s at continuous pumping, or 23 m /d. This can be regarded as the
worst case scenario and the long term sustainable yield calculated by the FC-Method is likely
more relevant. Therefore the sustainable yield calculated by this method is preferred.
Table 2: Results of Recovery-Method Yield Test Analyses
Recovery Method
Pumped Time (min)
Average Yield (L/s)
2880
0.53
Abstracted Volume (m3)
Recovery Time (min)
91.58
2880
Sustainable Yield (m3/d)
Sustainable Yield (L/s @ 24h/d)
22.90
0.27
The raw yield test data as well as the FC-Analyses are included in Appendix 1 at the back of this
report. No chemical analysis of the groundwater was supplied and therefore the groundwater
chemistry is not discussed in this report.
3. Recommendations
Based on the results and discussions the following is recommended for the Phillipskop borehole:
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1. The borehole can be utilized as a production borehole and should be equipped and
managed as indicated in
2.
3. Table 3 over page:
Table 3: Recommended Operation of Phillipskop Borehole
Bh No
PK1
Groundwater
Level
(mbgl)
Pump
Intake
(mbgl)
Max
Groundwater
Level
(mbgl)
0.00
25
17
Sustainable Yield
Comments
L/s
24h/d
0.85
3
m /d
73.4
Yield test conducted at low discharge rate
4. The borehole must be properly sealed at the surface to prevent surface pollution of
the groundwater. This measure will also prevent bees from invading boreholes.
5. The borehole must be equipped with functioning volume meter and allowance must
be made for access to measure groundwater levels. For this purpose, class 6
irrigation pipe can be attached to the rising pipes of the pump. This conduit pipe must
be installed to at least 1 m above the pump inlet and the bottom end has to be
partially blocked to guarantee that the probe of the dipmeter cannot be lowered below
this point where it can get stuck.
6. Rainfall must be recorded on a daily basis at a fixed time every day at a central
locality.
7. Rest groundwater levels (pumps to have been non-operative for a few hours at least)
as well as the volume meter readings of the borehole must be collected at least
monthly, but preferably on a weekly basis. Alternatively, boreholes can be equipped
with electronic water level loggers which can be programmed to record groundwater
levels at pre-selected intervals, but this is likely an over kill for the relative small
volumes that will be abstracted.
8. Groundwater samples must be collected at the borehole on a bi-annual basis and
send to a SANAS accredited laboratory for macro-chemical and micro-biological
analyses (if used for human consumption).
9. In order to evaluate the aquifer behaviour and to make timeously adjustments to the
abstraction rates, if necessary, all relevant monitoring data collected at production
and observation boreholes must be analysed by a SACNASP registered
hydrogeologist on at least a yearly basis.
10. In order to safeguard the groundwater supplies from contamination and equipment
from theft and damage, two zones of protection must be established around each
production borehole:
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Inner protection Zone
The inner protection zone is an area of at least 50m x 50m, centred on the actual borehole.
The following measures must be applied in this protection zone:
•
No pit latrines, VIPs, soak-aways or septic tanks – to prevent effluent from percolating
into the aquifer and borehole;
•
No storage of fuel, lubricants or other hazardous substances without a leak proof
bund;
•
Production boreholes for domestic use must be equipped with a sanitary seal – to
prevent contaminated surface water and spilled fuel from percolating down the casing
into the borehole;
•
The concrete collar around borehole casing must be at least 100mm higher than the
floor or surface level to prevent spilled fuel, water from leakages, wash water, etc.
entering the borehole;
•
No ponding of surface water must be allowed, i.e. the area must be sloped for surface
water to drain away from this zone;
•
Vegetation, other than trees and large bushes, should be maintained in this zone –
Note: Roots of bushes and trees growing near boreholes often grows into the
borehole where it can cause considerable problems;
•
The borehole and pumping equipment must be housed in a lockable pump house.
For this purpose, a removable cage manufactured out of galvanised steel mesh and
corrugated steel sheets is recommended. This cage, rather than a brick building, is
recommended as it can be readily removed in case the borehole is damaged or if it
needs to be re-developed and cleaned.
•
The production boreholes, as well as other monitoring boreholes in the area, must be
properly sealed to prevent entry of reptiles, insects, birds and small rodents.
•
The entire area should be properly fenced with a lockable gate to prevent
unauthorised entry and to exclude animals. The
gate must be positioned and of such a type that
allows easy vehicle access.
•
A signboard must be erected on the gate warning
people of the dangers and that unauthorised entry
is not allowed.
Outer protection Zone
The outer protection zone should cover an area of at least
500 m x 500 m and the following measures should be
applied within this zone:
•
No water-borne sewerage, soak-aways or new pit latrines;
•
No new stock watering points or pens;
•
No abattoirs and other hazardous industries such as workshops, metal plating
factories, petrol filling stations, etc;
•
No cemeteries or disposal of solid waste or sewage;
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•
Existing pit latrines and septic tanks within 100 m of the borehole must be properly
sealed;
•
If possible, no new housing or industry developments should be allowed in this zone.
I trust that this report effectively addresses the long term sustainable yield of the tested
borehole. However, should you need further clarity on any aspect of this report please do not
hesitate to contact the undersigned.
Yours sincerely,
C J Esterhuyse (Pr. Sci. Nat.)
Principal Hydrogeologist
Tel:
+27 (0) 53 861 5798
Moblie: +27 (0) 82 876 1961
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Appendix 1: Yield Test Data and FC-Analysis for
Phillipskop Borehole
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Yield Test Data:
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FC Analysis
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Derivative and Diagnostic Plots
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Cooper Jacob Method
Barker Method
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