Environ Earth Sci (2012) 66:1011–1020 DOI 10.1007/s12665-010-0710-4
ORIGINAL ARTICLE
Isotopic and geochemical characteristics of groundwater in the Senegal River delta aquifer: implication of recharge and flow regime Moctar Diaw • Serigne Faye • Willibald Stichler Piotr Maloszewski
•
Received: 21 July 2009 / Accepted: 13 August 2010 / Published online: 26 August 2010 Springer-Verlag 2010
Abstract Groundwater and surface water samples were collected to improve understanding of the Senegal River Lower Valley and Delta system, which is prone to salinization. Inorganic ion concentrations and environmental isotopes (18O, 2H and 3H) in groundwater, river, lake and precipitation were investigated to gain insight into the functioning of the system with regard to recharge sources and process, groundwater renewability, hydraulic interconnection and geochemical evolution. The geochemical characteristics of the system display mainly cation (Ca2? and/or Na?) bicarbonated waters, which evolve to chloride water type; this occurs during groundwater flow in the less mineralized part of the aquifer. In contrast, saline intrusion and secondary brines together with halite dissolution are likely to contaminate the groundwater to Na–Cl type. Halite, gypsum and calcite dissolution determine the major ion (Na?, Cl-, Ca2?, Mg2?, SO42- and HCO3-) chemistry, but other processes such as evaporation, salt deposition, ion exchange and reverse exchange reactions also control the groundwater chemistry. Both surface water and groundwater in the system show an evaporation effect, but high evaporated signatures in the groundwater may be due to direct evaporation from the ground, infiltration of evaporated water or enriched rainwater in this region. The stable
M. Diaw S. Faye (&) Geology Department, Faculty of Sciences and Techniques, University Cheikh Anta Diop (UCAD), PO Box 5005, Dakar, Senegal e-mail:
[email protected] W. Stichler P. Maloszewski German Research Center for Environmental Health, Institute of Groundwater Ecology, Helmholtz Zentrum Mu¨nchen, 85764 Neuherberg, Germany e-mail:
[email protected]
isotopes also reveal two types of groundwater in this system, which geomorphologically are distributed in the sand dunes (depleted isotopes) and in the flood plain (enriched isotopes). Consideration of the 3H content reinforces this grouping and suggests two mechanisms of recharge: contribution of enriched surface water in recharging the flood plain groundwater and, in the sand dunes area where water table is at depth between 8 and 13 m, slow recharge process characterized the submodern to mixed water. Keywords Hydrochemistry Environmental isotopes Senegal River Lower Valley and Delta Groundwater recharge Flow regime
Introduction Identifying the locations and the relative contributions of multiple recharge sources to aquifers in geologically complex, regional scale river basins has long challenged hydrologists (Palmer et al. 2007). In specific delta catchment under arid and semi-arid context such as the Senegal River Lower Valley and Delta (SRLVD) system, recharge mechanisms, surface water/groundwater relationship as well as long-term water quality are key concerns with regard to groundwater development and management. Salinization as a result of low and irregular rainfall, high temperatures and evaporation, and low river discharge may be crucial to the usable water resource and therefore affect socio-economic development in this region. Quality assessment of the groundwater system has received very little attention, and efforts to use geochemical and isotopic tools to solve particular problems are limited. Despite that, considerable works have been carried out in the system; they are related to the hydrologic
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functioning (Kane 1997), particle sediment transport (Gac and Kane 1986; Kane 1997), impact of the Diama Dam (OMVS/IRD 1998), flooding, river salinization (Barusseau et al. 1985) and surface/groundwater interaction (Audibert 1970; Illy 1973; Diagana 1994). The purpose of the present study was, thus, to use a multi-tracer method based on solute concentrations and environmental isotopes (18O, 2H, 3H) in water to make a new assessment of the availability of groundwater resources in this deltaic aquifer and to refine our understanding of the hydrogeologic features. This includes the following aspects: (a) an explanation of groundwater recharge sources and renewability; (b) identification of hydraulic connection between aquifer and river; (c) importance of geochemical reactions. This approach will lead to improve our conception of the groundwater flow regime in relation to the importance and distribution of recharge, mixing and the relative residence times. Such tools have been proved to be valuable for understanding many hydrogeological problems; they have been widely used over the past decades to address problems related to groundwater age, groundwater mixing and delineation of flow at the regional scale (Hendry and Schwartz 1990; Weaver and Bahr 1991; Sracek and Hirata 2002; Mahlknecht and Schneider 2004; Boronina et al. 2005).
Environ Earth Sci (2012) 66:1011–1020
depressions and lakes (Guiers Lake fed through a regulated channel). Since the mid-1980s, the flow regime has been regulated by two dams (Manantali in Mali, Diama located 27 km upstream of Saint Louis). The Diama Dam was designed to prevent the intrusion of ocean water and also to feed the reservoir used for irrigation and human needs. In the SRLVD system, besides pumping, the current practice of local industries is to discharge polluted water into the river. Geologic and hydrogeologic settings The subsurface sediments are originally marine and complex; they are composed of quartz, halite, montmorillonite, kaolinite, illite, gypsum, calcite as fossils (gastropods, lamellibranches), pyrite and ferrugineous minerals resulting from the influence of the hydroclimatological factors of the fluvial deltaic lagoon and continental media. The geomorphology of the area derived from both erosion and depositional sequences was used by Audibert (1970) to differentiate the aquifers present in the system: •
Study areas General description The geographical setting comprises the delta and the lower valley of the Senegal River; it lies between longitudes 15400 and 16350 W and latitudes 15450 and 16350 N. The SRLVD is limited to north by the Senegal River, in the west by the Atlantic Ocean, the east by the Ferlo (dried region) and in the south by the latitude 16400 (Fig. 1); the surface area is about 6,000 km2. The region lies in the semi-arid to arid Sahelian band characterized by a coastal climate with mean temperature around 27C (with minimum between 22 and 24C and maximum between 35 and 37C), potential evapotranspiration between 2,000 and 2,500 mm and precipitation between 200 and 250 mm. The Senegal River, with headlands located south in the Fouta Djalon Mountains of Guinea and in Mali (where precipitation of more than 2,000 mm/year is recorded), is 1,760 km long and flows through the western part of Mali, then along the Mauritania-Senegal frontier before reaching the delta and the ocean. The flow regime is unimodal and strongly influenced by seasonal rain; peak flows are recorded from August to November. In the lower valley and delta regions, the river discharges during high-water stage in many hydrologic systems comprising streams,
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•
•
The ancient ergs (vast NNE–SSW longitudinal dunes) edified during the ogolian period, a period of strong marine regression (-100 m) and extreme aridity. The sediments are composed of isometric sand grain (0.2 mm) often coated with ferruginous clay. The whole system overlies an Inchirian semi-permeable sandy clay layer, which is generally used as limit between the freshwater in the sand dunes aquifer and saline water in the Inchirian aquifer. The recent ergs (marine terraces) induced by the Nouakchottian transgression (Illy 1973) are composed of fine, white and isometric sand grains bearing some Anadara se´nilis shells and gravels overlaying argillaceous sand sediments. These are very characteristic in the lower valley zone and constitute the transition zone between the ancient erg and the flood plain. Thus, in the deltaic zone, the Ogolian and the Nouakchottian aquifer formations appear in the form of freshwater lenses with limited extension, whereas toward the south and the east of the Guiers Lake, where marine facies reach their extreme extension, the aquifer composed of ogolian red sands can bear important freshwater resources with thickness greater than 6 m. Following the Nouakchottian transgression, the delta was transformed into complex lagoons in the form of elongated golf evolving via an endorheic mode. In the east part of the golf, the alluvial sediments dominate lagoon sedimentation and limit the expansion of the mangroves in the delta. These sediments, which are composed of fine sands, clays and compacted yellow silts, incorporate sulfate acid and salt crusts. In low
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Fig. 1 Location and sampling points of the SRLVD system
altitude zone, the muddy clay sediments are marked by a higher salinity related to the successive marine invasions and the absence of drainage in the depression ground surface (Tricart 1961). This alluvial aquifer lies unconformably on the Eocene limestone or on the Maastrichtian sandstones; it is mainly fed by flooded water in the inundation plain. Transmissivity and hydraulic conductivity values are variable and they range from 10-2 to 4.8 9 10-6 m2/s and from 0.4 9 10-4 to 6 9 10-5 m/s, respectively. Groundwater table is shallow in the flood plain zone (1.6–6 m) and reach to 13 m depth in the sand dunes area (Diaw 2008). For this system, due to the limited number of monitoring wells, a piezometric map could not be established. The few measured head values vary between -1 and -10 m and the general and gradual trend indicates flow direction mainly from the river toward inland for both seasons (high-water and lowwater stages). However, the small depression observed south of the Diama Dam in the sand dunes zone may result from a high exploitation in this particular well. By comparing these values with the measured December 2005 values (high-water stage) from the same sampling points network, a groundwater rise between 1.4 and 3.8 m is observed in the flood plain area. This is likely due to rapid infiltration of the flooded water. In the sand dunes area, irregular head variability occurred, which could be attributed to the slow and diffuse infiltration process (Diaw 2008).
Field and analytical methods The sampling campaigns were conducted in July (dry season) and December 2005 (end of rainy season) for the SRLVD system. However, for the present paper, we focus mainly on the July 2005 campaign, where complete hydrochemical and isotopic data sets are available. The sampling campaigns include groundwater collected from 19 wells, and surface water collected from three sites in the Guiers Lake and at five sites along the Senegal River course. The locations of the sampling points are shown in Fig. 1. Geographical locations of wells, pH, water temperatures (TC) and electrical conductivity (EC) were determined on site using potable instrument according to standard methods (Greenberg et al. 1992). Alkalinity (as HCO3) was also measured by titration with 0.01 or 0.1 HCl against methyl orange and bromocresol green indicators at the sampling sites. Prior to sampling and field measurements, depths of water table were recorded for each well. For analysis of inorganic constituents, samples were 0.45 lm filtered into acid-washed polyethylene bottles, with one aliquot being acidified to pH less than 2 by addition of high purity HNO3 for analysis of cations. Other unacidified water samples were collected and coolly conserved for stable isotopes and tritium analyses. Major ions were measured by ion chromatography Dionex DX 120 at the Department of Geology/Dakar. Hydrogen and oxygen isotopes analysis were performed at the Institute of Groundwater Ecology (IGE)/Helmholtz
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Zentrum Mu¨nchen/Germany by, respectively, employing the standard CO2 equilibration (Epstein and Mayeda 1953) and the zinc reduction techniques (Coleman et al. 1982), followed by measurement by isotope ratio mass spectrometer. All oxygen and hydrogen isotopes analyses are expressed in the conventional d-per mil (d%) notation referenced from the Vienna-Standard Mean Oceanic Water (V-SMOW). The analytical reproducibility is ±0.1% for oxygen and ±1.0% for deuterium, respectively. Tritium analyses were performed by electrolytic enrichment and analyzed with a liquid scintillation counting method (Thatcher et al. 1977). The results are reported as tritium unit (TU) with a typical error of ±0.7 TU.
Results and discussion Hydrochemical characteristics of precipitation For the present study, precipitation waters were collected at Saint Louis station during the 2005 rainy season. The chemical compositions of the monthly aggregate rainfall samples were considered (only events [10 mm) and show a general decrease in solute concentration from July to September. The pH values are usually alkaline (8.6–8.9) and EC values decrease from 214 to 85 lS/cm, reflecting the progressive washout of the aerosol spray. The general trend observed in the solute contents (mg/L) of three samples is: HCO3- [ Ca2? [ SO42- [ Na? [ Cl- [ Mg2? [ K?. The high predominance of Ca2? over Na? and of SO42- over Cl- is likely the result of continental atmospheric pollution derived from desertic aerosols, atmospheric dust and/or industrial pollution. In fact, Gac and Orange (1990) reported that high Ca2? in precipitation was derived mainly from desertic aerosols in the form of carbonate particles, high SO42- was produced through oxidation of sulfur derived from industrial pollution and pesticides, while increase in Na? and Cl- concentrations were mainly derived from marine aerosol sprays. Hydrochemical characteristics of surface water and groundwater The principal characteristics of the surface and groundwater of the system are presented in Table 1. The surface water samples in the SRLVD system were collected at eight sites: five sites along the river course and three along an NS direction of the Guiers Lake. The pH values for these samples range from 6.6 to 8.5, indicating slightly acidic (river water) to alkaline (lake water) characteristics. These values shift to more alkaline characteristics after the rainy season, except at three points located
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downstream of the delta system. The average temperatures are close to ambient (24–30C) and EC values show a trend along the river course from 50 lS/cm in samples collected upstream the Diama Dam to 9,900 lS/cm in samples located downstream and in the delta area. The two EC values (79 and 9,900 lS/cm) measured at the neighbor sampling points (3 and 4) located upstream and downstream of the Diama Dam is an evidence of the primary role of this infrastructure in preventing saline water intrusion. The lake water EC values evolve quite similarly from 210 to 393 lS/cm, and this increase relative to the river water is essentially due to evaporation as the lake functions artificially as a close system (channel close) during this period of the year. The water types typically display HCO3- mixed cations (Ca/Na, Ca/Mg or Ca/Mg/Na) in the lake and freshwater upstream the Diama Dam, while Na–Cl type characterizes the highly mineralized water located downstream of the Diama Dam (Fig. 2). The groundwater samples exhibit pH values between 3.4 and 9.5 with the lowest acidic values (3–5) in the flood plain area, temperature 28–34C and EC values 50–10,580 lS/cm. This broad range of EC values differentiates two types of groundwater in the system: •
•
Freshwaters, which have EC values lower than 1,000 lS/cm. This group comprises water sampled from wells located below the dune formations and in the alluvial plain at the proximity of the hydraulic axis. The latter likely suggests a mixing between infiltrated water component and mineralized water component trapped at the base of the aquifer in the flood plain. Saline waters, located essentially in low topography area and marine terraces, exhibit EC values greater than 1,000 lS/cm. In some piezometers, water sampled after the rainy season shows values as high as 83,700 lS/cm, which is likely induced by contamination of the extremely saline Inchirian formations that evolved generally in a lens structure.
The groundwater facies encountered are the following (Fig. 2): •
•
The freshwater group has Ca–HCO3, Ca/Na–HCO3, Na–Cl and Ca/Na–Cl. These water types are located mainly at the vicinity of the river and lake, the flood plain and the sand dune areas. Water types evolve mainly with Na–Cl according to groundwater flow from the river toward inland. The saline water group has Na–Cl and Ca/Na–Cl water types located mainly in low topography area.
Slight seasonal variability is evident on comparing the chemical characteristics of groundwater sampled in July and December 2005; these concern mainly Na–Cl to Ca–Cl, Ca–Cl to Na–Cl, and Na–HCO3 to Ca/Na–HCO3 variations (Fig. 3).
2.5
–
–
–
–
2.36
4.65
4.10
12.9
8.31
4.76
4.95
5.90
–
6.8 2.8
1.6
12.5
–
5.7
–
–
4.94
4.85
8.6
3.95
2.00
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
26
27
GW groundwater
Depth (m)
No.
GW
GW
GW
GW
GW
Lake
Lake
GW
River
GW
GW
GW GW
Lake
GW
GW
GW
GW
GW
GW
GW
GW
River
River
River
River
GW
Type
8.5
7.4
6.2
9.5
7.8
7.9
7.7
6.1
6.8
6.2
6.7
7.3 3.4
8.3
7.1
5.3
7.1
6.7
7.0
6.6
6.2
6.9
7.8
6.9
7.7
6.7
7.3
pH
34.6
28.6
30
29.7
29.4
28.9
29.7
28.3
30.7
29.1
30.4
29 32
29.6
31.2
31
29.8
31
32
31
30.5
30.5
29.1
31.2
28.7
31.4
30.2
T (C)
5,920
1,754
661
392
6,970
219
243
886
81
788
460
10,580 3,080
393
7,010
112
471
198
10,350
529
1,106
189
50
9,900
79
646
3,430
EC (lS/cm)
1,106.8
201.1
56.3
40.6
1,018.5
21.1
19.8
71.2
7.2
69.1
56.3
1,062.5 481.2
41.1
1,009.6
13.8
18.7
19.5
1,740.2
67.5
125.5
37.5
2.5
2,131.2
7.6
78.1
507.3
Na? (mg/L)
62.5
27.6
9.1
8.9
50.1
4.5
6.6
19.6
1.5
12.2
14.8
141.2 35.5
7.5
421.1
5.2
4.6
2.1
62.5
6.3
6.3
2.4
1.0
72.1
1.2
13.2
40.2
K? (mg/L)
44.0
12.1
15.6
2.5
72.9
18.5
4.9
36.3
4.9
20.7
3.7
145.8 28.2
12.2
97.2
1.0
15.8
2.8
85.1
4.9
8.5
0.4
3.3
15.4
0.6
15.8
36.6
Mg2? (mg/L)
89.7
141.4
40.2
31.0
321.6
13.9
12.0
31.7
5.6
49.8
22.1
782.0 68.8
22.4
251.2
4.5
59.2
15.0
356.0
25.4
61.0
3.5
4.0
72.2
6.1
15.8
117.6
Ca2? (mg/L)
Table 1 Major ion chemistry and isotope data of the Senegal River Lower Valley and Delta
1,830.8
397.5
168.6
64.1
2,160.0
22.6
28.4
184.4
9.5
164.6
38.2
3,454.4 784.6
46.3
1,613.3
7.6
6.7
16.0
3,011.2
64.6
228.5
6.1
4.2
3,336.5
1.0
133.7
762.3
Cl(mg/L)
216.6
195.2
24.4
21.4
320.3
161.7
73.2
88.5
48.8
18.3
128.1
106.8 0.0
167.8
265.4
27.5
314.2
67.1
295.9
161.7
82.4
106.8
30.5
64.1
42.7
106.8
445.3
HCO3(mg/L)
138.8
56.0
12.0
9.1
87.6
7.2
4.6
18.1
0.2
8.5
40.6
148.8 162.9
9.9
451.4
18.2
1.0
16.1
554.0
22.6
33.6
0.4
0.0
270.3
0.0
20.9
177.6
SO42(mg/L)
23.3
86.6
41.2
65.3
12.9
0.4
0.2
82.3
1.2
160.3
5.7
65.9 0.8
0.5
874.4
0.9
2.1
0.9
10.5
3.0
65.4
0.2
0.3
1.7
0.3
7.4
0.1
NO3(mg/L)
Na–Cl
Na–Cl
Ca–Cl
Na–Cl
Na–Cl
Mixte–HCO3
Ca/Na–HCO3
Ca–Cl
Ca/Na–HCO3
Ca–Cl
Na–HCO3
Ca–Cl Na–Cl
Ca/Na–HCO3
Na–Cl
Na–Cl
Ca–HCO3
Ca–HCO3
Na–Cl
Na–HCO3
Na–Cl
Na–HCO3
Ca/Mg–HCO3
Na–Cl
Ca/Na–HCO3
Na–Cl
Na–Cl
Water types
-2.3
-5.0
-4.4
-4.9
-5.0
3.3
1.8
-1.1
0.3
-4.8
-2.9
-3.4 -4.7
6.0
-1.1
-1.4
-1.7
-1.4
-3.4
-0.6
-1.7
-2.3
-1.5
-0.4
-0.8
1.9
-1.1
d18O (%)
-22
-35
-34
-35
-38
9
0.1
-14
-9
-36
-25
-29 -33
25
-15
-17
-18
-17
-26
-12
-18
-21
-16
-8
-13
4
-14
d2H (%)
2.9
2.8
1.4
1.8
2.6
3.6
4.0
5.2
4.0
0.9
2.3
6.4 2.3
3.3
3.2
3.7
3.7
3.6
\1.3
3.3
3.9
3.5
3.9
3.4
3.9
3.6
3.5
H (TU)
3
Environ Earth Sci (2012) 66:1011–1020 1015
123
1016 Fig. 2 Water type distribution
Fig. 3 Piper diagram representing the two sets of samples
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Environ Earth Sci (2012) 66:1011–1020
1017
Ionic relations and sources of major components
18.0
10
Na/Cl (meq ratio)
8
6
4
2
0 0
20
40
60
80
100
120
Cl (meq/L)
Fig. 4 Relationship between Cl and Na/Cl in groundwater (seawater Na/Cl in dashed line, halite dissolution in continuous line)
[HCO3+SO4] (meq/L)
16.0
Dissolved species and their relations with each other can reveal the origin of solutes and the processes that generated the observed water composition. Positive or negative trends between pairs of parameters together with saturation index values of some common minerals can help identify the likely mechanisms occurring in the system. The Na?–Cl- relationship, often used to identify the mechanism that causes salinity and saline intrusion, shows good correlation, evidencing that part of Cl- and Na? in the saline water group is derived from halite dissolution. However, enrichment of Na? relative to Cl- in the less mineralized water (Fig. 4) with high molar ratios up to 8 is most likely controlled by ion exchange reactions following weathering of the silicate minerals. As a consequence of this process, Na? concentrations would increase preferentially compared with simple mixing, and Ca2? concentrations would decrease proportionately. But occurrence of high Ca2? in the system implies that ion exchange reaction is likely to proceed simultaneously with carbonate equilibrium reactions and/or gypsum dissolution. In contrast, lowering of Na?/Cl- ratio of the saline groundwater group probably results from reverse ion exchange of Na? for Ca2? and Mg2? in clays. In addition, leaching of salt precipitated through evaporation of surface water and soil moisture can be another dominant process in this context of semi-arid lands. In the SRLVD, high Na? and Cl- contents are generally attributed to the dissolution of halite (Audibert 1970) together with recurrent marine intrusions, which occurred even in recent periods (Gac and Kane 1986). Conversely, salinity in the sand dune area can be associated with secondary brines resulting from dissolution of halite disseminated in the aquifer matrix in association with the aerosols and marine spray contributions in the littoral zone.
14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0 0
10
20
30
40
50
60
[Ca+Mg] (meq/L)
Fig. 5 Relationship between [Ca ? Mg] and [HCO3 ? SO4]
The investigation of the Ca2?, Mg2?, HCO3- and SO42relationship allowed determining the importance of carbonate, sulfate and silicate (Feldspar) minerals in the dissolution processes. Specifically, a 1:1 stoichiometry ratio for [Ca2? ? Mg2?] to [HCO3- ? SO42-] should exist if these ions are derived from simple dissolution of calcite, dolomite and gypsum. The plots of the [Ca2? ? Mg2?] versus [HCO3- ? SO42-] displayed in Fig. 5 show a general trend falling below the 1:1 line, reflecting thus the requirement of cations from the weathering of silicates. The calcite and/or dolomite dissolution is ruled out because of a poor correlation between [Ca2? ? Mg2?] and [HCO3-] and the absence of these minerals in the reservoir materials. However, there is a positive trend of Ca2? versus SO42- and SIgypsum values ranging between -4.8 and -0.01 in the high mineralized waters, suggesting that gypsum dissolution contributes to the groundwater geochemistry (Diaw 2008). Cation exchange process, as stated above, may be a regulation factor of ionic concentrations in groundwater and therefore plays a temporary buffer role when ionic concentrations are variable. Increase in Na? concentration without an associated increase in Cl- concentrations can be explained by this process (Appelo and Potsma 1994). When calcite, dolomite and gypsum dissolution are associated processes, the possibility of exploring the importance of cation exchange can be examined using plots of [(Ca2? ? Mg2?)–(alkalinity ? SO4)] as a function of [Na?–Cl-]. The [Na?–Cl-] represents the amount of Na? gained or lost relative to that provided by gypsum, calcite and dolomite dissolution. Figure 6 indicates a negative correlation along a quite straight line (R2 0.9) with a slope of -1. This evidences that Na?, Ca2? and Mg2? participate in the ion exchange reaction and may be considered as the dominant process that would explain the less mineralized Na–HCO3 water type at the vicinity of the hydrologic axis of the SRLVD system.
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40 30 20 10 0 -10 -20 -60
-50
-40
-30
-20
-10
0
10
20
[Na-Cl] (meq/L)
Fig. 6 Relationship between [(Ca ? Mg)–(alkalinity ? SO4)] and [Na–Cl]
Isotope geochemistry Meteoric and surface waters The characterization of environmentally stable and radiogenic isotopes of local meteoric and surface waters is essential for the deduction of the sources and behavior of the shallow groundwater flow. In the SRLVD system, due to the highly heterogeneous nature of the aquifer matrix together with both temporal and spatial rainfall variability, the recharge mechanism may be complex and discontinuous, and local recharge rate may vary widely. This has been evidenced by early studies (Edmunds 1990; Gaye 1990), which used chloride mass balance (CMB) in the unsaturated zone at Louga in northern Senegal where computed recharge rates range from 0.5 to 34.4 mm/year (Edmunds and Gaye 1994). During the present investigation, no rainwater samples were isotopically analyzed to characterize the input signature of the stable isotopes and tritium. We have used the available rainfall stable isotopes data from Travi et al. (1987), Gaye (1990). These data, although relatively old (between 1986 and 1989), may be considered as suitable for this study since they underlined some influence of local oceanic vapor on the Senegalese coast. Moreover, correlations found from the d18O versus d2H pairs are relatively close to the global meteoric water line (GMWL) of Craig (1961). Tritium, with 12.43 years half-life, has been widely used for estimation of recent groundwater residence time. Very low 3H contents below detection limit mark waters below baseline of the pre-bomb tests and therefore indicate groundwater residence times of more than 50 years of age. In contrast, if measurable quantity of 3H values occurs, modern groundwater age and flow velocity can be determined. Active circulating groundwater and/or dispersion and mixing generally attenuate 3H content, while less
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active flow system may preserve 3H input of the bomb testing era (1952–1962). By using Leduc et al. (1996) approximation of 3H input signal in the present day Niger precipitation, the actual value in Senegal would range between 2 and 4 TU, representing a quite complete removal of the bomb tritium to the background level. These values are very consistent with the measured values obtained from the Dakar 2007 precipitation (1.8–2.8 TU) and measured values in surface water collected from the Guiers Lake and Senegal River (3.3–4.0 TU). Further, as tritium is a conservative tracer, changes due to evaporation are small relative to measurement errors. The d18O, d2H and 3H data of surface water and groundwater in the system are presented in Table 1. Surface water in the SRLVD system was only isotopically analyzed for samples collected in June 2005, corresponding to a period of high evaporation and low flow in the hydrologic system (low stage flow). The d18O values range from -1.5 to ?6.0%, and the d2H values from -16 to ?25%. All data plot along an evaporation line of Y = 5.6 9 -7.6 with a slope of 5.6 and R2 = 0.94 (Fig. 7). From this data set, it is noted that lake water samples are more evaporated (range ?1.8 to ?6.0% for d18O and 0 to ?25% for d2H); this is essentially due to the fact that the Guiers Lake evolves in a close system with no circulation during the period of low stage flow. The 3H contents in the river and the lake are similar and exhibit a narrow range of 3.4–4.0 TU with a mean of 3.7 TU, which probably reflects tritium content in local precipitation since evaporation does not affect its concentration. Groundwater By plotting d18O against d2H data of the SRLVD groundwaters, a distinguished trend is identified with the scattered line of Y = 5.3 9 -7.9 (R2 0.81, slope 5.3). The groundwater samples display d18O between -5.0 and 30 20 10 0
δ2H
[(Ca+Mg)-(HCO3+SO4)] (meq/L)
50
-10 Evaporation lines
-20 B
-30 A
-40 -50 -8
-6
-4
-2
0
2
4
6
8
δ 18O
Fig. 7 d18O versus d2H plots (empty square SRLVD Lake and River waters, filled square SRLVD groundwater)
Environ Earth Sci (2012) 66:1011–1020
1019
7 GW SW
6 5
3H
4 3
Lake and River water
2 1 Sand dunes GW
Flood plain GW
0 -7
-5
-3
-1
1
δ18O
Fig. 8 d18O versus 3H plots in SRLVD system
3
5
7
8 6 4
A 2
D-excess
-0.6% and d2H between -38 and -11% (Fig. 7). These ranges evidence that groundwater exhibits more evaporated signature, which may be due to direct evaporation from the ground, infiltration of percolated water or enriched rainfall in this region. Despite this fact, two (2) distinct groups of groundwater isotopic data are present in the SRLVD system: group A, characterized by more depleted stable isotope values with d18O -5.0 to -4.4% and d2H -38 to -32% and with 3H values ranging between 0.9 and 2.8 TU, geographically distributed in the sand dune geomorphological setting (water table depth 8–13 m); and group B, representing enriched isotopes, which characterized groundwater below the alluvial flood plain (water table depth 1–6 m) geomorphological setting. This latter group displays stable isotopes values between -3.4 and -0.6% for d18O and between -29 and -11% for d2H and 3 H contents between 1.3 and 6.4 TU, with 70% of these values ranging between 3.2 and 3.9 TU. By comparing d18O against 3H values (Fig. 8), scattering of the two previous groups are confirmed where group B overlaps with surface water signature, evidencing therefore a likely contribution of surface water. The relatively light stable isotope values and low 3H content in the sand dunes context reflect relatively submodern water, which may be mixed by recharging water to give this range. The Deuterium-excess (d-excess), used as a proxy for identifying secondary processes that influence the atmospheric vapor content in the evaporation–condensation cycle in nature (Craig 1961; Daansgaard 1964), indicates the evaporation effect on the physicochemical characteristics of water (d-excess decreases with inverse evaporation). The measured values range from -17.2 to -3.6% for the Senegal River water, while groundwater exhibits a range of [-7, ?5.4%]. From the latter, it can be deduced that the groundwater in this system originates from water with different d-excess values probably derived from rainfall, surface water, flood and irrigation return flow.
0 -2 -4
B
-6 -8 -7
-6
-5
-4
-3
-2
-1
0
δ18O
Fig. 9 d18O versus d-excess plots in the SRLVD system
Figure 9, which shows a negative correlation between d18O and d-excess for the whole set of samples, confirms high evaporation affecting groundwater and grouping according to groundwaters sampled in the flood plain and in the sand dunes areas. The enriched stable isotopes values associated with low d-excess in the SRLVD system is an indication that evaporation occurred during the recharge process.
Conclusion The integrated analysis of the reservoir material, hydrogeological, hydrochemical and isotopic data in the SRLVD system, has allowed the definition of the principal characteristics of the aquifer system, the recharge location and mechanisms, the hydraulic interconnection between surface water and groundwater, and the spatial and temporal hydrochemical behavior that governs the groundwater quality. The system reveals relatively simple hydrochemical facies with mainly Na–HCO3, Na–Cl, Ca–HCO3, Ca/ Na–HCO3 and Ca/Na–Cl freshwater types, and locally Na– Cl and Ca/Na–Cl in the saline water at the vicinity of the lower reach of the delta system. Environmental isotopes (18O, 2H, 3H) were used in this study to investigate groundwater typing, water origin, recharge mechanism processes and mixing according to flow. Clustering of the SRLVD groundwater data evidenced that the enriched signature likely derived from direct evaporation from the ground, infiltration of percolated water or enriched rainfall in this region. Moreover, by combining with the 3H content, recharge processes relative to the geomorphological settings can be identified. In the sand dunes zone, depleted stable isotopes values and low 3 H content reflect relatively submodern water, which may be mixed by recharging water, while enriched stable isotopes, low D-excess and 3H contents between 1.3 and
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6.4 TU in the flood plain area are governed by surface water contribution. Acknowledgments This was supported by French cooperation through the CORUS/GESCAN program and by the Alexander von Humboldt (AvH) Foundation. We gratefully acknowledge the personnel of the Hydrochemistry Laboratory at the Geology Department (Dakar-UCAD) and the Helmholtz Zentrum Mu¨nchen (formerly GSF research centre–Munich/Germany) and the IAEA (Vienna-Austria) for the isotopes analyses.
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