Application of Tracers in Arid Zone Hydrology (Proceedings of the Vienna Symposium, August 1994). IAHS Publ. no. 232, 1995. 363 Use of chemical and isotopic tracers in studying the recharge processes of the upper Cretaceous aquifer of the Garoua basin, northern Cameroon R. NJITCHOUA, J. Ch. FONTES & G. M. ZUPPI Laboratoire d'Hydrologie et de Géochimie Isotopique, Université de Paris-Sud, F-91405 Orsay Cedex, France J. F. ARANYOSSY Projet IAEA RAF/8/012, PNUD, BP 154, Dakar, Senegal E. NAAH Institut de Recherches Géologiques et Minières, BP 4110, Yaounde, Cameroon Abstract Precipitation and groundwater from the Garoua sandstone aquifer in northern Cameroon have been analysed for their chemical and isotopic contents. The isotopic composition of daily precipitation displays a large range of values, from —1.61%o to — 10.18%o foro 18 0, and from +1.3 %o to - 70.4 %o for <5 2 H. Precipitations falling in the study area are generally of oceanic origin. Chemical and isotopic data give evidence of two distinct groundwater systems within the aquifer: a shallow and a deep groundwater system. The former is characterized by dilute waters, which were recharged by direct infiltration of rainwater through some favourable zones of the aquifer, and by lateral infiltration of free surface water bodies. The deep groundwater system reflects a mixing between shallow water and an old groundwater that was probably recharged under more humid climatic conditions than at present. INTRODUCTION The northern part of Cameroon is characterized by a very low groundwater resource potential. Due to harsh climatic conditions, there is no permanent resource of surface water available; all the development projects performed in this part of the country depend entirely on groundwater resources. The extensive exploitation of the ground- water reserve for domestic, agro-pastoral and industrial purposes, together with the pluviométrie deficit observed these last decades in this part of the country, have provoked a substantial decrease of the groundwater resource. As a result of this, since 1990, the Institute for Geological and Mining Research of Cameroon (IGMR), in coope- ration with the International Atomic Energy Agency (IAEA), has carried out an extensive programme in northern Cameroon with the aim of investigating the ground- water resources. The purpose of this paper is to investigate the recharge processes occurring within the upper Cretaceous sandstone aquifer of the Garoua basin, using both chemical and environmental isotope (oxygen-18, deuterium, tritium) tracers.
Transcript
Application of Tracers in Arid Zone Hydrology (Proceedings of the Vienna Symposium, August 1994). IAHS Publ. no. 232, 1995. 363
Use of chemical and isotopic tracers in studying the recharge processes of the upper Cretaceous aquifer of the Garoua basin, northern Cameroon
R. NJITCHOUA, J. Ch. FONTES & G. M. ZUPPI Laboratoire d'Hydrologie et de Géochimie Isotopique, Université de Paris-Sud, F-91405 Orsay Cedex, France
J. F. ARANYOSSY Projet IAEA RAF/8/012, PNUD, BP 154, Dakar, Senegal
E. NAAH Institut de Recherches Géologiques et Minières, BP 4110, Yaounde, Cameroon
Abstract Precipitation and groundwater from the Garoua sandstone aquifer in northern Cameroon have been analysed for their chemical and isotopic contents. The isotopic composition of daily precipitation displays a large range of values, from —1.61%o to — 10.18%o foro180, and from +1.3 %o to - 70.4 %o for <52H. Precipitations falling in the study area are generally of oceanic origin. Chemical and isotopic data give evidence of two distinct groundwater systems within the aquifer: a shallow and a deep groundwater system. The former is characterized by dilute waters, which were recharged by direct infiltration of rainwater through some favourable zones of the aquifer, and by lateral infiltration of free surface water bodies. The deep groundwater system reflects a mixing between shallow water and an old groundwater that was probably recharged under more humid climatic conditions than at present.
INTRODUCTION
The northern part of Cameroon is characterized by a very low groundwater resource potential. Due to harsh climatic conditions, there is no permanent resource of surface water available; all the development projects performed in this part of the country depend entirely on groundwater resources. The extensive exploitation of the groundwater reserve for domestic, agro-pastoral and industrial purposes, together with the pluviométrie deficit observed these last decades in this part of the country, have provoked a substantial decrease of the groundwater resource. As a result of this, since 1990, the Institute for Geological and Mining Research of Cameroon (IGMR), in cooperation with the International Atomic Energy Agency (IAEA), has carried out an extensive programme in northern Cameroon with the aim of investigating the groundwater resources.
The purpose of this paper is to investigate the recharge processes occurring within the upper Cretaceous sandstone aquifer of the Garoua basin, using both chemical and environmental isotope (oxygen-18, deuterium, tritium) tracers.
364 R. Njitchoua et al.
THE STUDY AREA
The Garoua basin is located in the Sudano-Sahelian region of northern Cameroon, between the latitudes of 9° and 10°N. It covers an area of about 4700 km2 and it is drained by the Benue river, which is the most important river of the area. The climate is semiarid, with a mean annual temperature of 28°C. The rainy season is from May to September. The mean annual rainfall amounts to 1018 mm (1951-1989), with about 70 % of rain falling between July and September. Precipitation occurs either as low altitude monsoon rains or as occasional high altitude squally showers. Despite the high value of the annual rainfall, only a small fraction of precipitation contributes to the groundwater recharge because of the high annual potential évapotranspiration (about 1800 mm), which is about double the mean annual rainfall.
The Garoua basin forms the northwestern part of the Benue sedimentary trough (Fig. 1). According to Maurin & Guiraud (1990), the Benue trough is to be related to the opening of the Gulf of Guinea. The basin is formed within the middle to upper Proterozoic granito-gneissic basement, and is entirely filled by continental sediments of middle to upper Cretaceous ages (Roch, 1953). Groundwater samples analysed in this study are from the upper Cretaceous series, also known as the "Garoua Sandstones". This formation is principally composed of sandstone banks interbedded by numerous clay lenses. In the western part of the basin, the Garoua Sandstone is pierced by numerous Tertiary volcanic intrusions composed mainly of trachytes. Moreover, close to the Benue river and its tributaries, the Garoua Sandstone complex is overlain by some Quaternary alluvial deposits of sands, gravels and clays.
The Garoua Sandstone aquifer is well described by Tillement (1972). Its thickness ranges from up to 400 m in the central part of the basin, decreasing towards its margins. The aquifer is unconfined in most parts of the basin, nevertheless the occurrence of numerous clayey levels within the series imposes a semi-confinement on the underlying deposits. Measured transmissivities in the upper horizon of the aquifer are between 10"1
and 10"5 m2 s"1.
SAMPLING AND ANALYTICAL PROCEDURES
Both precipitation and groundwater samples were collected for the purpose of this study. Precipitation was collected daily from June to September of 1991 and 1992 at the Garoua Meteorological Service (9°18'N, 13°23'E). The sampling was performed at the end of each rainy event in order to minimize the effect of evaporation. Groundwater was sampled in 1991 and 1993 from drilled wells available within the basin (Fig. 1).
Field measurements of pH, electrical conductivity and alkalinity were performed on all the groundwater samples. Other major chemical species were analysed at the Laboratoire d'Hydrologie et de Géochimie Isotopique (LHGI) at the Orsay University in France for the anions, and at the Société Anonyme de Gestion des Eaux de Paris (SAGEP) for the cations.
The stable isotope ratios of all the water samples were determined at LHGI, following the methods described by Epstein & Mayeda (1953) for oxygen-18 and by Coleman et al. (1982) for deuterium. Isotope contents are expressed in b values, the per mil deviation from the Vienna-SMOW standard (Gonfiantini, 1978). Analytical accuracy
Recharge processes of the upper Cretaceous aquifer of the Garoua basin, Cameroon 365
Fig. 1 Geological map (from Tillement, 1972) and geographical locations of the sampling points.
is within ±0.2%o for ô180 and ±2%o for ô2H. Tritium was determined by liquid scintillation counting preceded by electrolytic enrichment either at the Centre de Recherches
366 R. Njitchoua et al.
Géodynamiques in Thonon-les-Bains (France), or at the Isotope Hydrology Laboratory of the IAEA in Vienna (Austria). Tritium concentrations are expressed in tritium units (TU) and the estimated standard deviation is given for each individual analysis.
RESULTS AND DISCUSSION
Isotopic characteristics of local precipitation
Table 1 shows the isotopic compositions of the daily rainfall at Garoua, as well as the associated rainfall amounts. The isotopic compositions of daily rainfall vary between -1.61 and -10.18%o for <5180 and between +1.3 and -70.4%0 for Ô2H. The rainfall
Table 1 Isotopic composition of daily precipitation at Garoua.
Nos. Dates of Amounts g l ^ o g2jj d^
sampling of (%c) (%o) (%c) rainfall (mm)
p l l pi 2 pi 3 pi 4 pi 5 pi 6 pi 7 pi 8 pi 9 pi 10 pi 11 pi 12 pi 13 pi 14 pi 15 pi 16 pi 17 pi 18 pi 19 pi 20 pi 21 pi 22 pi 23 pi 24 pi 25 pi 26 pi 27 pi 28 pi 29
Data are expressed relative to Vienna-SMOW (Gonfiantini, 1978). ' d = deuterium excess: d = 8-̂ H - 85l°0
Recharge processes of the upper Cretaceous aquifer of the Garoua basin, Cameroon 367
events collected in 1991 show a wide range of values than those sampled in 1992. In the former case, both <5180 and ô2H range from —1.61 to -10.18%o and from -5 .8 to -70.4%o respectively. Rainfalls of 1992 are relatively enriched in their isotope contents, with values varying between —0.65 and —6.47%o for ô180 and between +1.3 and -39.3%0for<52H.
The plot of data points in a diagram <5180 vs amounts of rainfall (Fig. 2) shows a different isotopic behaviour of rainwaters collected in 1991 and 1992. With the exception of samples 4, 5, 8 and 12, the isotopic variation of rainfall sampled in 1991 may be attributed to a "mass effect". In fact, the important precipitations which occur at the heart of the rainy season show more negative values of ô180, whereas the beginning and end of the rainy season are characterized by lighter precipitations with more enriched ô180. Such a variation, which reflects the temporal evolution of the InterTropical Convergence Zone (ITCZ), is characteristic of tropical regions (Fontes et al, 1970; IAEA, 1981; Yurtsever & Gat, 1981). On the contrary, samples 4, 5, 8 and 12 show lower isotopic compositions for amounts of rainfall less than 5 mm (Fig. 2). These low isotope contents may be attributed to condensation at high altitude (low temperature condensation) of convective showers generated by the squall lines (Joseph & Aranyossy, 1989; Fontes et al., 1993; Mathieu et al., 1993, Rozanski et al.., 1992). On the other hand, the effect is not very evident for most of the rain events collected in 1992, which display slightly enriched <5180 whatever the amount of precipitation. Such enrichment may be related to continuous monsoon rains (warm rains) which are generally generated at low altitude (Fontes et al, 1993, Yurtsever & Gat, 1981, Rozanski et al., 1992).
As seen in a ô2H vs ô180 diagram (Fig. 3), most of the data points, with the exception of samples 4,5,8 and 11, plot close to the global meteoric water line (GMWL) 5rH = 8<5180 + 10 (Craig, 1961), suggesting that the atmospheric vapour at the origin of these rain events has an origin both unique and oceanic, the Gulf of Guinea. This fact also indicates that apart from the precipitations at the beginning and end of the rainy
V-S
MO
W
1 oo
Oxy
gen-
1
- 2 -
- 4 -
- 6 -
- 8 -
- 1 0 -
-1 2-
1 «O
® \
•pi 12 \
•pi 8
• p i s
' 1
o
o
sé>#
1
o o
o
o o
o
— 1 1 . 1—
• 1991
O 1992
1
1 I •
10 20 30 40
Amount of rainfall (mm)
50 60
Fig. 2 Stable isotopic composition of daily precipitation at Garoua. Samples were collected at the Garoua Meteorological station, "d" refers to deuterium excess (ô = 52H - 8S , 80).
368 R. Njitchoua et al.
-150H 1 1 . 1 . 1 . 1 -15 -10 -5 0 5
Oxygen-18 (%o)
Fig. 3 Correlation between oxygen-18 and the amount of rainfall of daily precipitation at Garoua.
season, there was not any significant evaporation from precipitations during their fall. The stable isotope contents of samples 4,5,8 and 12 are located below the GMWL.
This may be attributed to low values in the deuterium excess of precipitation (—15%o > d < -5%o) which would probably have characterized a condensation process under cooler conditions (high altitude condensation) rather than being due to evaporation.
Chemical and isotopic compositions of groundwater
The chemical and isotopic compositions of groundwaters investigated for the purpose of this study are given in Table 2. From the whole data, two distinct groundwater systems are identified in the Garoua sandstone aquifer: a low-salinity groundwater system, with electrical conductivities (EC) varying between 24 and 331 /xS cm"1, and high-salinity water system, with EC varying between 571 and 812 fj.S cm"1.
The low-salinity waters are very heterogeneous in their chemical compositions, being dominated by both Ca2+ and Na+ for the cations, and by HCO J for the anions (Fig. 4(a) and (b)). However several boreholes display significant amounts of NO J (see Table 2 and Fig. 4). The occurrence of significant amounts of NO 3 in several wells is attributed to anthropogenic pollution, indicating therefore a shallow groundwater system.
The isotopic compositions of these waters (except sample 3, see Table 2), vary from -3.72 to -4.86%o and from -22.2 to -29.5%0 for ô180 and S2H respectively. The mean isotope values are -4.35 ± 0.3%o for ô180, and -26.5 ± 1.9%o for Ô2H. The low values of the standard deviations suggest that this groundwater system is spatially very homogeneous. Most of the wells display significant tritium levels (Table 2), which clearly indicates that corresponding groundwaters are recharged under present-day
Recharge processes of the upper Cretaceous aquifer of the Garoua basin, Cameroon 369
conditions. On a ô2H vs ô180 diagram (Fig. 5), these groundwater samples show some scatter
around the GMWL, suggesting that groundwaters were probably replenished by monsoon rains coming from the Gulf of Guinea. The location of some samples on the GMWL indicates that the isotopic compositions of corresponding groundwater have not been modified by evaporation prior to or during the infiltration of the recharge water within the aquifer. This fact suggests that recharge is an easy and rapid process throughout the aquifer. On the contrary, the location of other samples below the GMWL may be attributed to evaporation and/or infiltration of evaporatively enriched waters from the free surface water bodies. For this purpose, the occurrence of elevated amounts of tritium in wells 1, 4, 13, 16, 18 and 21, which are situated closer to Benue, Goulongo and Gourna rivers (Fig. 1), probably gives evidence of the contribution of these rivers to the recharge processes. For groundwater from well 3, the slight depletion of both ô180 and ô2H (Fig. 5, and Table 2) may probably be related to a different climatic condition at the time of recharge. This assumption is supported by the low tritium level ( < 1 TU) which clearly suggests a greater flow time after the infiltration. Moreover, the
Table 2 Chemical and isotopic composition of groundwaters from the Garoua sandstone aquifer in the Garoua basin (northern Cameroon).
No
GS1
GS2
GS3
GS4
GS5
GS6
GS7
GS8
GS9
GS10
GS11
GS12
GS13
GS14
GS15
GS16
GS17
GS18
OS 19
GS20
GS21
GS22
GS23
GS24
GS25
GS26
GS27
GS28
GS29
GS30
GS31
Sampling
sites
Gaschiga
Tapare
BS Manga
Kolléré
Pomla Manga
Pomla Pete!
Ouro Harissou
Nakong
Hosseré Faourou
Poukouloukou
Sonayo
Djalingo
Djamboutou
Sanguéré Paul
Sanguéré Ngaoundéré
Doult
Ouro Donka
Karewa
OuroLabo
Ngong
Bamé
L. Manawassi
L. Modibo
L. Bamaliki
L. Madilda
L. Dispensaire
L. Ecole
L. Fadil
L. Marché
Tchéboa
Touroua
Dates
07/91
07/91
12/91
12/91
12/91
12/91
12/91
07/91
07/91
07/91
12/91
12/91
12/91
07/91
07/91
07/91
07/91
07/91
07/9!
07/91
07/91
08/91
07/91
08/91
08/91
07/91
07/91
07/91
07/91
07/91
12/91
depths
(m)
30.5
_ 32.4
27.5
32.4
26.3
28.4
. 57.0
44.0
. 27.0
27.0
33.6
31.0
61.0
62.0
31.0
35.5
26.6
30.0
75.0
73.0
.
. 45.0
75.0
73.0
69.0
„
41.0
EC
us cm"'
218
138
294
331
160
105
28
60
72
112
204
32
24
90
63
402
60
106
133
120
229
_ 598
571
_ 767
756
701
812
_ 121
Ca
17.2
13.5
28.4
39.7
13.4
11.5
14.2
2.8
2.9
7.9
18.2
6.9
6.5
n.m
1.2
n.m
0.9
7.2
10.8
5.1
15.2
29.8
24.7
35.0
35.1
15.5
25.5
12.2
12.7
n.m
13.4
Mg
4.4
3.3
12.2
8.3
2.6
1.6
2.2
0.7
0.7
24
4.0
0.2
0.1
n.m
0.5
n.m
0.3
1.7
2.1
1.9
2.4
10.0
27.9
30.0
16.0
12.4
27.6
3.5
4.2
n.m
2.5
Na
20.5
6.4
11.8
7.0
3.8
9.2
3.0
2.4
2.6
3.5
11.2
4.1
3.8
n.m
2.3
n.m
2.1
4.0
9.9
4.1
31.9
23.0
68.5
47.5
80.0
137.6
103.5
159.5
159.5
n.m
6.6
K
1.8
14.8
19.0
21.1
15.1
4.8
3.9
6.1
6.2
9.1
4.6
5.2
4.8
n.m
5.2
n.m
5.9
4.2
2.8
7.4
3.9
5.9
7.7
6.2
6.1
6.9
7.2
4.8
6.4
n.m
9.3
Alk.
125.1
84.2
176.9
103.7
38.4
48.8
15.9
27.5
25.4
58.0
94.6
33.8
31.7
n.m
18.5
n.m
15.9
30.8
36.6
11.0
98.8
244.0
372.7
451.4
405.0
445.9
470.4
447.6
432.0
n.m
31.1
CI
1.1
0.3
2.8
21.4
6.4
2.5
4.8
0.3
0.2
0.3
0.7
0.6
0.5
1.4
0.2
0.8
0.3
3.3
3.4
2.5
3.3
0.5
0.2
0.3
1.1
0.2
0.2
1.1
3.3
5.9
3.9
SO4
0.8
1.3
11.5
5.3
0.9
2.4
0.9
0.9
0.7
0.9
1.4
1.0
0.8
0.9
0.7
3.1
0.7
1.4
1.0
1.3
3.8
0.6
1.2
0.4
1.0
2.8
1.7
6.7
10.6
7.3
1.4
NO3
1.7
2.0
2.5
50.7
38.4
12.4
41.1
0.8
0.8
2.7
7.6
3.3
1.8
6.6
0.7
16.9
0.9
20.5
28.5
32.9
22.8
0.0
0.7
0.0
0.0
1.7
1.8
1.8
0.9
3.8
42.3
51*0
-4.54
-4.36
-5.30
-3.84
-4.70
-3.95
-4.48
-4.8!
-4.69
-4.89
-4.34
-4.60
-3.72
-4.54
-4.51
-4.41
-4.25
-4.25
-4.21
-3.87
-4.16
-4.51
-4.74
-4.97
-5.16
-4.95
-5.39
-4.85
-5.11
-4.11
-4.12
52H
-29.3
-29.5
-33.1
-23.3
-27.6
-25.7
26.5
29.0
27.1
29.3
24.3
27.9
25.6
28.4
27.7
28.1
26.3
25.1
25.0
25.5
27.0
31.1
32.9
35.1
34.5
31.7
36.4
32.7
34.5
26.7
22.2
TU
21 ± 1
< 1
<1
14 ± 1
n.m
n.m
n.m
1.7 ± 0.4
n.m
<1.0
3.4 ± 0.4
nm
19.2 ± 0.7
3.0 ± 0.4
3.3 ± 0.4
8.6 ± 0.6
4.5 ± 0.5
12.8 ±0.6
n.m
24 ± 1
6.4 ± 0.5
0.7 ±0.4
a m
nrn
njn
2.4 ± 0.6
<1
0.7 ± 0.4
< 1
11 ± 1
2.2 ± 0.4
Chemical and isotope data arc expressed in mg I"* and relative to Vienna-SMOW respectively. EC « electrical conductivity L. = signifies Lamoudan. njn = not measured.
370 R. Njitchoua et al.
Fig. 4 Chemical composition of groundwaters from the upper Cretaceous Sandstone of the Garoua basin: (a) cation and (b) anions.
O
-lO-i
-20-
-30-
-40-
- 5 0 -
O low-salinity groundwaters
• Relatively higlvsalinity groundwaters
°J
y4Ss JÈ%
S'y j/m
(&1S0 = -6.6%o)
' "T ' "1 '
<éy y •JÙ- jS s-
f|ra>§/
, , ! , , -7 -2
Oxygen-18(%ovs. SMOW)
Fig. 5 Isotopic composition of groundwaters from the Garoua Sandstone aquifer.
available carbon-14 measurement gives an apparent age of 8500 years BP (Njitchoua, in preparation), suggesting a recharge period during the Holocene (Servant, 1973).
The high-salinity waters are chemically very homogeneous, being mainly dominated by Na+ and HCO J, accompanied sometimes by significant amounts of Mg2+ (Figs 4(a) and (b)). The concentrations of NO 3 in these groundwaters are low (Table 2), suggesting that this groundwater system is well protected from the surface water body. This groundwater system displays slightly depleted isotopic compositions, with values ranging from -4.74 to -5.16%o for ô180, and from -31.1 to -36.4%o for Ô2H (Table 2). In the ô2H vs ô180 diagram (Fig. 5), all of these samples are located along a defined trend having a slope of 5.7, which intercepts the GMWL at -6.52%o for oxygen-18. A similar value was encountered for paleo-waters from the Chad basin (Dray
Recharge processes of the upper Cretaceous aquifer of the Garoua basin, Cameroon 371
et al., 1983) located some hundreds of kilometres to the north of the study site. The assumption of old groundwater in the Garoua sandstone aquifer is supported by the carbon-14 content in the groundwater sample from Lamoudan Ecole, which gives an apparent age of 11000 years BP (Njitchoua et al., 1993), suggesting therefore a recharge during the Holocene. However, the occurrence of detectable amount of tritium in some wells (Table 2) probably indicates a mixing with a modern component water body.
CONCLUSIONS
Both the chemical and isotopic compositions of the groundwater from the Garoua sandstone aquifer indicate that, in most parts of the aquifer, the recharge took place under present-day conditions, either by direct infiltration of meteoric waters, or by infiltration of evaporatively enriched surface water bodies. This assumption is in good agreement with the unconfmed character of the aquifer. The isotopic study also gives evidence of mixing between an old component groundwater that was recharged during one of the humid periods of the Holocene, and probably an evaporatively enriched surface water body.
Acknowledgement The cooperation of the Section of Isotopic Hydrology of IAEA, the Centre de Recherches Géodynamiques de Thonon-les-Bains and the Société Anonyme de Gestion des Eaux de Paris (SAGEP) for their participation for some of the analytical programmes is acknowledged. R. Njitchoua expresses his sincere thanks to: P. Meyebe of the Garoua Meteorological service for the collection of rainwater samples; S. Watat and J. Njock both from the northern Cameroon provincial delegation of Mines, Water and Energy Ministry; M. Maucolin and A. Dudepant of the S AGEP.
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