The Wadi-Fira region in eastern Chad is facing dramatic water supply problems, related to the climatic semi-arid context and the reception of refugees from the Darfour, which has increased the local population by 22% these last years. Expansion of agglomerations (temporary new towns), development of agricultural and pastoral practices together with the augmentation of the population have led to dramatic water needs. The basement aquifer of Wadi-Fira constitutes the main source of water supply. However, little is known about this system. Within this context, this work aims at better understanding and identifying hydrogeochemical processes and their relations to groundwater quality within this complex environment, and groundwater recharge mechanisms. 31 groundwater samples were collected at two sites, Am Zoer and Guereda-Iriba, from hand dug wells and deep wells. Major chemical elements were analyzed on all samples and stables isotopes (oxygen-18 and deuterium) on 17 samples. Various methods were used to interpret the hydrochemical data (descriptive and multivariate statistics, Piper and Schoeller diagrams, scatter plots, minerals saturation indices). The stable isotopes were interpreted using conventional IAEA methods. The results permitted to differentiate the laterite reservoir from the deep fractured reservoir. The main process controlling groundwater mineralization is water-rocks interaction and natural minerals dissolution. Ion exchanges, evaporation and anthropogenic activities have also a moderate impact on groundwater quality. Based on isotopes data, it is concluded that groundwater in the basement aquifer is related with modern rainfall. These results provide further insights into this basement aquifer, which is a vital resource for the region of Wadi-Fira.
The Wadi-Fira region in eastern Chad is facing dramatic water supply prob-lems, related to the climatic semi-arid context and the reception of refugees from the Darfour, which has increased the local population by 22% these last years. Expansion of agglomerations (temporary new towns), development of agricultural and pastoral practices together with the augmentation of the pop-ulation have led to dramatic water needs. The basement aquifer of Wadi-Fira constitutes the main source of water supply. However, little is known about this system. Within this context, this work aims at better understanding and identifying hydrogeochemical processes and their relations to groundwater quality within this complex environment, and groundwater recharge mecha-nisms. 31 groundwater samples were collected at two sites, Am Zoer and Guereda-Iriba, from hand dug wells and deep wells. Major chemical elements were analyzed on all samples and stables isotopes (oxygen-18 and deuterium) on 17 samples. Various methods were used to interpret the hydrochemical data (descriptive and multivariate statistics, Piper and Schoeller diagrams, scatter plots, minerals saturation indices). The stable isotopes were interpreted using conventional IAEA methods. The results permitted to differentiate the laterite reservoir from the deep fractured reservoir. The main process controlling groundwater mineralization is water-rocks interaction and natural minerals dissolution. Ion exchanges, evaporation and anthropogenic activities have also a moderate impact on groundwater quality. Based on isotopes data, it is concluded that groundwater in the basement aquifer is related with modern rainfall. These results provide further insights into this basement aquifer, which is a vital resource for the region of Wadi-Fira.
Keywords:
Wadi-Fira, Chad, Basement Aquifer, Hydrochemistry, Stable Isotopes
The Wadi-Fira region in eastern Chad, adjacent to Sudan (
However, the basement aquifer of Wadi-Fira constitutes the main source of water supply for the populations of this region. The aquifer system has two main types of reservoirs, the alterites reservoir and the fractured reservoir. Most of the underground water resources exploited by deep wells and hand dug wells are located in alterites and basement fractured rocks. Sedimentary formations play a negligible role.
In 2003, the Wadi-Fira region hosted »201,000 refugees from Darfur out of a total of »924,000 residents that is 22% of the population. This migratory flow led to an expansion of agglomerations (temporary new towns), an increase in agricultural and pastoral practices. The groundwater of the crystalline system is thus now heavily exploited to meet the dramatically increasing water requirements in recent years.
The rational exploitation of this vital resource requires a prior assessment of
the processes of mineralization of the groundwater in order not to degrade its quality by a disorderly overexploitation.
Within this framework, the present investigation aims at better understanding and identifying hydrogeochemical processes and their relations to groundwater quality within this complex environment, and groundwater recharge mechanisms.
The geological formations encountered in the study area are mainly granitoids, schists, migmatites and volcano-sedimentary formations (basalts and alluvium) covering the crystalline basement. Two regions were sampled to conduct this work, the Am Zoer region and the Guereda-Iriba region.
The study area lies between latitudes 14˚ and 16˚North, and longitudes 21˚ and 23˚East. It covers an area of approximately 50,000 km². It is characterized by a Sahelo-Saharan climate, with a great spatial and temporal variability in precipitation, i.e. 150 to 400 mm of rain per year, between the North and the South. Most of the rainfall is concentrated over two months, between July and August. The Am Zoer site is located in the south-west, and that of Guereda-Iriba in the northeast (
The geological context of the study area (
The formations of the Precambrian basement identified in Chad, of which the Ouaddaï massif forms part, covers between 15% and 20% of the territory of the country [
From recent results, it is observed that in basement hard rocks, aquifers are located within the first tens of meters in the rocks [
In the Wadi-Fira study area, groundwater is exploited by shallow large-di- ameter wells (average depth »10 m) dug in the alterites and deeper wells penetrating the fractured aquifer (mean depth »45 m).
The hydrodynamic properties are very variable in space, showing the strong heterogeneity of this system. Transmissivities estimated by pumping tests on deep wells range between 1.9 × 10−6 to 8.8 × 10−4 m2/h. Discharge rates are generally low. More than 80% of wells have discharge rates between 0.5 and 5 m3/h. Flow rates exceeding 10 m3∙h−1 were obtained at Diker (Am Zoer site) and on two wells at Guereda. Such high discharges are linked respectively to a thick sedimentary layer (20 to 40 m) and a well-developed (10 and 26 m) weathered and/or fractured horizon.
The piezometric map is reported in
Two sampling campaigns were conducted in 2011 and 2015 for a total of 31
groundwater samples. The first campaign involved 15 deep wells. The analyses were carried out at the National Laboratory of Water Analysis (LNAE) in Ndjamena (Chad). The second field mission in July 2015 resulted in the sampling of 16 water samples from boreholes (9) and dug wells (7). The chemical and isotopic analyzes were carried out at the Center of Hydrogeology and Geothermics (CHYN) of the University of Neuchâtel in Switzerland.
During both field missions, the physical parameters (electrical conductivity, pH and temperature) of the groundwater were measured in situ. The data of the conductimeter during these two campaigns proved unfortunately wrong.
Other complementary parameters were also measured, the location and altitude of the water points using a Garmin GPS, the piezometric level and the depth of the wells respectively by a piezometric probe and a decametric tape.
Analyzes of the major ions, cations and anions were carried out respectively by ion chromatography Dionex DX-120 and Dionex-ICS-1600. The determination of the bicarbonate ion ( HCO 3 − ) was carried out by the titrimetry method using an apparatus of the Metrohm 805 Dosimat type. The quality of the analyses was verified by the calculation of the ion balance.
The isotopic analyzes were carried out only on the samples collected during the 2nd campaign in 2015. The stable isotopes contents of the water molecule are measured by the Picarro L2140-i analyzer, which performs simultaneous measurements of δ18O and δ2H with an accuracy of ±0.0015‰.
All stable isotopic composition (18O and ²H) are reported in standard δ‰ notation (part per thousand versus VSMOW (Vienna Standard Mean Ocean Water)) as follows:
δ ‰ = [ ( Rsample / Rstandard ) − 1 ] × 1000
where Rsample and Rstandard represent the ratio of heavy to light isotopes of the samples and standard, respectively [
The results of the physico-chemical and isotopic analyzes carried out on the 31 groundwater samples are interpreted using statistical, graphical and hydrogeochemical methods. Statistical interpretations were performed using box-plots, correlation matrice and Principal Component Analysis (PCA).
The hydrochemical classification of water is developed using Piper's trilinear diagram [
The Phreeqc geochemical program allowed to evaluate the saturation indices, to characterize the geochemical processes at the origin of the mineralization of groundwater.
Isotopic data on stable isotopes (oxygen-18 and deuterium) in groundwater have been interpreted using conventional IAEA methods [
The application of all these different methods to the hydrochemical and isotope data allowed to characterize the mechanisms of mineralization of groundwater and the recharge processes in the basement aquifer of the Wadi-Fira area.
The physico-chemical data of the groundwaters samples are reported in
Name | Long E | Lat N | Depth (m) | T˚C | pH | Ca2+ | Mg2+ | Na+ | K+ | HCO 3 − | Cl− | SO 4 2 − | F− | NO 3 − |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
AZ-F1 | 21.18 | 14.25 | 31.0 | 27.6 | 7.2 | 44.0 | 11.0 | 20.0 | 2.0 | 165.9 | 25.0 | 2.0 | 1.0 | 18.2 |
AZ-F2 | 21.2 | 14.25 | 33.0 | 27.6 | 7.1 | 61.0 | 11.5 | 30.0 | 3.0 | 244.0 | 25.0 | 13.0 | 1.0 | 20.0 |
AZ-F3 | 21.32 | 14.2 | 53.0 | 27.4 | 8.0 | 57.8 | 3.9 | 39.0 | 3.9 | 173.2 | 25.0 | 46.0 | 1.3 | 30.0 |
AZ-F4 | 21.27 | 14.22 | 70.0 | 30.7 | 8.7 | 56.0 | 4.9 | 32.0 | 3.2 | 195.2 | 30.0 | 13.0 | 0.8 | 20.2 |
AZ-F5 | 21.33 | 14.05 | 33.0 | 26.5 | 7.1 | 32.8 | 10.0 | 36.0 | 3.6 | 124.0 | 11.0 | 2.0 | 1.1 | 59.0 |
AZ-F6 | 21.32 | 14.18 | 51.0 | 32.9 | 7.1 | 88.8 | 0.5 | 24.0 | 2.4 | 317.2 | 30.0 | 2.0 | 1.4 | 20.2 |
AZ-F7 | 21.1 | 14.23 | 41.0 | 29.3 | 8.8 | 44.7 | 10.0 | 18.0 | 1.9 | 165.9 | 17.0 | 11.0 | 1.0 | 15.0 |
AZ-F8 | 21.15 | 14.33 | 41.0 | 30.4 | 7.6 | 33.6 | 8.7 | 27.0 | 5.2 | 159.8 | 15.0 | 15.0 | 1.0 | 2.7 |
AZ-F9 | 21.18 | 14.3 | 65.0 | 31.9 | 7.8 | 26.4 | 5.8 | 30.0 | 6.0 | 122.0 | 23.0 | 27.0 | 0.1 | 2.1 |
AZ-F10 | 21.18 | 14.3 | 59.0 | 32.4 | 7.6 | 42.4 | 4.6 | 28.0 | 5.0 | 170.8 | 20.0 | 20.0 | 0.2 | 3.4 |
AZ-F11 | 21.07 | 14.37 | 45.0 | 25.9 | 8.8 | 52.0 | 6.8 | 28.0 | 3.0 | 190.3 | 13.0 | 15.0 | 1.0 | 30.0 |
AZ-F12 | 21.08 | 14.37 | 45.0 | 28.0 | 8.5 | 79.2 | 30.0 | 42.0 | 9.0 | 366.0 | 45.0 | 54.0 | 1.0 | 2.3 |
AZ-F13 | 21.08 | 14.37 | 43.0 | 25.9 | 8.8 | 53.6 | 8.0 | 22.0 | 2.0 | 195.0 | 15.0 | 9.0 | 1.0 | 14.0 |
AZ-F14 | 21.38 | 14.17 | 29.0 | 30.4 | 8.9 | 69.6 | 3.4 | 25.0 | 2.5 | 219.6 | 30.0 | 15.3 | 1.3 | 10.1 |
AZ-F15 | 21.07 | 14.27 | 31.0 | 29.6 | 8.7 | 59.2 | 16.0 | 34.0 | 2.4 | 317.0 | 10.0 | 17.0 | 1.0 | 4.4 |
AZ-F16 | 21.39 | 14.22 | 37.0 | 28.5 | 6.1 | 24.5 | 7.3 | 18.2 | 1.1 | 125.2 | 12.1 | 9.6 | 1.4 | 9.4 |
AZ-F17 | 21.39 | 14.22 | 35.0 | 29.8 | 5.9 | 47.5 | 13.3 | 29.2 | 2.5 | 131.8 | 24.2 | 20.9 | 1.2 | 113.1 |
AZ-F18 | 21.07 | 14.43 | 37.8 | 27.5 | 7.2 | 25.3 | 5.1 | 32.2 | 1.6 | 125.2 | 3.7 | 10.0 | 1.3 | 26.4 |
AZ-P1 | 21.27 | 13.98 | 13.0 | 27.5 | 7.0 | 69.5 | 22.9 | 49.9 | 2.6 | 430.1 | 7.9 | 15.6 | 1.1 | 21.6 |
AZ-P2 | 21.39 | 14.23 | 5.6 | 27.0 | 5.6 | 13.9 | 3.7 | 7.0 | 6.4 | 71.1 | 3.2 | 4.6 | 0.3 | 26.6 |
GI-P3 | 22.24 | 15.12 | 13.7 | 24.7 | 6.6 | 24.1 | 4.9 | 12.6 | 4.4 | 132.1 | 2.9 | 6.2 | 0.4 | 8.1 |
GI-P4 | 22.24 | 15.13 | 11.9 | 26.1 | 6.5 | 28.7 | 7.0 | 21.3 | 4.5 | 185.9 | 2.7 | 3.0 | 0.5 | 2.9 |
GI-P5 | 22.23 | 15.14 | 9.6 | 26.1 | 5.2 | 22.8 | 5.7 | 10.7 | 8.3 | 79.3 | 12.7 | 15.1 | 0.3 | 28.0 |
GI-P6 | 22.34 | 14.81 | 10.5 | 26.5 | 5.1 | 14.6 | 4.7 | 9.6 | 4.5 | 78.0 | 4.2 | 7.0 | 0.3 | 24.4 |
GI-P7 | 22.08 | 14.51 | 6.9 | 26.1 | 4.6 | 10.6 | 3.0 | 6.3 | 6.3 | 53.4 | 3.6 | 5.3 | 0.3 | 17.2 |
GI-F19 | 22.23 | 15.09 | 42.0 | 27.8 | 6.7 | 27.9 | 9.0 | 33.0 | 2.3 | 199.5 | 4.0 | 7.2 | 0.5 | 17.5 |
GI-F20 | 22.11 | 15.13 | 62.0 | 30.9 | 6.0 | 49.4 | 12.9 | 47.9 | 1.8 | 312.2 | 6.5 | 13.2 | 2.2 | 17.2 |
GI-F21 | 22.12 | 15.12 | 39.0 | 29.5 | 5.9 | 32.4 | 9.7 | 31.7 | 1.3 | 224.5 | 3.6 | 5.8 | 1.6 | 7.6 |
GI-F22 | 22.07 | 14.51 | 66.0 | 16.5 | 5.1 | 14.9 | 3.4 | 16.1 | 2.3 | 103.0 | 3.0 | 4.1 | 1.4 | 5.7 |
GI-F23 | 22.11 | 14.51 | 35.0 | 28.7 | 6.2 | 48.0 | 13.7 | 92.1 | 0.8 | 423.5 | 11.1 | 11.9 | 2.9 | 11.5 |
GI-F24 | 21.95 | 14.48 | 58.0 | 28.0 | 6.5 | 65.1 | 18.3 | 32.1 | 2.6 | 282.1 | 23.6 | 13.7 | 1.1 | 51.6 |
The descriptive statistics (average, variance, coefficient of variation) are reported in
The pH of the groundwater, for both hand dug wells and deep wells ranges between 4.6 and 8.9 with an average of 7.0. More than half of the pH values (51%) are in the range 5.5 - 7.5. Such intermediate natural-water pH values implies a balance between the production of H+ ions from the dissociation of weak acids, and H+ (and CO2) consumption by weathering reactions, as the weathering of albite (Na-feldspar) to kaolinite [
2NaAlSi 3 O 8 + 2CO 2 + 11H 2 O → Al 2 Si 2 O 5 ( OH ) 4 + 2Na + + 4H 4 SiO 4 + 2HCO 3 − (1)
The weak acids involved in these processes are carbonic acid and organic acids such as fulvic acid [
Higher pH values are also observed in the study area with values in the range 7.5 - 8.5 common in groundwaters in hard rocks (basalt and granite). The maximum observed pH value for groundwaters is 8.9. High pH values occur in waters whose composition is dominated by minerals such as silicates and aluminosilicates, which tend to raise the pH [
The groundwater temperature varies from 16˚C to 33˚C with an average of 28˚C. These temperatures are close to the air temperature in the region of 29˚C, showing that recharge takes place by infiltration of rainfall, in the absence of
Parameter | Minimum | Maximum | Average | S.D. | CV % | WHO Standard |
---|---|---|---|---|---|---|
Ca2+ (mg/l) | 10.6 | 88.8 | 42.6 | 20.4 | 47.9 | − |
Mg2+ (mg/l) | 0.5 | 30.0 | 9.0 | 6.3 | 69.7 | 50 - 500 |
Na+ (mg/l) | 6.3 | 92.1 | 28.5 | 16.1 | 56.5 | 150 - 200 |
K+ (mg/l) | 0.8 | 9.0 | 3.5 | 2.0 | 58.2 | No-12 |
Cl− (mg/l) | 2.7 | 45.0 | 14.9 | 10.9 | 72.6 | 200 - 250 |
SO 4 2 − (mg/l) | 2.0 | 54.0 | 13.4 | 11.5 | 86.4 | 250 |
HCO 3 − (mg/l) | 53.4 | 430.1 | 196.2 | 99.1 | 50.5 | − |
NO 3 − (mg/l) | 2.1 | 113.1 | 20.7 | 21.7 | 105.2 | 50 |
F− (mg/l) | 0.1 | 2.9 | 1.0 | 0.6 | 58.7 | 1.5 |
T˚ (˚C) | 16.5 | 32.9 | 28.0 | 3.0 | 10.6 | |
pH | 4.6 | 8.9 | 7.0 | 1.3 | 18.0 |
very deep circulations.
An analysis of the hydrochemical data, carried out using Box-plots, is shown in
The box-plot (
Parameters | Depth | T˚C | pH | Ca2+ | Mg2+ | Na+ | K+ | HCO 3 − | Cl− | SO 4 2 − | F− | NO 3 − | TDS |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
DW. | 45.1 | 28.5 | 7.4 | 47.3 | 9.5 | 32.0 | 3.0 | 210.5 | 17.7 | 14.9 | 1.2 | 21.3 | 357 |
HDW. | 10.2 | 26.3 | 5.8 | 26.3 | 7.4 | 16.7 | 5.3 | 147.1 | 5.3 | 8.1 | 0.5 | 18.4 | 235 |
All box-plots show that concentrations of the elements are higher in deep wells than in wells dug in the alterites except for K+ and NO 3 − . This highlights the increasing mineralization of infiltrating waters and that mineralization processes are more pronounced in fractured rock. The average mineralization of dug wells water is lower (average TDS = 235 mg/l) than that of deep wells (average TDS = 357 mg/l).
Pearson correlation matrice [
Strong correlations are found between Ca2+ and HCO 3 − (R = 0.77), Mg2+ and HCO 3 − (R = 0.68), Na+ and HCO 3 − (R = 0.78), Ca2+ and Cl− (R = 0.73), Na+ and F− (R = 0.73). Strong relationships between Ca2+, Mg2+, HCO 3 − indicate contribution from dissolution of ( HCO 3 − ) 2 Ca 2 + or ( HCO 3 − ) 2 Mg . Relations between Ca2+ and Cl−, Na+ and HCO 3 − would result from complex processes discussed below.
Fluoride content is rather an important issue regarding potability of groundwater. According to the World Health Organization [
The high correlation between F− and Na+ should be noticed, providing an indication on the possible source of F−. Dissolution of fluoride-bearing minerals
Variable | Depth | T˚C | pH | Ca2+ | Mg2+ | Na+ | K+ | HCO 3 − | Cl− | SO 4 2 − | F− | NO 3 − |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth | 1.00 | |||||||||||
T˚C | 0.23 | 1.00 | ||||||||||
pH | 0.40 | 0.37 | 1.00 | |||||||||
Ca2+ | 0.35 | 0.44 | 0.61 | 1.00 | ||||||||
Mg2+ | 0.02 | 0.09 | 0.18 | 0.47 | 1.00 | |||||||
Na+ | 0.31 | 0.28 | 0.21 | 0.45 | 0.51 | 1.00 | ||||||
K+ | −0.28 | −0.05 | −0.12 | −0.19 | 0.05 | −0.34 | 1.00 | |||||
HCO 3 − | 0.21 | 0.32 | 0.33 | 0.77 | 0.68 | 0.77 | −0.29 | 1.00 | ||||
Cl− | 0.43 | 0.45 | 0.56 | 0.73 | 0.30 | 0.16 | 0.19 | 0.31 | 1.00 | |||
SO 4 2 − | 0.31 | 0.22 | 0.40 | 0.42 | 0.45 | 0.31 | 0.42 | 0.28 | 0.61 | 1.00 | ||
F− | 0.31 | 0.06 | 0.03 | 0.38 | 0.27 | 0.73 | −0.65 | 0.59 | 0.05 | 0.01 | 1.00 | |
NO 3 − | −0.06 | 0.02 | −0.20 | 0.07 | 0.10 | 0.01 | −0.12 | −0.16 | 0.11 | 0.00 | 0.07 | 1.00 |
such as cryolite (Na3AlF6) was depicted as a source of F- in hard rocks groundwater [
Cations that can exchange through ion exchange processes exhibit moderate correlation (Na+-Ca2+: R = 0.45; Na+-Mg2+: R = 0.51). This indicates that cation exchange processes can affect the groundwater chemical composition, but are not dominant processes.
Principal Component analysis is a well-established and widely used multivariate method in hydrochemical studies. As groundwater mineralization is a multivariate concept, analyzing the hydrochemical parameters pairwise, may lead to ignore relevant relationships [
The data processed by the ACP consists of 12 hydrochemical elements measured on 31 samples from the Am Zoer and Guéréda-Iriba sites. The eigenvalues and the percentages of variance explained by each Principal Component (PC) are shown in
The PC1 component with an expressed variance of 37.2% is the most important of all the factorial axes. The PC2 component expresses the greatest residual variance, i.e. 18.6% of the total variance. The factor plane PC1-PC2 expresses 58.8% of the cumulative variance. As can be seen on the screeplot (
PC1 | PC2 | PC3 | |
---|---|---|---|
T˚C | 0.480 | 0.130 | −0.077 |
pH | 0.776 | 0.095 | −0.335 |
Ca2+ | 0.812 | 0.426 | 0.064 |
Mg2+ | 0.403 | 0.339 | 0.640 |
Na+ | 0.285 | 0.787 | 0.288 |
K+ | 0.192 | −0.726 | 0.300 |
Cl− | 0.855 | −0.079 | 0.143 |
SO 4 2 − | 0.668 | −0.132 | 0.397 |
HCO 3 − | 0.513 | 0.748 | 0.186 |
NO 3 − | −0.157 | 0.009 | 0.545 |
F− | 0.015 | 0.862 | 0.087 |
The loadings of the variables are reported in
The results of the PCA are shown in
The component PC1, which has the highest variance (37.2%), is strongly associated with major chemical elements (Ca2+, Mg2+, Na+, HCO 3 − , Cl−). Correlations with the other elements (F− and SO 4 2 − ) also remain significant. It is noted that the physical parameters (temperature, pH, depth) are also well associated with this axis. NO 3 − and K+ are poorly represented on this axis. The component PC1 thus expresses the mineralization of the water in the study region.
The second component PC2 is positively associated with F− (0.71), and negatively with potassium (−0.86).
The analysis of the projection of the variables on the factor plane (PC1-PC2) reveals that the factorial axis PC1 gathers almost all the elements in the positive pole with the exception of potassium (K+) and NO 3 − . These two elements have no significant correlation with the set of variables represented in the correlation matrix. The axis CP2 is negatively correlated with the variables; Ca2+, Cl−, SO 4 2 − , pH, T˚C, and K+, the positive pole is represented by the variables; HCO 3 − , Na+, Mg2+, F−.
The spatial analysis of the variables of the PC1-PC2 plane makes it possible to classify, according to the affinities of the ions, two groups of variables. The first group is determined by HCO 3 − , Na+ and F−. The second group contains the variables pH, T˚C, SO 4 2 − and Cl−. The Ca2+, Mg2+ and Depth variables are intermediate to these 2 groups.
These links between the chemical elements are in agreement with the chemical facies derived from the Piper diagram, which are Ca 2 + -HCO 3 − , Na + -HCO 3 − , and Ca 2 + -SO 4 2 − -Cl − . This shows the acid hydrolysis of the silicate minerals present in the aquifer system and at the origin of the different ions. This cluster reflects a mineralization of groundwater due mostly to the water-rock interaction.
Nitrate and potassium do not correlate with the other elements, which presuppose that they are dissolved by different mechanisms. Nitrates have a superficial origin and are evidence of anthropogenic pollution.
The projection of the samples on the plane PC1-PC2 makes it possible to differentiate 2 very distinct clusters according to their mineralization. Waters flowing through the fractured reservoir are more mineralized than waters in the alterites. The latter also have a lower temperature. It is interesting to note that waters in the alterites remain undifferentiated on both sites (Am Zoer and Guereda-Iriba).
On the other hand, certain discrimination can be observed on the deep waters between the two sites. Accordingly, it can be noted that most of the deep wells at Am Zoer site have warmer, more basic waters, differenciated by SO 4 2 − and Cl− ions. Guereda-Iriba waters are more associated with Na+ and HCO 3 − elements, and especially with F−.
The alkaline earths (Ca2+, Mg2+) are present in both sites and do not discriminate between these two sites.
The analysis of the data from the 31 water samples shows the order of abundance of major ions as follows: calcium (51%) and bicarbonate (80%) are respectively the main cation and anion in the groundwater. The relative abundance in descending order for cations in groundwater is Ca2+ > Na+ > Mg2+ > K+ while that of the anions is HCO 3 − > Cl − > SO 4 2 − > NO 3 − . Since nitrates are not significantly correlated with any other element (indicating anthropogenic origin), they ae not considered in the Piper diagram (
The Piper diagram shows that the dominant anion is HCO 3 − at both sites. For the waters of most deep wells at Am Zoer site, the dominant cation is Ca2+. Waters from dug wells and some boreholes at Guereda-Iriba, are marked by the poles Ca2+ and Mg2+. A single well (F23) at Guereda-Iriba, is characterized by the Na+ pole.
The Piper diagram thus shows the following hydrochemical types:
- bicarbonate-calcium type secondary bicarbonate-sodium and bicarbonate- magnesium
- bicarbonate calcium type secondarily magnesium
- bicarbonate sodium type secondary bicarbonate-calcium and magnesium
These hydrochemical types are also represented on a semi-logarithmic Schoeller diagram [
Interpretation of ionic relations in the form of scatter plots can greatly help to understand the origin of solutes and the various hydrogeochemical processes involved in the groundwater chemistry.
These scatter plots are reported in
shows that in all samples these elements are very weakly correlated. All samples display an enrichment with Na+ compared to Cl−, indicating that these elements are not derived from the same source. The Na+/Cl− molar ratio is higher than 1 (1.2 < Na+/Cl− < 13.8), reflecting that silicate weathering can be the main contributor for Na+ [
The relation of Na+ associated to base exchanges process is discussed below. The presence of Cl− can be attributed to another source for this ion than the dissolution of halite and could be associated, at least partly, to anthropogenic sources (agricultural fertilizers).
The source for alkaline earth elements (Ca2+ and Mg2+) may be dissolution of silicate minerals (e.g., plagioclase feldspar, chlorite, or biotite), carbonates (dolomite or calcite), gypsum, and/or cation exchange of Na+ for Ca2+ and Mg2+ on clay minerals. HCO 3 − vs. Ca2+ (
The scatter plot HCO 3 − vs. Mg2+ (
In crystalline rocks, Ca2+ is found in the feldspar network, associated with sodium and potassium, while Mg2+ is associated with iron in ferromagnesian materials (micas, amphiboles, pyroxenes, peridots). Thus excess of Ca2+ in granitic rocks (
CaAl 2 Si 2 O 8 + 3H 2 O + 2CO 2 → Ca 2 + + 2HCO 3 − + Al 2 Si 2 O 5 ( OH ) 4 (2)
Weathering of ferromagnesian minerals such as biotite may be responsible for the release of Mg2+ in groundwater in basement rocks (reaction 3):
2K ( Fe , Mg ) 3 AlSi 3 O 1 0 ( OH ) 2 + 17H 2 O → Si 2 O 5 Al 2 ( OH ) 4 + 6FeO ( OH ) + 4Si ( OH ) 4 + 2K + + 2Mg 2 + + 2OH − ( Biotite ) ( Kaolinite ) ( Limonite / Goethite ) (3)
Additional geochemical processes such as the Ca2+ ↔ Mg2+ exchanges during precipitation reactions or calcite recrystallization [
Saturation indices of minerals indicate to which extent water mineralization is controlled by equilibrium with solid phases [
The software PHREEQC [
The saturation index of a mineral is obtained from Equation (5) [
SI = log ( IAP / Kt ) (4)
where IAP is the ion activity product of the dissociated chemical species in solution, Kt is the equilibrium solubility product for the chemical involved at the sample temperature.
The saturation indices of the following minerals in all water samples were evaluated: calcite, dolomite, gypsum and halite. The average values of the SI for each mineral are given in
The saturation indices regarding calcite and dolomite have high and negative values, expressing that the groundwater is undersaturated with these minerals.
The evaporitic minerals (gypsum, halite) also have negative SI values. These results show that the minerals in question (calcite, dolomite, gypsum and halite) are not the main source of the groundwater mineralization. This is in agreement with the conclusions derived above.
As the study is located in a semi-arid climatological context, it may expected that evaporation could also affect groundwater mineralization. The role of evapora
Mineral | Calcite | Dolomite | Gypsum | Halite |
---|---|---|---|---|
Average S.I. | −3.5 | −7.1 | −5.5 | −11.0 |
tion has been assessed using the Gibbs diagrams [
Gibbs diagrams represent the ratios of (Na+ + K+)/(Na+ + K+ + Ca2+) and Cl−/(Cl− + HCO 3 − ) as a function of TDS. They assume that evaporation increases salinity by increasing Na+ and Cl- contents in relation with increasing TDS. The diagram differentiates 3 areas highlighting various mechanisms: precipitation dominance, rocks dominance, evaporation dominance.
The plots of the hydrochemical data in Gibbs diagram are reported in
This figure shows that chemical weathering of rock-forming minerals is the most important functional source of dissolved elements in almost all groundwater samples. A few samples fall in the evaporation dominance area. These samples were collected from hand dug wells.
We may thus conclude that evaporation does not represent a major hydrochemical process in the study area.
Ions exchanges processes between the groundwater and its host rocks during its flow, can be analyzed using the plot of (Ca2+ + Mg2+) versus ( SO 4 2 − + HCO 3 − ). In this scatter diagram, the points close to the 1:1line (Ca2+ + Mg2+ = SO 4 2 − + HCO 3 − ) suggest that these ions have resulted from minerals dissolutions [
When groundwater has been in contact with clayey materials able to release the interchangeable alkaline-earth ions easily, then Na+ contained in groundwater can exchange with these alkaline-earth ions (Ca2+ and Mg2+) sorbed on the exchangeable sites. In such cases, groundwater is enriched with Ca2+ and Mg2+ and depleted in Na+. This process is called reverse ion exchange. Direct ion exchange may also occur (exchange between adsorbed Na+ or K+ with Mg2+ or Ca2+ in the groundwater).
The fact that ions exchanges processes between groundwater and the crystalline aquifer are relatively limited, is well in agreement with the previous findings.
The environment isotopes of oxygen d18O and hydrogen d2H are excellent tracers for determining the origin of groundwater and widely used in studying the natural water circulation and groundwater movement.
The groundwater has δ18 O values in the range of −5.06‰ to −0.39‰ and δ²H from −30.18‰ to +6.0‰ (
The local meteoric water line (LMWL) is defined at the Geneina station (capital of West Darfur in Sudan). Its equation is:
δ 2 H = 6 . 35 δ 18 O + 4 . 73 (5).
The stable isotopes values are plotted in
δ 2 H = 8.0 δ 18 O + 10 (6)
This diagram shows that the groundwater data are close to the GMWL meaning that the groundwater is in relation to modern rainfall.
This finding indicates that the groundwater recharge resource is mainly from precipitation and is weakly affected by evaporation.
Suc result is quite important since we can deduce that the renewal of groundwater is sufficiently rapid. Dating using C isotopes (13C, 14C) would help to determine the residence time of water in the aquifer system.
Locality | Code | Long | Lat | Alt (m) | δ²H vs. SMOW (‰) | δ18O vs. VSMOW (‰) |
---|---|---|---|---|---|---|
Am Zoer | F16 | 21.39 | 14.22 | 830 | −16.17 | −3.27 |
Am Zoer | F17 | 21.39 | 14.22 | 823 | −19.61 | −3.67 |
Diker | F18 | 21.07 | 14.43 | 598 | −30.18 | −5.06 |
Iraba | P1 | 21.27 | 13.98 | 727 | −22.79 | −4.06 |
Am Zoer | P2 | 21.39 | 14.23 | 823 | 6 | −0.39 |
Iriba | P3 | 22.24 | 15.12 | 923 | −22.82 | −4.16 |
Iriba | P4 | 22.24 | 15.13 | 938 | −23.6 | −4.35 |
Iriba | P5 | 22.23 | 15.14 | 942 | −24.53 | −4.51 |
Am nabak | P6 | 22.34 | 14.81 | 962 | −22.86 | −4.21 |
Guéréda | P7 | 22.08 | 14.51 | 986 | −7.11 | −2.13 |
Iriba | F19 | 22.23 | 15.09 | 914 | −24.79 | −4.51 |
Iridimi | F20 | 22.11 | 15.13 | 950 | −23.44 | −4.12 |
Iridimi | F21 | 22.12 | 15.12 | 954 | −20.56 | −3.86 |
Guéréda | F22 | 22.07 | 14.51 | 991 | −14.37 | −3.13 |
Guéréda | F23 | 22.11 | 14.51 | 1001 | −15.12 | −3.03 |
kounogo | F24 | 21.95 | 14.48 | 965 | −19.81 | −3.74 |
Groundwater is a vital resource for the Wadi-Fira region in eastern Chad. The water needs of this region of Chad have increased dramatically in recent years, given the increase in population and the development of agricultural and urban activities caused by an influx of refugees from neighboring Darfur.
The aquifer system exploited in this region is represented by the rocks of the crystalline basement. Groundwater is exploited using wells dug in the alterites and deep wells penetrating the fractured rocks. However, the available knowledge about the hydrogeology and the water quality of this system is still very low.
The objective of this work was to study the processes of groundwater mineralization and recharge mechanisms. Hydrochemical and isotopic analyses were carried out on water samples collected during two field campaigns organized in 2011 and 2015. The results of the analyses and their interpretations have shown that groundwater is on the whole fairly gentle. However, the mineralization of the deep waters of the fractured crystalline reservoir is more important than that of the waters of the alterites. The predominant process of mineralization is the interaction between water and rock and the solution of minerals. Other processes intervene (basic exchanges, precipitation, evaporation), but their impact is moderate. The isotopic data have shown that the groundwater of this system is related to modern precipitation. Recharging the system is relatively fast. This study has allowed to significantly improve knowledge on this complex system and laid the bases to consider a more rational exploitation of the resource.