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Mahlknecht et al., 2006, Env. Geol. (author’s version) Intrinsic aquifer vulnerability assessment: validation by
environmental tracers in San Miguel de Allende, Mexico
Jürgen Mahlknecht a,1, Ma. Guadalupe Medina-Mejía b, Jamie Gárfias-Solis c, Irene
Cano-Aguilera d
a
Centro de Estudios del Agua, Instituto Tecnológico y de Estudios Superiores de Monterrey, Ave. Eugenio Garza Sada No.
2501, C.P. 64849, Monterrey, Nuevo León, Mexico, Tel/Fax: +52 81 81 58 22 61, Email: [email protected]
b
Centro de Investigaciones en Química Inorgánica, Universidad de Guanajuato, Guanajuato, Mexico
c
Centro Interamericano de Investigaciones de Recursos del Agua, Universidad Autónoma del Estado de México, Toluca,
Edo. de México, Mexico
d
Facultad de Química, Universidad de Guanajuato, Guanajuato, Mexico
This is the author’s version prior publishing of the paper: Mahlknecht J., Medina-Mejía M. G., Gárfias-Solis J., Cano-Aguilera I. (2006) Intrinsic aquifer
vulnerability assessment: validation by environmental tracers in San Miguel de Allende, Mexico.
Environmental Geology, vol. 51(3), p. 477-491, Springer-Verlag Heidelberg ISSN: 0943-0105
http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s00254-006-0344-8
Abstract
Vulnerability maps are important tools for water decision makers and land-use planners for
protection of aquifers against contamination. The vulnerability map, according to the parametric
method SINTACX for assessing intrinsic aquifer vulnerability, was validated in a case study with
chlorofluorocarbon tracer technologies (CFC-11, CFC-12, and CFC-113) of groundwater. The
tested area was the 1,295 km2 volcano-sedimentary area of San Miguel de Allende, Mexico. From
the results of this area, it appears that the vulnerability map is in parts inconsistent with the
underlying groundwater flow system. Thus, the vulnerability map was corrected with tracer
information. The validated vulnerability map indicates that the degree of vulnerability varies from low
1
Corresponding author
1 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) to moderate-high. Low vulnerability values are found in the graben extending from north to south
along the San Miguel de Allende fault system and high values in recharge areas southeast and
northwest of the study area. The investigation is a demonstration that the scientific reliability of
results of the parametric method can be improved by validation with tracer techniques representing
the groundwater dynamics. The flexible structure of SINTACX allows revising and adjusting scores
and weights of the parameter maps to rebuild a new vulnerability map consistent with the
hydrological system.
Keywords
Aquifer vulnerability · SINTACX · CFCs · Semi-arid regions · Guanajuato ·
Mexico
Introduction
The assessment of groundwater resources vulnerability has become an important tool for
water resources decision makers and land-use planners to protect aquifers from
contamination. In Latin America, the most widely used methods for the assessment of
intrinsic vulnerability - i.e. the vulnerability which takes into account the geological,
hydrological and hydrogeological characteristics of an area but is independent of the nature
of the contaminants and the contamination scenario - are parametric (or rating-and-overlay)
methods, such as DRASTIC (Aller et al. 1985), GOD (Foster 1987), AVI (Van Stempvoort
et al. 1995), and SINTACX (also: SINTACS) (Civita 1994). Parametric methods assign
numerical scores (ratings) directly to various physical attributes (e.g. the vadose zone
thickness rating is based on the principle that the thicker the vadose zone, the lower the
vulnerability) to build parameter or base maps. These parameter maps are then combined in
an overlay process to develop a vulnerability map with a range of vulnerability categories.
Some parametric methods are more complex (e.g. SINTACX, DRASTIC) because they use
more parameters than others (e.g. AVI, GOD). In contrast to groundwater flow models and
statistical methods, the parametric methods are a popular approach to groundwater
vulnerability assessments because they are relatively inexpensive, straightforward, use data
that are commonly available or estimated, and produce an end product that is easily
interpreted and incorporated into the decision-making process (Focazio et al. 2002). The
main drawbacks of these methods are: a) parameter ratings may vary within a wide range,
2 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) b) the weighting of the parameter maps is qualitative, with no physical basis; and c) the
categorization of vulnerability classes is subjective. The subjectivity of these methods may
strongly affect the results and make results from different methods incomparable. A way to
deal with this is performing a sensitivity analysis where the single parameter influence is
evaluated on the aquifer vulnerability (Lodwik et al. 1990; Napolitano and Fabbri 1996).
However, according to Gogu and Dassargues (2000) regarding future trends in vulnerability
assessment, integrated tools are expected and according to needed parameters data are to be
collected and better quantified. Goldscheider et al. (2001) used artificial tracers and Perrin
et al. (2004) applied artificial and natural tracers at catchment scale to test the adequacy of
the subsystems conceptual model used by EPIK method developed by Doerfliger et al.
(1999) for karst systems.
The SINTACX method (Civita 1994), based on the popular DRASTIC (Aller at al. 1985)
approach, is proposed for the vulnerability assessment of the San Miguel de Allende area in
central Mexico. It has been shown by different authors (e.g. Gogu and Dassargues 2000;
Lobo-Ferreira and Oliveira 2004) that DRASTIC and SINTACX results are similar, due to
the fact that SINTACX uses the same seven parameters like DRASTIC. The advantage of
the SINTACX system over DRASTIC is that it is more flexible: it provides four weight
classifications but it also allows the creation of new ones (Civita 1994; Gogu and
Dassargues 2000) which is useful in the present vulnerability assessment. SINTACX
subdivides the catchment area into a regular square grid raster, and calculates the
vulnerability parametric Iv for each grid from seven previously quantified parameters:
Iv = ΣP(1,7) · W(1,n)
Iv
= SINTACX vulnerability parametric
P(1,7)
= the rating of each of the seven parameters used: depth-to-groundwater (S), net
recharge (I), vadose zone (N), soil media (T), aquifer media (A), hydraulic
conductivity (C), topography (X)
W(1,n) = the associated weight
n
= the number of weight classification arrays
3 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) On the other hand, chlorofluorocarbon (CFC) tracers offer an innovative alternative for
assessment of contamination potential of groundwater resources. The potential of CFCs as
age-dating tools and transport tracers of natural waters was detected already in the 1970s in
hydrological (Thompson and Hayes 1979) and oceanographic studies (Lovelock et al.
1973). The measurement of CFCs has become routine in the 1980s (Bullister and Weiss
1988). The use of these environmental tracers in groundwater studies are reported in
Busenberg and Plummer (1992). Since the increase of atmospheric concentration of CFCs
during the last decades due to industrial development is well reported (Plummer and
Busenberg 1999), and their dissolution in recharging water is proportional to atmospheric
concentration at the moment of infiltration, these conservative gases are considered
comfortable residence time indicators for modern water. The vulnerability assessment with
CFCs is based on the principle that the more modern is groundwater, the more susceptible
to groundwater contamination and vice versa. In this context, the intrinsic vulnerability can
be assessed by the transit time of infiltrated water from the surface to the point of
discharge.
This study is a contribution to the understanding of vulnerability. The parametric method
SINTACX is applied in a volcano-sedimentary catchment; its results are critically
reviewed; and it is demonstrated how the validation approach of parametric methods with
chemical constituents and modern water tracer technologies is increasing the usefulness of
results.
Study area
General description and hydrology
San Miguel de Allende (SMA), in the Central Mexican Altiplano (Fig. 1), is an
intermountain sedimentary area in a semi-arid region with an extension of about 1,295 km2.
It is part of the 6,840 km2 rural hydrological Alto Rio Laja or Independence Basin
(Mahlknecht et al. 2004a). The annual groundwater extraction in SMA is 135.6 million m3
4 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) (UGTO 2004). Eighty-three percent of the extracted volume is used for irrigation purposes
(corn, chili, broccoli, beans). According to the Instituto Nacional de Estadística, Geografía
e Informática (INEGI 1999), the SMA municipality counted 70,124 inhabitants in 1999.
Fig. 2 shows the land-use map for the SMA area: 42.6% of the area is irrigation agriculture,
31.6% woodland, 21.8% pasture, 2.0% erosion and 2.0 % surface water bodies. The
intensive groundwater extraction for irrigation purposes provoked significant water quality
deterioration during the last tens of years. Although in most parts of the study area, the
water quality meets the Mexican maximum admissible concentration (MAC) for drinking
water (SSA 2000), the SMA urban area faces tendencies of increasing nitrate contents and
coliformes in groundwater (Mahlknecht et al. 2004c). This fact gave rise to a major concern
about the susceptibility to contamination of the aquifer below the urbanized area of SMA.
The demand for safe drinking water and healthy ecosystems has led to the assessment of
water resources in touristic San Miguel de Allende.
Figure 1: A) Location of the study area. B) Topographic relief and important features
from the region. GTO = Guanajuato City, DH = Dolores Hidalgo, SMA = San Miguel de
Allende, SMOc = Sierra Madre Occidental, SMOr = Sierra Madre Oriental, TMVB =
Transmexican Volcano Belt, MC = Mesa Central.
5 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 2: Land use of the SMA area according to the Instituto Nacional de Estadística,
Geografía e Informática.
The climate is semiarid with the rainy season from May to October and the dry season
during the months of November to April. The mean annual temperature is 17ºC. The
minimum elevation is 1,900 m and the maximum is 2,600 m above sea level (m a.s.l.). The
study area represents a plain surrounded by the Sierra de Guanajuato in the western part,
the Palo Huérfano and La Joya volcanoes in the south and southeast, the Sierra de Cuarzos
at the northeast, and the plains of Laguna Seca at the north (Fig. 1).
Geology and hydrogeology
The SMA area is at the border of three geological provinces: the Sierra Madre Occidental
(SMOc), the Sierra Madre Oriental (SMOr) and the Transmexican Volcano Belt (TMVB;
Fig. 1). Four different tectonic deformation events are documented between the Oligocene
6 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) and Pliocene-Recent. The fault pattern generated during each of the post-Eocene events
were controlled by three major fault systems that correspond to structural boundaries of
major geological provinces: a) El Bajío fault separates the Mesa Central from El Bajío
Basin, La Joya and Ixtla faults separate the Mesa Central from the TMVB, and the TaxcoSMA fault system separates the Mesa Central from the SMOr (Alaniz-Álvarez et al. 2001).
The SMA fault (Fig. 3) is part of the Taxco-SMA system.
Figure 3: Surficial geology (modified from Alaníz-Álvarez et al. 2001), piezometry (m
a.s.l) and groundwater flow direction (arrows) of the SMA area. Explanation: ArCg =
Cenozoic sandstones and conglomerates, TmJ = La Joya andesites and basalts from
Miocene; Tig = Rhyolitic ignimbrites from Oligo-Miocene; ToA = El Cedro andesites
from Oligocene; KiCa = Mesozoic limestones and sandstones; Kvs = Mesozoic
volcanosediments; SMA fault = San Miguel de Allende fault.
The oldest outcropping rock formations date back to the Mesozoic Era. This unit is
considered as the basement of the regional aquifer and is in the study area at a maximum
depth of more than 600 m. This unit is covered by fractured geological formations (basaltic
to andesitic lavas, acid ignimbrites, tuff deposits, andesitic-dacitic spills and lahar/breccia
deposits) with a maximum thickness of about 400 m, followed by granular-fractured
aquifer material (conglomerates, sandstones) intercalated with alluvial sediments, and
7 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) granular material (alluvial and fluvial sediments) (Figs. 3 and 4) (Flores-Orozco 2004).
Figure 4: Schematic vertical W-E section of the study area (location indicated in
Figure 3). Geometry of rock formations modified from Flores-Orozco (2004).
The present study refers to the uppermost unit. Spatially, the aquifer is heterogeneous with
a range of hydraulic conductivities between 5·10-3 and 10-5 m/s, as inferred from pumping
test data. The aquifer conditions for the granular-fractured material are unconfined to semiconfined (UGTO 2004).
SMA represents the southernmost area and outlet of the Alto Rio Laja or so-called
Independence Basin and is considered important due to its high capacity of recharge
formation (Mahlknecht et al. 2004b). The regional recharge zones are in the eastern (flanks
of the Sierra de Guanajuato), southern (Palo Huerfano and La Joya volcanoes) of the study
area. Minor recharge zones are also found at elevations in the northeast of Coral de Piedras
de Arriba locality (Figs 1 and 3). The recharge rate ranges between 37 (plains) and 513
8 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) mm/year (mountainous zones) (UGTO 2004). The main river - Rio Laja - with its northsouth direction is intermittent and discharges into the Allende dam at the southern limit of
the basin (Fig. 2). Groundwater levels at the border of Rio Laja and Allende dam indicate
that the depth to groundwater along these surface waters within the study area is
approximately 70 and 50 m, respectively. The area in the north of the Allende dam is
considered as discharge area given the heavy groundwater exploitation by irrigation wells
(Fig. 3).
Materials and methods
Parameter estimation for parametric methods
The quantification of the vulnerability parameters in the SMA area is based on different
data sources: the depth-to-water parameter S was computed using the difference of the
potentiometric level from control wells and the DEM, and interpolating point data by the
kriging method; the net-recharge parameter I was calculated using the chloride mass
balance method applied in Mahlknecht et al. (2004b); the vadose-zone parameter N was
assessed using geological/geophysical logs and mineralogical/petrographic studies (FloresOrozco 2004; Medina-Mejía 2005); for evaluation of soil media parameter T, the 1:250,000
edaphological and land use maps of INEGI (1990) were used and improved by using
LANDSAT TM+ satellite imagery and field checks (UGTO 2004); data for evaluating the
aquifer media parameter A were obtained from previous geological and hydrogeological
studies (COREMI 1999; CEASG 1999; Alaniz-Álvarez et al. 2001; Flores-Orozco 2004;
UGTO 2004); the hydraulic conductivity parameter C was calculated from aquifer test data,
conducted in the hydrogeological studies of CEASG (1999) and UGTO (2004); the
topography layer parameter X is calculated using a DEM based on 1:50,000 INEGI
topography maps. To create the vulnerability map, the seven parameters were weighted
according to Civita and De Maio (2001), i.e. 5(S)-4(I)-5(N)-4(T)-3(A)-3(C)-2(X) for
untilled areas with unsaturated matrix permeability; 5(S)-5(I)-4(N)-5(T)-3(A)-2(C)-2(X)
for areas with potential anthropogenic impact; and 3(S)-3(I)-3(N)-4(T)-4(A)-5(C)-4(X) for
fractured rock aquifers. The vulnerability assessment procedure is shown in Fig. 5.
9 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 5: Development scheme of vulnerability map using ArvView GIS (ESRI®).
INEGI=Instituto Nacional de Estadística, Geografía e Informática, Comisión Estatal del
Agua de Guanajuato, CNA = Comisión Nacional del Agua; UGTO = Universidad de
Guanajuato; CODEREG = Consejo de Desarrollo Regional de Guanajuato; COREMI =
Consejo de Recursos Minerales
Field and laboratory techniques
Water samples from 15 water wells in the SMA area were collected during June 2003 to
determine the content with respect to modern water tracers 3H, CFC-11, CFC-12, and CFC113, and chemical elements (major and trace element As). The oxygen concentration, water
temperature, electrical conductivity, alkalinity and pH were field-measured. The
10 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) potentiometric head, well completion depth and well screen were also reported. These
samples were completed with 10 samples from water wells taken during May 2002 (UGTO
2002). Four sampling sites coincide in both sampling campaigns (Tables 1 and 2).
Prior to sampling, the water well and all fittings were flushed with groundwater of at least
three times the well volume. All samples were taken directly at the wellhead. The anion and
cation samples were filtered (0.45mm) and stored with tri-distillate water, in new pre-rinsed
HDPE bottles at a constant temperature of 4°C. Additionally, cation samples were acidified
to a pH<2 by ultrapure nitric acid. The cations and anions were determined at the Instituto
Nacional de Investigaciones Forestales, Agrícolas y Pecuarias in Celaya, Mexico, using
inductive-coupled plasma optical emission spectrometry (cations), ion chromatography,
selective ion electrode method (fluorine), argentometric titration method (chloride), and
turbimetric method (sulfate). Samples with a charge balance error above 5% were
eliminated from the dataset.
The tritium analyses were performed by enriched liquid scintillation counting at the
Environmental Isotope Laboratory of University of Waterloo, Canada, and the results are
expressed in tritium units (T.U.).
A special 500-ml glass bottle (NS 24) with glass fittings was used for CFC sampling. To
restrict air contamination and to remove all gas bubbles, the glass bottle was filled directly
using a copper tube, and flushed about ten times its volume. For secure transport, the
tightly tapped bottle was placed into a lamina box which itself was filled with sample water
and sealed. The analyses were performed at Spurenstofflabor, Wachenheim, Germany, by
gas chromatography with electron-capture detector after pre-concentration using a purgeand-trap technique (Oster et al. 1996). The results are reported in pmol l-1.
Interpretation of CFCs
For the interpretation of groundwater residence time, an exponential model was initially
applied (Maloszewski 1996). This model assumes an ideally exponential distribution of the
residence time from the site of recharge to the withdrawal at the water production well. The
11 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) apparent age corresponds to the mean residence time in the groundwater reservoir:
t
cout (t ) =
∫c
in
−∞
1
⎡ t − t'⎤
(t ') ⋅ exp ⎢−
dt '
τ
⎣ τ ⎥⎦
where t is the time, cin/out the input/output concentration, 1/τ the first-order or decay
constant, and t - t’ the mean residence time. The mean residence time was determined
independently for each tracer, CFC-11, CFC-12 and CFC-113, respectively. Values in the
proximity of the minimum detection level are not considered because their analytical error
is relatively high.
The screen length of wells corresponds in most cases the difference between the depth to
water table and the well depth (Table 1). Isotopic data and well characteristics indicate that
groundwater of different residence time mixes within the wells. Thus, a simple mixing
model was applied alternatively to the exponential model, where the existence of a mixture
between a modern (<10 years) and an old (> 50 years) component leads to the measured
concentration. With the help of this model, it was possible to calculate the percentage of
modern water in samples.
The equilibrium concentration of the CFC tracers in water was obtained by using Darcy´s
law constant according to Warner and Weiss (1985) and a barometric correction factor. The
recharge temperature is 17° C (i.e. the annual average air temperature) and the infiltration
height 2,100 m and 2,200 m a.s.l., respectively. According to the equilibrium calculation,
the modern water composition is 1.5 pmol l-1, 3.2 pmol l-1, and 0.28 pmol l-1, for CFC-11,
CFC-12, and CFC-113, respectively.
12 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Table 1: Physical characteristics of the sampled water wells, 3H, 2H and CFC data for
the SMA area.
Sample
Sampling
Altit.
Well
Well
Depth to
3
No.
date
(masl)
type
depth (m)
water (m)
(TU)
7-May-02
9-May-02
9-May-02
9-May-02
10-May-02
10-May-02
10-May-02
10-May-02
11-May-02
11-May-02
08-Jun-03
08-Jun-03
08-Jun-03
08-Jun-03
08-Jun-03
08-Jun-03
09-Jun-03
09-Jun-03
09-Jun-03
09-Jun-03
08-Jun-03
10-Jun-03
10-Jun-03
10-Jun-03
12-Jun-03
1887
2186
1912
2076
1940
2110
2052
2066
1860
1914
2082
2019
2088
1875
1887
2014
2027
2076
2065
1860
1902
2072
2104
2066
2125
open
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
cased
spring
16
nd
100
300
250
200 (?)
150 (?)
100
200 (?)
250
215
300
nd
140
16
nd
250
300
200
100
200
300
200 (?)
200 (?)
0
14
105
40
95
80
85
109
66
107
68
107
95
106
36
14
123
110
95
111
66
60
114
85
107
0
<0.6
<0.6
<0.6
<0.6
-<0.6
<0.6
<0.6
<0.6
<0.6
<0.8
<0.8
<0.8
<0.8
<0.8
<0.8
<0.8
<0.8
20
<0.8
--<0.8
<0.8
1.4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Note:
ID
1
RL-16
RL-20
RL-31
RL-382
RL-39
RL-41
RL-43
RL-404
RL-63
SMA-6
SMA-19
SMA-22
SMA-23
SMA-18
SMA-11
SMA-21
SMA-27
SMA-32
SMA-14
SMA-36
SMA-31
SMA-29
SMA-23
SM-94
SMA-37
1,2,3,4
H
±
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.5
0.5
0.5
0.5
0.5
0.5
0.6
0.5
1.5
0.5
0.4
0.5
0.4
CFC-12
(pmol l-1)
0.90
0.16
0.29
0.06
-0.19
0.25
0.19
0.53
0.15
0.50
0.06
0.24
0.04
0.80
0.80
1.00
-0.21
-0.45
0.15
0.60
-1.20
±
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.10
0.05
0.10
0.05
0.05
0.05
0.10
0.10
0.10
0.05
0.05
0.05
0.10
0.10
CFC-11
(pmol l-1)
1.40
0.47
0.33
0.07
-0.23
0.3
0.22
0.65
0.88
0.60
0.09
0.34
0.09
1.30
1.00
1.50
-0.32
-2.80
0.90
1.00
-2.00
±
0.20
0.05
0.05
0.05
0.05
0.05
0.05
0.1
0.1
0.10
0.05
0.05
0.05
0.20
0.10
0.20
0.05
0.30
0.10
0.10
0.20
CFC-113
(pmol l-1)
0.15
0.03
0.06
0.01
-0.04
0.05
0.04
0.06
0.02
0.05
0.01
0.04
<0.01
0.12
0.17
0.14
-0.03
-<0.01
0.02
0.1
-0.18
±
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
corresponding boreholes
Results and discussion
Groundwater chemistry
Groundwater of the SMA area is relatively low in salinity and pH neutral (6.3-8.2). The
temperature varies between 21.5 and 38.5 ºC; many of the water production wells present
mixture with thermal water. The groundwater type is between Ca-Na-HCO3 and Na-CaHCO3 with local dominance of Mg at the southern part. The major ion chemistry of
groundwater in SMA is summarized on a Piper diagram in Fig. 6. Numerical geochemical
models indicate that the major groundwater chemical processes are: 1) interaction of deep
thermal water with silicates at the lower end of the flow lines; 2) natural and irrigational
induced evapotranspiration, 3) carbonate dissolution, and 4) dissolution of Na, Ca and K
13 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) silicates (Mahlknecht 2003; Mahlknecht et al. 2004a).
Figure 6: Piper diagram of the chemical samples collected in the study area.
Chemical constituents of groundwater may be used to indicate groundwater flow system
conditions. Fig. 7 shows the distribution of selected elements throughout the aquifer in the
study area. The distribution of Cl throughout the study area demonstrates elevated
concentrations up to 28.0 mg l-1 in the basin center (north and west of Allende dam).
Moderate concentrations predominate east from the SMA fault and low concentrations west
from the SMA fault. According to a regional study, groundwater is of meteoric origin
(Mahlknecht et al. 2004b) and under the assumption that no other chloride salinization
effect exists, it may be used as residence time indicator. In this case, the higher Cl contents
indicate older groundwater, with the exception of the urban area. The higher Cl
concentration in the urban area of SMA indicate rather surface influence as elevated NO3-N
contents (Fig. 7) and shallower groundwater levels indicate.
14 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 7: Distribution of Cl, NO3-N, F and As concentration in groundwater of SMA.
Nitrate is ascribed to anthropogenic inputs. The concentration of NO3-N ranges between
0.01 and 11.5 mg l-1. Elevated contents are found in the eastern part of the study area and in
the SMA urban area (Fig. 7). The elevated NO3-N concentrations in the eastern area
coincide with areas of intensive farming practices (fertilizers and slurry application), which
should be the likely source of nitrate. The elevated NO3-N concentration in the urban area
of SMA is probably due to infiltration of waste water from leaky sewers. The western area
is characterized by mostly uncultivated lands and pastures (Fig. 2) with associated lower
NO3-N concentrations. The diverse soil use practices are probably the reason that no unique
relationship between residence time and NO3-N is found for the study area. Therefore, it is
rather difficult to use nitrate as a vulnerability indicator.
The maximum F concentration is 5.9 mg l-1, which corresponds as four times the Mexican
maximum admissible concentration (MAC) for drinking water (SSA 2000) (1.5 mg l-1).
15 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) The geographical distribution indicates that the elevated F contents are north and south of
the Allende dam (Fig. 7). Anthropogenic F inputs from the surface may be excluded
because of the rural character of the area, the water table configuration suggests that the
water with elevated F contents is relatively evolved. The fact that dissolved F is positively
associated with Na, Cl and water temperature, supports the hypothesis that the origin of the
elevated F concentration is geological. The dissolution of F-rich rhyolite rocks or volcanic
ashes with water of meteoric origin, the exchange of Na for Ca at the surfaces of rhyolitic
clays and metallic oxides, and the increase of the temperature along with the flow path,
results in a decrease of dissolved Ca and increase of F along the flow path (Mahlknecht et
al. 2004a).
The maximum As concentration in groundwater is 16 µg l-1 which is below the Mexican
MAC (25 µg l-1); 20% of the sampled water is, however, higher in As than the permitted 10
µg l-1 from the WHO guideline for drinking water (WHO 1998). The elevated As
concentration is probably due to mixing with regional groundwater with a deep circulation
and the formation of Mg oxyhydroxides, evidenced by a significant positive correlation
between groundwater temperature and As, a significant negative correlation between Mg
and As, and mostly undersaturation with respect to the mineral Mg3(AsO4)2 according to
speciation/saturation models (Mahlknecht et al. 2004c).
Degree of vulnerability
Following the SINTACX vulnerability mapping protocol, the final map is computed as
weighted sum of the seven base maps. The final map in Fig. 8 uses a 200 m per 200 m
resolution and shows three classes of pollution potential: a) low, b) medium and c)
moderate to high. The low pollution potential covers 2.6%, the medium pollution potential
49.8%, and the moderate to high pollution potential 47.6% of the area extent. The moderate
to high pollution potential coincides mainly with recharge areas in the south and southwest.
The Sierra de Cuarzos, a Mesozoic formation and recharge zone at the east, reveals
surprisingly low vulnerability values. The map also shows spots of low vulnerability
southeast of Corral de Piedras de Arriba (Fig. 8).
16 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 8: Vulnerability map using weights according to Civita and De Maio (2001).
To know the parameter sensitivity, the map-removal sensitivity measure according to
Lodwik et al. (1990) was applied:
S xi =
Vi V xi
−
Ni
n
where:
Sxi = sensitivity of the ith grid associated with the removal of one parameter
Vi = final vulnerability of the ith grid
Ni = total number of parameters (7 maps)
Vxi = vulnerability for the ith grid without considering one parameter
n = number of parameters used in perturbed suitability (6 maps)
The results in Fig. 9 indicate that the influence on the final vulnerability index is
17 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) I>C>S>T=N=X>A. Flux parameters I (from 0.0-4.9; average Φ=2.3) and C (0.3-3.7;
Φ=2.1) exert an important influence on the vulnerability map. The unsaturated soil zone
parameters S (0.2-3.5; Φ=1.9), T (0.4-5.3; Φ=1.8), N (0.0-3.2; Φ=1.8) and topographic
parameter X (0.5-3.8; Φ=1.8) are also important, while the sensitivity to aquifer parameter
A (0.0-1.8; Φ=0.6) is insignificant. Spatial differences exist; e.g the influence of parameter
S on the overall vulnerability is lower at the western part of the study area than at the
eastern part.
Residence time and modern water percentage
Tritium is not detected (<0.6 T.U.) in all except two. The CFC-11 concentration varies
between 0.09 and 2.80 pmol l-1; CFC-12 between 0.04 and 1.20 pmol l-1; and CFC-113
between 0.01 an 0.18 pmol l-1 (Table 2). As expected for a mainly rural area, all samples
have lower concentrations than present-day atmospheric equilibration, with the exemption
of CFC-11 in sample 20 and 24. As there is no other CFC-11 source nearby, these samples
from rural areas indicate that the CFC-11 atmospheric concentration is probably slightly
higher than originally assumed. The CFC data suggest that groundwater within the SMA
area range from modern (<10 years) to old groundwater (>50 years). The apparent mean
residence time according to the exponential model approach varies between 7 and ~490
years (Table 3).
18 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 9: Parameter sensitivity according to the map removal technique (Lodwik et al.
1990).
19 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Table 2: Hydrochemical data for the SMA area. “--“ indicates that the parameter was
not measured.
Sample pH
Temp. Diss. Ox.
No.
(ºC)
(mg l-1)
(µS cm-1)
22.3
22.8
34.2
29.1
25.2
27.4
23.9
27.0
26.6
29.7
32.1
26.8
30.0
38.5
23.4
30.3
26.3
30.3
26.5
26.6
28.6
33.9
26.2
21.5
22.8
5.0
-1.5
2.4
5.7
2.2
1.9
--1.7
4.6
5.3
5.6
4.7
6.5
5.2
8.4
5.8
5.0
-7.1
3.2
4.7
6.4
1.1
505
120
470
423
745
432
439
463
532
726
459
506
446
426
514
430
512
420
434
532
423
376
428
573
292
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
7.4
6.3
7.3
7.4
7.3
7.5
7.4
7.2
7.2
7.3
7.4
7.4
7.5
7.5
7.7
7.3
7.9
7.8
7.6
7.2
7.5
7.7
7.6
8.0
8.2
SEC
Na
K
Mg
Ca
HCO3 SiO2
Cl
F
NO3-N
SO4
P
As
(mg l-1)
31.6
38.9
69.6
32.3
59.8
22.0
28.6
38.7
35.0
92.9
44.8
52.9
42.3
69.9
46.9
27.8
46.9
40.9
43.5
35.0
46.0
40.9
26.0
43.5
25.9
11.3
7.9
11.3
10.8
12.9
8.3
10.1
11.6
10.9
9.4
9.4
10.9
6.7
4.3
9.4
7.4
7.8
7.8
5.9
10.9
8.2
6.7
6.7
9.4
10.9
3.7
8.9
1.8
10.4
20.1
15.9
9.7
10.2
22.9
5.5
4.1
10.1
10.0
1.6
5.0
10.0
9.4
6.1
9.2
22.9
5.4
13.9
19.5
15.8
6.2
58.6
24.7
40.0
35.3
35.3
47.2
42.3
44.5
49.6
47.1
34.5
38.1
33.9
24.2
44.3
45.7
41.5
40.9
40.7
49.6
30.1
22.0
41.7
30.1
23.3
270
199
273
245
231
243
252
268
360
255
190
277
209
218
186
203
269
221
245
360
188
203
215
195
181
73.4
82.2
92.1
69.9
-84.7
75.5
80.9
57.2
--37.3
-38.9
-48.4
--46.3
27.5
48.8
52.2
--45.0
4.8
2.3
9.3
3.3
18.8
3.8
3.4
4.7
1.9
22.7
11.3
7.5
5.7
28.0
9.6
6.0
6.4
7.8
10.3
1.9
3.9
7.8
6.7
11.3
4.6
Charge
imbal. (%)
1.0
0.2
1.0
0.3
0.4
0.3
0.3
<0.1
1.0
2.1
1.1
0.5
0.5
<0.1
1.9
0.3
0.4
0.4
0.5
1.0
5.9
0.7
0.4
0.8
0.2
0.1
1.1
1.0
2.0
11.5
1.7
2.3
1.4
1.9
4.4
2.5
1.1
3.0
1.8
6.2
3.1
3.6
2.6
2.9
1.9
1.4
0.6
2.2
3.4
1.3
16.0
12.6
25.2
9.1
73.0
22.0
9.5
24.5
4.9
79.7
20.7
28.3
25.9
0.5
55.7
29.3
12.5
34.6
26.9
4.9
17.3
25.0
64.8
64.4
4.7
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
0.004
0.002
0.016
0.003
0.011
0.002
0.004
0.007
0.003
0.013
0.008
0.006
0.008
0.016
0.003
0.002
0.006
0.001
0.002
0.003
0.014
0.004
0.002
<0.001
0.004
-0.5
3.3
1.2
-0.5
3
2.8
-0.5
0.0
0.4
3.1
3.6
-0.6
3.7
1.2
2.4
4.0
1.8
-1.1
-0.9
0.4
3.4
1.0
-1.1
-0.2
-2.4
Table 3: Residence time and modern water percentage in groundwater samples from
the SMA area.
Sample Residence time
No.
(years)
1
15
2
150
3
130
4
360
5
200
6
90
7
200
8
50
9
150
10
44
11
390
12
94
13
490
14
16
15
24
16
12
18
105
20
44
21
155
22 a g e28 20 | P
24
7
Modern water in sample (%)
±
CFC-12
CFC-11
CFC-113
Average
Used Tracer
3
5
5
9
4
4
4
6
-6
10
1
110
3
3
2
4
--2
1
28
5
9
2
6
8
6
17
5
16
2
8
1
25
25
31
7
14
5
19
38
93
31
22
5
15
20
15
43
59
40
6
23
6
87
67
100
21
187
60
67
133
54
11
21
4
14
14
14
21
7
18
4
14
2
43
61
50
11
2
7
36
64
58.3
18
16
3
10.6
14
10.3
30
5
28
4
15
4
52
51
60
14
14
5
40
51
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11, CFC-113
CFC-12
CFC-12, CFC-11, CFC-113
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11
CFC-12, CFC-11, CFC-113
CFC-12, CFC-11, CFC-113
CFC-12, CFC-11, CFC-113
CFC-12, CFC-11
CFC-12
CFC-12
CFC-12, CFC-11, CFC-113
CFC-12, CFC-11
Mahlknecht et al., 2006, Env. Geol. (author’s version) The diffusive movement of the CFC tracers throughout the unsaturated zone may affect the
‘age’ when the water table is deep. Values from water samples taken from a deep water
table are rather uncertain and must be taken as qualitative information rather than
quantitative data. Cook and Solomon (1995) observed in the Surgeon Falls surficial aquifer,
Ontario, an error of 6-10 years for 45 m of aeration zone. The maximum observed depth to
water table in the SMA area is 123 m, which poses serious restrictions on the use of CFCs
as residence time estimation tool.
A more efficient way to assess the aquifer vulnerability is the modern water contribution
expressed in percentage, where groundwater with a high modern water contribution
corresponds to an elevated aquifer susceptibility/vulnerability, and low modern water
percentage represents a low aquifer susceptibility/vulnerability. A good estimate of the
modern water contribution is the average of the contents of all three CFC tracers, with the
exclusion of values in excess of 100% or values in the order of the minimum detection
limit. MacDonald et al. (2003) demonstrate, however, that in the absence of evidence of
reducing conditions - such as in this study area - the lower percentages are considered the
most representative ones. Our CFCs data reveal that, the lowest values are represented in all
samples by the CFC-12 tracer. The average modern water percentage ranges between 4 and
60, while the minimum modern water percentage varies between 1 and 38 (Table 3).
The distribution of the minimum modern water percentage in Fig. 9 indicates that
groundwater from the east (Sierra de Guanajuato massif) and the northwest (intermountain
elevations) contains a large component of modern groundwater, tending to lower
concentrations in the plain areas of the study area. According to the configuration, the
lowest aquifer susceptibility is found in a trough which extends along the east of the SMA
fault in the northern study area through the upper urban area of SMA and continues in a
southeast direction. The other area with a similar low degree of susceptibility can be
appreciated in the southeast of Corral de Piedras de Abajo. On the other extreme, the Sierra
de Guanajuato in the east and the area south of Los Rodriguez are highly susceptible to
contamination. This is quite consistent with the groundwater flow concept about the study
21 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) area (Fig. 3) described above. The recharge zones coincide with the areas above the 15%contour line, and areas below the 5%-contour line agree with local discharge areas.
Validation of vulnerability map
A comparison between vulnerability map (Fig. 8) and modern water contribution map (Fig.
10) shows that the patterns are, in general, consistent: moderate to high vulnerable areas
agree with groundwater of elevated modern water contribution, and areas with low
vulnerability are consistent with more evolved and less modern groundwater. A closer look
reveals that in the western part, the 15%-contour line matches the limit between medium to
moderate-high vulnerability. In the eastern part the association is less evident: apparently
the same vulnerability limit lies between the 5- and 10%-contour lines. With respect to the
lowest vulnerable zones in the south and east of SMA and in the southeast of Corral de
Piedras de Arriba, the association between vulnerability map and modern water distribution
is only approximate but the tendency is similar. The differences between both maps are
probably due to errors in the vulnerability conceptualization.
22 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) Figure 10: Contour lines of modern water contribution (in %) in samples of
groundwater. The contour lines were determined by the kriging method. The sampling
sites (with number) are also matched.
One of the major challenges of vulnerability mapping using parameter maps is the
compatibility of the results with the groundwater dynamic system. Inconsistencies between
the vulnerability map and the modern water contribution map are evident in the southeast of
Corral de Piedra: the vulnerability map indicates very low vulnerability while the
hydrological concept based on satellite image interpretation (UGTO 2004) and tracer data
suggests a vulnerable area (Figs. 8 and 10). In consequence, the vulnerability map was
revised and corrected until it matched the groundwater dynamic system. Each parameter
map was adjusted, and a new vulnerability map generated by changing the scores by
iterative processes such as illustrated in the scheme of Fig. 5. It revealed that the
contribution of parameter N and A of the mesozoic volcano-sediments to the vulnerability
have been underevaluated. The areas where these geological formations crop out are
23 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) recognized as recharge areas. A field revision shows that the fissuring of these units was
not sufficiently acknowledged. The final vulnerability map (Fig. 11) was obtained after
matching satisfactorily the modern water percentage pattern. By this means, scores from
uncertain and less constrained data - i.e. the influence of the unsaturated and aquifer media
on vulnerability - may be identified and corrected.
Figure 11: Validated vulnerability map after correction with tracer information.
It is important to define to what extent the correction of the scores changes the original
vulnerability values, to know how significant the results are. It shows that in areas where
the correction applies (mesozoic vulcanosediments), the average increase of the score was
30, which represents 33% of the original average score (92). This increase of the score is
important and practically sufficient to provoke the change from a “low” to a “medium”
pollution potential class. At this point, the question arises if these results are case specific
(not sufficiently acknowledged fissuring of geological formations) or if the SINTACX
method demonstrates deficiency with respect to the parameters N and A. It is no doubt,
when the SINTACX method was developed in the 1990s, the authors tried to improve or
24 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) make some of the parameter scores more sensitive to the physical framework than in the
existing DRASTIC method. SINTACX was proven and extensively applied in Italy (Civita
and De Maio 2001) and other countries during the last decade. However, the results have
never been proven against environmental or artificial tracers in a case study. The results of
this case are ambiguous: the general correlation between the vulnerability classes and tracer
results is significant, however, at a local level some exceptions and deviations are observed.
Taking in account the limitations of the tracer methods, the weak correlation could indicate
that the SINTACX method needs a further revision. The results have to be confirmed by
additional investigations in other test areas. These investigations should also consider the
possiblity of using other parametric methods that are less prescriptive and less complex.
Conclusion
The assessment of the 1,295 km2 volcano-sedimentary area of San Miguel de Allende area
reveals that its aquifer vulnerability is low to moderate-high. Low vulnerability values are
found in the graben extending from north to south along San Miguel de Allende fault
system; and the high values in regional recharge areas southeast and west of the study area.
The general good agreement of the vulnerability map with the hydrological system
indicates that the methods used for generation of parameter maps are quite useful (e.g.
SINTACX method, recharge map). Some inconsistencies are found in the western area
southeast from Corral de Piedra. Thus, the vulnerability map was validated with
independent data from tracers.
Quantitative tracer data is a formidable indicator of intrinsic vulnerability: the evaluation of
environmental tracers assesses the underlying processes of the groundwater movement.
CFC gases are appropriate for estimation of residence time and modern water percentage in
samples. These parameters are key components of aquifer susceptibility to contamination,
i.e. a groundwater resource that has recharged more than 50 years ago and/or has not mixed
with modern water, is practically not susceptible to anthropogenic contamination, and vice
versa. Thus, environmental tracers represent surrogate indicators of intrinsic vulnerability
25 | P a g e Mahlknecht et al., 2006, Env. Geol. (author’s version) to modern inputs of anthropogenic contamination.
The validation process of parametric maps with environmental tracers consists of the
comparison of results obtained by both techniques, and the adjustment process, i.e. reevaluation of scores and, if necessary, the parameter weights of the rating system until both
maps are congruent and the final vulnerability map is compatible with the hydrological
system.
The scientific defensibility of vulnerability maps - as claimed by numerous authors (e.g.
Gogu and Dassargues 2000; Focacio et al. 2002) - can be accomplished when validated
against data from the underlying groundwater flow system. It is proposed to apply the same
methodology to neighbor catchments to determine if the same weights are applicable to a
larger region or if the SINTACX method needs to be revised.
Acknowledgements
The analytical results used in this article were obtained during the project investigation under
contract no. 02/16-CEAG/SDA/CONCyTEG-064, co-funded by Comisión Estatal del Estado de
Guanajuato (CEAG), Consejo de Ciencia y Tecnología de Guanajuato (CONCyTEG) and
Secretaría de Desarrollo Agropecuario (SDA). We thank Lars GE Backstrom for his comments to
the manuscript.
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