Hydrochemical and Isotopic Assessment of Groundwater in the Goda Mountains Range System. Republic of Djibouti (Horn of Africa)

: The hydrogeological system of the Goda Mountains Range (GMR) in the Republic of Djibouti (Horn of Africa), hosted by volcanic and sedimentary formations, is the only water resource in the Tadjourah region for more than 85,000 inhabitants. Water needs are expected to drastically increase in the coming years, due to fast socio-economic development of the region. Accordingly, this system is under high pressure and should sustainably be exploited. However, little is known about the hydrogeology of this system. This study aims to improve the understanding of the hydrochemistry and the recharge processes of this system. The study is based on the combined interpretation of major ions, stable isotopes ( 18 O, 2 H), and radiogenic isotopes ( 3 H, 14 C). The interpretation of major ions contents using classical hydrochemical methods and principal component analysis highlighted that alteration of volcanic rocks minerals, coastal rainfall inﬁltration, and evaporation are the main processes from which groundwater acquires mineralization. Stable isotopes revealed that groundwater is of meteoric origin and has undergone high evaporation during inﬁltration. Radiogenic isotopes showed that groundwater in the basalts is mostly submodern to old, in relation with low hydraulic conductivity of the rocks and / or longer pathways through ﬁssures from outcrop to subsurface. Groundwater in the rhyolites is much younger compared to the basalts due to faster inﬁltration. The sedimentary part, in connection with the rhyolites, has younger waters compared to the basalts, but older compared to the rhyolites. The overall results show that GMR is a fairly complex hydrogeological system, containing a resource made up of a mixture of waters of di ﬀ erent ages. This study has made signiﬁcant progress in understanding this system and is an initial step towards the sustainable exploitation of resources.


Introduction
The Republic of Djibouti, located in the Horn of Africa, is facing precipitation scarcity and recurrent drought due to predominantly arid climate in East Africa. River flows are intermittent and therefore groundwater remains the unique water supply for the entire country. In addition to the climatic stress, the intensive exploitation caused by the fast growing of the population and of the socio-economic context leads to overexploitation and deterioration of the groundwater resource. The Goda Mountains Range (GMR), where this study is undertaken, is situated in the western part of the Tadjourah region and constitutes the only water resource [1] for all purposes (drinking, agriculture) for more than This series presents numerous apparent fractures and faults, mainly in the 160° N direction [33,34]. The sediments fill coastal plains with a mean thickness of 300 m, and mainly consist of the erosion products of both Mablas and Dalha series forming sands, clays, and conglomerates. In the study area, the GMR peaks at 1800 m at Day hills and dips toward the east to the Gulf of Tadjourah and towards the south to Asal lake (−155 m/sea level) ( Figure 2). Three main geomorphological entities match with the geological formations outcrops: (i) the Dalha series forms a plateau landscape, with some uplifted units (100 m high escarpment cliffs) and narrow valleys along tectonic lineaments; (ii) the Mablas series corresponds to the eastern hillslope of the GMR with a steep slope (>20°) and several marked valleys; and (iii) the sediments are related to a flat landscape with large spreading features at the outlet of the mountain valleys. The precipitation is sporadic and related to thunderstorm events. The hydrogeological system of the GMR includes three interconnected aquifers corresponding each to a given geological formation. From upstream to downstream, we find the basalts aquifer, the rhyolites aquifer, and the sedimentary aquifer ending at the sea.
The mean annual rainfall ranges from 350 mm at the top of hill to 150 mm in the coastal area ( Figure 2). Annual rainfall displays high variablility with years of drought followed by extreme rain events. These extreme pluviometric events in fact represent very favorable episodes to the recharge of aquifers in such arid contexts [35][36][37][38]  This series presents numerous apparent fractures and faults, mainly in the 160 • N direction [33,34]. The sediments fill coastal plains with a mean thickness of 300 m, and mainly consist of the erosion products of both Mablas and Dalha series forming sands, clays, and conglomerates. In the study area, the GMR peaks at 1800 m at Day hills and dips toward the east to the Gulf of Tadjourah and towards the south to Asal lake (−155 m/sea level) ( Figure 2). Three main geomorphological entities match with the geological formations outcrops: (i) the Dalha series forms a plateau landscape, with some uplifted units (100 m high escarpment cliffs) and narrow valleys along tectonic lineaments; (ii) the Mablas series corresponds to the eastern hillslope of the GMR with a steep slope (>20 • ) and several marked valleys; and (iii) the sediments are related to a flat landscape with large spreading features at the outlet of the mountain valleys. The precipitation is sporadic and related to thunderstorm events. The hydrogeological system of the GMR includes three interconnected aquifers corresponding each to a given geological formation. From upstream to downstream, we find the basalts aquifer, the rhyolites aquifer, and the sedimentary aquifer ending at the sea.
The mean annual rainfall ranges from 350 mm at the top of hill to 150 mm in the coastal area ( Figure 2). Annual rainfall displays high variablility with years of drought followed by extreme rain The mean annual temperature in the city of Tadjourah is about 29 °C and the minimum potential evapotranspiration is estimated to be 1500 mm year −1 in the GMR. This arid climatic condition does not allow any development of the vegetal cover, except on the eastern hillslope of the GMR where the largest Primary forest of the country still exists (Day forest, 900 ha) ( Figure 2). Surface flows are temporary and occur in the main valleys only after storm events. In addition, several springs emerge from fractures or from the Dalha-Mablas interface. Water supply for rural population and livestock thus comes from shallow wells in the GMR valleys and some deeper boreholes in the Dalha series. In the coastal area, some boreholes exploit the sedimentary basin, for water supply of Tadjourah city.

Materials and Methods
A list of 43 water points was sampled in the study area, located in the Mablas series (13 water points), in the Dalha series (21), and in the sedimentary unit (9) (Figure 2). These water points correspond to 13 deep boreholes (depth > 50 m), 21 shallow wells (depth < 30 m), and nine springs. Water temperature, pH, and electrical conductivity (EC) were measured in-situ with a multiparameters device (Multi 340i). Water was sampled in polyethylene flacons of 500 mL, previously washed with nitric acid at 10% and rinsed with deionized ultra-pure water. For boreholes and wells, the water was collected after 1 h of pumping in order to clear out the stored water. All samples were analyzed in the Laboratory of Geochemistry of CERD (Centre d'Etudes et de Recherche de Djibouti, National Research Center of Djibouti) for major ions with ion chromatography (model ICS Dionex300). Each collected sample was filtered through 0.45 µm membrane filters to avoid the suspended particles and the flacons filled for cations analysis were acidified with 1 M HCl. The bicarbonate ion (HCO3 − ) was determined by titration with 0.1 M HCl and the silica content was determined by spectrophotometry (Model Jenway 6300). The geochemical data are provided in Table  1. All samples were checked for accuracy by evaluating their Charge Balance Error (CBE) [40] given by Equation (1): The mean annual temperature in the city of Tadjourah is about 29 • C and the minimum potential evapotranspiration is estimated to be 1500 mm year −1 in the GMR. This arid climatic condition does not allow any development of the vegetal cover, except on the eastern hillslope of the GMR where the largest Primary forest of the country still exists (Day forest, 900 ha) ( Figure 2). Surface flows are temporary and occur in the main valleys only after storm events. In addition, several springs emerge from fractures or from the Dalha-Mablas interface. Water supply for rural population and livestock thus comes from shallow wells in the GMR valleys and some deeper boreholes in the Dalha series. In the coastal area, some boreholes exploit the sedimentary basin, for water supply of Tadjourah city.

Materials and Methods
A list of 43 water points was sampled in the study area, located in the Mablas series (13 water points), in the Dalha series (21), and in the sedimentary unit (9) (Figure 2). These water points correspond to 13 deep boreholes (depth > 50 m), 21 shallow wells (depth < 30 m), and nine springs. Water temperature, pH, and electrical conductivity (EC) were measured in-situ with a multi-parameters device (Multi 340i). Water was sampled in polyethylene flacons of 500 mL, previously washed with nitric acid at 10% and rinsed with deionized ultra-pure water. For boreholes and wells, the water was collected after 1 h of pumping in order to clear out the stored water. All samples were analyzed in the Laboratory of Geochemistry of CERD (Centre d'Etudes et de Recherche de Djibouti, National Research Center of Djibouti) for major ions with ion chromatography (model ICS Dionex300). Each collected sample was filtered through 0.45 µm membrane filters to avoid the suspended particles and the flacons filled for cations analysis were acidified with 1 M HCl. The bicarbonate ion (HCO 3 − ) was determined by titration with 0.1 M HCl and the silica content was determined by spectrophotometry (Model Jenway 6300). The geochemical data are provided in Table 1. All samples were checked for accuracy by evaluating their Charge Balance Error (CBE) [40] given by Equation (1):  14 C) were performed at the Environmental Isotope Laboratory of University of Waterloo (Canada). The activity of 14 C were measured by AMS and the results are expressed as percentage of modern carbon (pMC) with an average error of ±0.39 pMC. Tritium content was investigated until the limit of 0.8 TU (Tritium Unit). Isotopes data are given in Tables 2 and 3.

Descriptive Statistics
Descriptive statistics of the variables are given in Table 4. Electrical conductivity (EC) varies between 582 and 2470 µS/cm with an average value of 1283 µS/cm and a standard deviation of 497 µS/cm. The variation coefficient amounts to 39% showing that EC values are not much variable at the scale of the whole system. We can however note a spatial differentiation according to the type of rock. Saline waters showing higher electrical conductivity values are found in the sedimentary aquifer (average: 1443 µS/cm). Waters with lower electrical conductivity are located in the central rhyolites aquifer (average: 1124 µS/cm), while the upstream basalts aquifer, contains slightly saline water (average: 1313 µS/cm). pH is a parameter which distribution remains almost homogeneous throughout the massif. Average values of pH range between 7.6 and 7.7 for the whole system. Table 4. Descriptive statistics of hydrochemical data. GMR: Goda Mountains Range. SD: standard deviation. CV: coefficient of variation. All chemical variables are expressed in mg/L. The groundwater temperature varies between 19 and 47 • C, with an average of 27 • C. The maximum temperature is quite high (47 • C) and was recorded at Ngoubeth well. We note that the groundwater temperature exceeds 30 • C in six wells. High groundwater temperature has been observed in other aquifers in Djibouti, the Gulf basalts aquifer [41], the SW Dalha basalts aquifer [10]. These high temperatures have been associated with the geodynamic context of Djibouti and possible heat sources. In the case of GMR, the stable isotopes of Ngoubet well ( 18 O = 3.5% and 2 H = 15% ) show that these are meteoric waters, which excludes the hypothesis of thermal water rise. Additional data will be needed (including deep drilling) to explore this question further.
Bicarbonate ion (HCO 3 − ) is the dominant anion, followed by chloride (Cl − ) and sulphate (SO 4 2− ) in the whole system and within each rock type. HCO 3 − concentration ranges between 128 and 417 mg/L. Three out of 13 rhyolites waters (about 23%) show values above the WHO-admitted nominal level of 50 mg/L. In basalts, 14 out of 21 waters (67%) exceeds this limit and in the sedimentary aquifer, five out of nine waters (about 56%). In low-populated arid climate areas with low agricultural activity, high nitrate concentrations are not well elucidated [42][43][44][45]. Given the significant depths of boreholes (100-250 m) where such high concentrations are found, anthropogenic pollution is unlikely, as agricultural activities using chemical fertilizers are for the time being not significant. Population density remains low in the Goda massif and there are no large-scale sewers or septic tanks. High nitrate concentrations in arid environment without significant agriculture have been associated to the degradation of organic matter in groundwaters. Indeed, some plants such as Acacias, which are widespread on the Goda massif and have a high nitrogen fixation capacity and whose roots can go down to significant depths (more than 40 m) could be the cause of nitrate concentration excess in deep boreholes [44,46]. With the exception of a few water points in the volcanic formations with potassium (K + ) content above 10 mg/L, the overall concentration of K + is lower than 4 mg/L. Average TDS (Total Dissolved Solids) for the whole GMR system is 860 mg/L. High TDS is found in the sedimentary rocks (average: 926 mg/L) and low TDS in the rhyolites (average: 764 mg/L). Statistical analysis of the ions content in the groundwater enables to conclude that diluted waters presenting low mineralization are located in the rhyolitic formation compared to the waters extracted from adjacent basaltic and sedimentary geological formations.

Correlation Matrix
The correlation matrix between all hydrochemical parameters is provided in Table 5. As the dataset size is n = 43, the correlation coefficient R is significant at a level of 1%, if its value is ≥0. 39. At a level of 0.1%, R is significant if its value is ≥0.48 [47]. Several relevant hydrochemical relationships (R ≥ 0.39), shown by values highlighted in bold, can accordingly be distinguished in Table 5. Electrical conductivity (EC) is significantly and highly correlated with Cl − (R = 0.93),

Principal Components Analysis
Principal components analysis (PCA) is a widely applied multivariate statistical technique in hydrochemistry. Its purpose is to identify important components or factors accounting for most of the variance in a dataset [48][49][50]. To maximize the variance of the principal factors, a rotation of the factors can be performed. The Varimax normalized rotation was applied in this study [51]. To perform PCA, TDS (total dissolved solutes) and pH were removed from the dataset, as the former holds the same information as EC (electrical conductivity) and the latter shows no variability in the study area (pH CV = 4%).
PCA of the dataset resulted in three factors accounting for 75% of the total variance of the dataset. The factors loadings, and their respective explained variance, are shown in  Figure 3 shows the plot of variables on the plane associated with factors F1 and F2. The plot of the variables on the plane F1-F2 highlights clustering of the variables in connection with their origin. The variables Cl − , SO 4 2− , and Na + , strongly associated with EC, are the main factor controlling the chemical composition of waters. Ca 2+ and Mg 2+ can be grouped in association with a common origin. HCO 3 − and SiO 2 form a group opposite to temperature, expressing that waters with lower temperature are much characterized with these elements. NO 3 − is not significant on the plane F1-F2. Figure 4 shows the samples plot on the plane associated with factors F1-F2. Factor 1 clearly differentiates waters according to their mineralization. Waters of the sedimentary aquifer are mainly located towards the pole (Na + , Cl − , SO 4 2− ) and rhyolites waters are mainly located towards the opposite pole characterizing waters with lower mineralization. Basalt waters are evenly distributed between both poles. On factor 2, we note that rhyolites waters are mainly located towards the pole (HCO 3 − , SiO 2 ).
Water 2020, 12, x FOR PEER REVIEW 11 of 24 The plot of the variables on the plane F1-F2 highlights clustering of the variables in connection with their origin. The variables Cl − , SO4 2− , and Na + , strongly associated with EC, are the main factor controlling the chemical composition of waters. Ca 2+ and Mg 2+ can be grouped in association with a common origin. HCO3 − and SiO2 form a group opposite to temperature, expressing that waters with lower temperature are much characterized with these elements. NO3 − is not significant on the plane F1-F2. Figure 4 shows the samples plot on the plane associated with factors F1-F2. Factor 1 clearly differentiates waters according to their mineralization. Waters of the sedimentary aquifer are mainly located towards the pole (Na + , Cl − , SO4 2− ) and rhyolites waters are mainly located towards the opposite pole characterizing waters with lower mineralization. Basalt waters are evenly distributed between both poles. On factor 2, we note that rhyolites waters are mainly located towards the pole (HCO3 − , SiO2).

Piper Diagram
The Piper diagram [39] is a commonly used method to represent several water samples simultaneously. It allows to compare groups of samples according to the relative content of ions with each other and indicates the types of dominant cations and anions. The MGR water samples are characterized by three main facies ( Figure 5).

Piper Diagram
The Piper diagram [39] is a commonly used method to represent several water samples simultaneously. It allows to compare groups of samples according to the relative content of ions with each other and indicates the types of dominant cations and anions. The MGR water samples are characterized by three main facies ( Figure 5).

Piper Diagram
The Piper diagram [39] is a commonly used method to represent several water samples simultaneously. It allows to compare groups of samples according to the relative content of ions with each other and indicates the types of dominant cations and anions. The MGR water samples are characterized by three main facies ( Figure 5).  Chloride-Sodium facies (Cl-Na) is the dominant facies with 37% of the samples which are generally located in basaltic and sedimentary formations. About 32% of the samples have a mixed Chloride-Calcium-Magnesium facies (Cl-Ca-Mg) and are mostly clustered in the basaltic aquifer. A total of 31% of the samples, located mainly in the rhyolitic aquifer, present a HCO 3 -Ca-Mg facies. The Chloride-Sodium facies is observed from upstream to downstream. The transition from Cl-Na facies to mixed facies Cl-Ca-Mg may be caused by the process of direct ion exchanges between water and rocks. This process was investigated using the chloro-alkaline indices [52] CA1 and CA2, which are expressed following Equations (2) and (3): Positive values of these indices indicate that alkaline ions (Na + and K + ) in groundwater are exchanged by alkaline earths ions (Ca 2+ and Mg 2+ ) contained in weathered rocks. These two indices have positive (or close to zero) values ( Table 1) and confirm that Na + and K + contained in the water are replaced by the Ca 2+ and Mg 2+ of the matrix following ion exchanges process. Na + in groundwater, as discussed below, is attributed to coastal rainfall and atmospheric deposition. Calcic/magnesic minerals like anorthite and biotite may release Ca 2+ and Mg 2+ following these exchange reactions.
The samples displaying a Bicarbonate-Calcium-Magnesium facies (HCO 3 -Ca-Mg) are located mainly in the rhyolites aquifer of the Goda massif, between 1000 and 400 m. The rhyolites aquifer, containing waters with lower mineralization and low residence time (see Isotopes interpretation below), constitutes a preferential recharge area of the whole GMR hydrogeological system. The reaction of rainwater with the CO 2 of the soil, during infiltration, releases bicarbonates (HCO 3 − ) [53]: Indeed, waters in the rhyolites aquifer have the highest concentrations in HCO 3 − compared to the basalts aquifer and the sedimentary aquifer of the GMR system.

Hydrogeochemical Processes
The results from the chemical analyses were used to determine the hydrogeochemical processes which are responsible for the mineralization of the GMR groundwater. In arid and semi-arid regions, it is expected that the evaporation process plays an important role and increases the concentrations of all elements in groundwater. When evaporation is dominant, the molar ratio Na/Cl is unchanged [54]. In such cases, the plot of Na/Cl versus EC is represented by a horizontal line. The Na/Cl versus EC plot ( Figure 6) displays a linear trend around a horizontal line (Na/Cl = 1) meaning that evaporation is a significant process in this area. However, as data are more or less scattered around the horizontal line, other processes are also involved in water mineralization. This is also confirmed by the Gibbs diagrams [55] where TDS is plotted versus Na + /(Na + + Ca ++ ) and versus Cl − /(Cl − + HCO 3 − ). This diagram divides the space into three fields according to the evolution of the ratios Na + /(Na + + Ca ++ ) and Cl − /(Cl − + HCO 3 − ) (known as Gibbs ratios) vs. TDS. Each of the fields is characteristic of a dominant process governing groundwater mineralization (rainfall dominance, water-rocks dominance, evaporation dominance). This approach has been quite often applied to assess groundwater mineralization [56,57]. Both diagrams (Figure 7) indicates that water-rocks interaction (weathering) also makes a significant contribution to water mineralization.  The mineralization processes were analyzed using bivariate plots, which display relationships between dissolved elements in groundwater. These plots provide significant information about the possible processes which account for groundwater chemistry. The plot of Na + versus Cl − is shown in Figure 8a. When halite dissolution is responsible for sodium and chloride, the Na/Cl molar ratio is approximately equal to one. When this ratio is greater than one, it typically indicates a Na + release from silicate weathering reactions [58,59].  The mineralization processes were analyzed using bivariate plots, which display relationships between dissolved elements in groundwater. These plots provide significant information about the possible processes which account for groundwater chemistry. The plot of Na + versus Cl − is shown in Figure 8a. When halite dissolution is responsible for sodium and chloride, the Na/Cl molar ratio is approximately equal to one. When this ratio is greater than one, it typically indicates a Na + release from silicate weathering reactions [58,59]. The mineralization processes were analyzed using bivariate plots, which display relationships between dissolved elements in groundwater. These plots provide significant information about the possible processes which account for groundwater chemistry. The plot of Na + versus Cl − is shown in . When halite dissolution is responsible for sodium and chloride, the Na/Cl molar ratio is approximately equal to one. When this ratio is greater than one, it typically indicates a Na + release from silicate weathering reactions [58,59]. In the present study, the Na + -Cl − diagram (Figure 8a) displays a linear trend between these elements but reveals an excess of Na + over Cl − . Most of the samples fall above the seawater line (Na In the present study, the Na + -Cl − diagram (Figure 8a) displays a linear trend between these elements but reveals an excess of Na + over Cl − . Most of the samples fall above the seawater line (Na = 0.86 × Cl, [60,61]) and also above the halite dissolution line (1:1). Contribution of halite to these elements in groundwaters can be excluded in the context of the GMR. Coastal rainfall and atmospheric deposition appear as a major source of both sodium and chloride, whose concentrations increase under evaporation action. In the coastal sedimentary aquifer of the GMR seawater intrusion due to overexploitation of the aquifer can also be a source of sodium and chloride. This is discussed below.
The role of evaporation is further confirmed by analyzing the plot of Cl − versus Br − (Figure 8b). Chloride and Bromide have chemically similar properties. Both are conservative ions that do not participate in exchange reactions in the aquifer, are not affected by redox reactions and do not form solid compounds [62][63][64]. In seawater, Cl − is the most abundant chemical constituent and the concentration of Br-is much less. Their SMOW (Standard Mean Ocean Water) concentrations are Cl = 545.13 meq/L and Br = 0.84 meq/L. The SMOW Cl/Br ratio is about 649. Note that for halite, this ratio is very high (Cl/Br > 4000, [62]). In the present study, the ratio Cl/Br ranges from 125 to 755, excluding any halite dissolution. The plot of Cl − versus Br − (Figure 8b) shows that most samples (90%) are close to the seawater line (Cl − = 649 × Br − ), indicating again that the source of these elements in groundwater comes from coastal rainfall and evaporation effect. The plot also indicates that seawater intrusion in the sedimentary aquifer can be a source for these elements.
Samples which are much below the SMOW line, located in the basalts (two samples), in the rhyolites (two samples), and the sediments (one sample), are probably more affected by anthropogenic causes (waste waters, agricultural activities). The average value of the ratio Cl/Br in the basalts, rhyolites and sediments is, respectively, 491, 488, and 599. We note that the ratio value is almost the same in the basalts and the rhyolites. However, it is significantly higher in the sediments.
This may be a clue for seawater intrusion in the sedimentary aquifer of the system [65,66]. The plot of the Cl/Br ratio versus the distance to the sea of the water points ( Figure 9) shows that the wells in the sedimentary rocks within 1 km distance to the sea have a ratio close to the seawater ratio. These wells are likely to be affected by seawater intrusion. A previous study [37] focused on the sedimentary aquifer showed that seawater intrusion is effectively a serious issue to be considered within the sustainable exploitation of the system. = 0.86 × Cl, [60,61]) and also above the halite dissolution line (1:1). Contribution of halite to these elements in groundwaters can be excluded in the context of the GMR. Coastal rainfall and atmospheric deposition appear as a major source of both sodium and chloride, whose concentrations increase under evaporation action. In the coastal sedimentary aquifer of the GMR seawater intrusion due to overexploitation of the aquifer can also be a source of sodium and chloride. This is discussed below.
The role of evaporation is further confirmed by analyzing the plot of Cl − versus Br − (Figure 8b). Chloride and Bromide have chemically similar properties. Both are conservative ions that do not participate in exchange reactions in the aquifer, are not affected by redox reactions and do not form solid compounds [62][63][64]. In seawater, Cl − is the most abundant chemical constituent and the concentration of Br-is much less. Their SMOW (Standard Mean Ocean Water) concentrations are Cl = 545.13 meq/L and Br = 0.84 meq/L. The SMOW Cl/Br ratio is about 649. Note that for halite, this ratio is very high (Cl/Br > 4000, [62]). In the present study, the ratio Cl/Br ranges from 125 to 755, excluding any halite dissolution. The plot of Cl − versus Br − (Figure 8b) shows that most samples (90%) are close to the seawater line (Cl -= 649 × Br − ), indicating again that the source of these elements in groundwater comes from coastal rainfall and evaporation effect. The plot also indicates that seawater intrusion in the sedimentary aquifer can be a source for these elements.
Samples which are much below the SMOW line, located in the basalts (two samples), in the rhyolites (two samples), and the sediments (one sample), are probably more affected by anthropogenic causes (waste waters, agricultural activities). The average value of the ratio Cl/Br in the basalts, rhyolites and sediments is, respectively, 491, 488, and 599. We note that the ratio value is almost the same in the basalts and the rhyolites. However, it is significantly higher in the sediments.
This may be a clue for seawater intrusion in the sedimentary aquifer of the system [65,66]. The plot of the Cl/Br ratio versus the distance to the sea of the water points ( Figure 9) shows that the wells in the sedimentary rocks within 1 km distance to the sea have a ratio close to the seawater ratio. These wells are likely to be affected by seawater intrusion. A previous study [37] focused on the sedimentary aquifer showed that seawater intrusion is effectively a serious issue to be considered within the sustainable exploitation of the system. The Na/Cl molar ratio is unbalanced in favor of sodium, the excess of this element is due to the weathering of the volcanic rocks. In volcanic formations sodium is a significant element of alkali feldspars, as albite [67][68][69]. The reactions of silicate hydrolysis, like albite, are slow and occur in the presence of carbon dioxide (CO2) and produce an amorphous phase (Kaolinite), alkaline cations (Na + , Ca 2+ ), and bicarbonate (HCO3 − ): The Na/Cl molar ratio is unbalanced in favor of sodium, the excess of this element is due to the weathering of the volcanic rocks. In volcanic formations sodium is a significant element of alkali feldspars, as albite [67][68][69]. The reactions of silicate hydrolysis, like albite, are slow and occur in the presence of carbon dioxide (CO 2 ) and produce an amorphous phase (Kaolinite), alkaline cations (Na + , Ca 2+ ), and bicarbonate (HCO 3 − ): 2 When silicate weathering is a major process, the bicarbonate ion HCO 3 − is dominant in groundwater (Equations (5)- (7)) [70]. In the GMR system, bicarbonate ion is indeed the dominant anion, which highlights the major role of silicate hydrolysis in groundwater mineralization. The silicate minerals present in volcanic rocks, the alteration of which can lead to the release of Ca 2+ and Mg 2+ , are mainly anorthite and biotite [33]. The hydrolysis equations for anorthite and biotite are written, respectively [71,72]: The common origin of Ca 2+ and Mg 2+ , from the alteration of silicate minerals, is confirmed by the strong correlation between the contents of these two elements in GMR groundwater (R = 0.60). The plot of Ca 2+ versus Mg 2+ (Figure 8c) shows an excess of Ca 2+ in almost all samples. Calcium can also come from a source other than the hydrolysis of silicate minerals. Previous studies [73] have shown that fractures in outcropping volcanic rocks can be filled with calcite deposits. The dissolution of calcite during infiltration may explain this excess of Ca 2+ compared to Mg 2+ .
Further, when the silicate weathering controls principally the ionic concentration of groundwater, the ratio of HCO 3 − to the total cations concentration (TC) in groundwater would be around one [74].
The plot of HCO 3 − versus TC (Total Cations) reveals that all samples fall below the line 1:1 (Figure 8d) indicating that other minerals, in addition to silicates, are also involved, to a lesser extent, in the mineralization processes. This militates in favor of calcite dissolution present in the rock fractures. The correlation between sulphate and calcium is weak (R = 0.36). It is not significant even at a 1% threshold (R > 0.39). This excludes the dissolution of a mineral such as gypsum. The correlation table (Table 5)  deposits of sodium sulphate (Thenardite Na 2 SO 4 , Glaserite K 2 Na 2 SO 4 ) can occur [75]. The dissolution of these deposits may contribute SO 4 2− in the groundwater of the GMR. Figure 8e shows the plot of Na + versus SO 4 2− .

Stable Isotopes
The contents of stable isotopes (See Table 2) are varying in space. δ 18 O values vary between −2.31% and 0.06% while those of δ 2 H vary between −9.96% and 3.38% . Due to the lack of reference meteoric data in Djibouti, we took as reference Local Meteoric Water Line (LMWL), the line defined from isotopic reference data at Addis Ababa station (Ethiopia) near the study area, measured by the IAEA (International Atomic Energy Agency) [76]. The Global Meteoric Water Line (GMWL [77]) was also considered. Equations of both reference lines are written as follows: LMWL: δ 2 H = 7.2 δ 18 O + 12 A plot of the stable isotopes data is shown in Figure 10. Groundwater samples of the GMR are located below or close to both lines LMWL and GMWL, indicating that the GMR groundwater is of meteoric origin. Two clusters can be identified. Most of the samples belong to cluster 1, in which waters are enriched in stable isotopes. This highlights the role of evaporation in the recharge processes. The same conclusion was reached previously from major ions analysis. Two samples, from the basalts, are grouped in cluster 2. They are located below and close to the Global Meteoric Water Line (GMWL) and are depleted in heavy isotopes. These samples are from Tewele and Itki wells located, respectively, at 885 and 1458 m/asl. Low contents in stable isotopes may indicate that they are affected by slight evaporation or a high recharge altitude.
Water 2020, 12, x FOR PEER REVIEW 18 of 24 the basalts, are grouped in cluster 2. They are located below and close to the Global Meteoric Water Line (GMWL) and are depleted in heavy isotopes. These samples are from Tewele and Itki wells located, respectively, at 885 and 1458 m/asl. Low contents in stable isotopes may indicate that they are affected by slight evaporation or a high recharge altitude.

Radioactive Isotopes 3 H and 14 C
Tritium ( 3 H) and radiocarbon ( 14 C) are radioactive isotopes widely used in hydrogeology to assess the 'age' of groundwater, i.e., the time passed since the recharge or the average subsurface residence time. The groundwater residence time is an important factor as it is related to the renewability of the groundwater resources. Groundwaters are classified as 'modern' if their age is roughly less than 50 years. Groundwaters between about 50 and 1000 years old are called 'submodern'. Beyond 1000 years, they are termed "old" [28]. Knowing whether waters are modern, submodern, or old is critical for their management and sustainability. Tritium (3H) is a radioactive isotope of hydrogen with a half-life estimated at 12.32 years. Its content in waters are expressed in tritium unit (TU). Tritium is naturally generated in the upper atmosphere and rainwater contains a natural concentration of the order of a few TU (tritium unit). During the 1950s and 1960s, nuclear tests in the atmosphere dramatically increased the tritium content in rainwater. These high contents have long been used as a marker to trace and date groundwater. Nowadays tritium concentrations in precipitation have stabilized at levels close to those of natural production before nuclear tests. Tritium is used to date groundwater recharged after the 1950s, years of thermonuclear testing onset [26].
Radiocarbon ( 14 C) is a radioactive isotope of carbon. It has an estimated half-life of 5730 years. It is produced in the upper atmosphere and becomes part of the carbon and hydrological cycles. As its half-life is much longer compared to tritium, radiocarbon 14 C is considered as an ideal tracer to estimate groundwater age for up to ≈30,000 years. Radiocarbon has been applied to groundwater studies throughout the world since the early 1960s [78,79]. 14 C concentrations are expressed in pMC (percentage of Modern Carbon) and are provided along with an estimation of apparent groundwater age.

Radioactive Isotopes 3 H and 14 C
Tritium ( 3 H) and radiocarbon ( 14 C) are radioactive isotopes widely used in hydrogeology to assess the 'age' of groundwater, i.e., the time passed since the recharge or the average subsurface residence time. The groundwater residence time is an important factor as it is related to the renewability of the groundwater resources. Groundwaters are classified as 'modern' if their age is roughly less than 50 years. Groundwaters between about 50 and 1000 years old are called 'submodern'. Beyond 1000 years, they are termed "old" [28]. Knowing whether waters are modern, submodern, or old is critical for their management and sustainability. Tritium (3H) is a radioactive isotope of hydrogen with a half-life estimated at 12.32 years. Its content in waters are expressed in tritium unit (TU). Tritium is naturally generated in the upper atmosphere and rainwater contains a natural concentration of the order of a few TU (tritium unit). During the 1950s and 1960s, nuclear tests in the atmosphere dramatically increased the tritium content in rainwater. These high contents have long been used as a marker to trace and date groundwater. Nowadays tritium concentrations in precipitation have stabilized at levels close to those of natural production before nuclear tests. Tritium is used to date groundwater recharged after the 1950s, years of thermonuclear testing onset [26].
Radiocarbon ( 14 C) is a radioactive isotope of carbon. It has an estimated half-life of 5730 years. It is produced in the upper atmosphere and becomes part of the carbon and hydrological cycles. As its half-life is much longer compared to tritium, radiocarbon 14 C is considered as an ideal tracer to estimate groundwater age for up to ≈30,000 years. Radiocarbon has been applied to groundwater studies throughout the world since the early 1960s [78,79]. 14 C concentrations are expressed in pMC (percentage of Modern Carbon) and are provided along with an estimation of apparent groundwater age.
During field campaigns in 2015, 18 samples were collected for Tritium analyses (10 from the basalts, five from the rhyolites, and three from the sedimentary formations) and 15 for Radiocarbon analysis (10 from the basalts, three from the rhyolites, and two from the sedimentary formations). Analyses data are given in Tables 2 and 3. The waters of the GMR are free of Tritium except a single borehole located downstream of the central valley which displays 1 TU (Table 2). When Tritium is not detectable in groundwaters, this may evidence that these groundwaters have been recharged prior to thermonuclear weapons testing in the 1950s-1960s. Given the disintegration of Tritium from natural pre-bomb levels, it cannot normally be present in groundwater recharged until around 1950. Accordingly, the GMR groundwaters are over 50 years old and are classified as submodern or old. The radiocarbon analysis results provide additional precision on the GMR groundwater age. The 14 C concentrations in GRM groundwater vary between 17 and 102 pMC. Two samples, located in the volcanic rocks, have 14 C concentrations higher than 100 pMC, 101 pMC in basalts, and 102 pMC in rhyolites. Groundwater at both points is classified as modern (i.e., with ages < 50 years). For the remaining samples, apparent ages range from 160 to 14,395 years (See Table 3). It is worth noting that waters in the same formation do not belong to a single category. In each type of rock, the waters have different ages. The lowest values of 14 C activities are found in basaltic formations, meaning that the oldest waters belong to this formation.
Groundwater dating in the GMR, using isotopes, highlights the complexity of this hydrogeological system. In the Dalha basalts (average 79 pMC), groundwater is mostly submodern to old, indicating a longer residence time which may result from lower hydraulic conductivity of the rocks and/or longer pathways through fissures from outcrop to subsurface. 14 C concentrations are higher in rhyolites (average 97 pMC) indicating much younger waters and faster infiltration compared to the basalts. The sedimentary aquifer of the GMR (average 89 pMC) is connected with the rhyolites and presents younger waters compared to the basalts, but noticeably older compared to the rhyolites.

Conclusions
The GMR is located in the Republic of Djibouti (Horn of Africa) in a semi-arid climatic context, where rainfall is scarce and surface flows are short-lived. Groundwater remains the only resource for food and other needs, including agriculture. Given the very important socio-economic growth forecast for this region, groundwater is under very high pressure. Knowledge of the hydrogeological system of the GMR was to date very limited in order to plan a rational exploitation which would avoid over-exploitation and degradation of the resource. The overall objective of this work was to fill this gap, to provide the first elements of knowledge of the functioning of this system for sustainable exploitation. The study was based on a coupled approach of the major elements hydrochemistry and stable ( 18 O, 2 H) and radiogenic ( 3 H, 14 C) isotopes analysis. A conceptual model summarizing the functioning of the hydrogeological system of the Goda Mountains Range (GMR) is presented in Figure 11. This system is hosted by volcanic (basalts and rhyolites) and sedimentary formations. The upper part of the GMR is covered by basalts and the middle part by rhyolites. The downstream sedimentary part is bounded by the sea in the Gulf of Tadjourah. Results showed that groundwater is of meteoric origin and acquires its mineralization through alteration of volcanic rocks minerals and high evaporation. Waters presenting lower mineralization are located in the rhyolitic formation compared to waters extracted from adjacent basaltic and sedimentary geological formations. In the sedimentary aquifer, there is a proven risk of seawater intrusion.
An important outcome of this study concerns the age of groundwater in the system. Radiogenic isotopes showed that overall the GMR system contains waters of very different ages, from modern (<50 years) to old (>1000 years), though samples with modern water are very few (two out of 15). In the Dalha basalts, groundwater is mostly submodern to old (50 years > age > 1000 years), indicating a longer residence time which may result from lower hydraulic conductivity of the rocks and/or longer pathways through fissures from outcrop to subsurface. In the Mabla rhyolites, waters are much younger indicating faster infiltration and groundflow compared to the basalts. The sedimentary aquifer of the GMR is connected with the rhyolites and presents younger waters compared to the basalts, but noticeably older compared to the rhyolites. These interpretations lead to the conclusion that the rhyolites constitute for this system a preferential recharge zone.

Conclusions
The GMR is located in the Republic of Djibouti (Horn of Africa) in a semi-arid climatic context, where rainfall is scarce and surface flows are short-lived. Groundwater remains the only resource for food and other needs, including agriculture. Given the very important socio-economic growth forecast for this region, groundwater is under very high pressure. Knowledge of the hydrogeological system of the GMR was to date very limited in order to plan a rational exploitation which would avoid over-exploitation and degradation of the resource. The overall objective of this work was to fill this gap, to provide the first elements of knowledge of the functioning of this system for sustainable exploitation. The study was based on a coupled approach of the major elements hydrochemistry and stable ( 18 O, 2 H) and radiogenic ( 3 H, 14 C) isotopes analysis. A conceptual model summarizing the functioning of the hydrogeological system of the Goda Mountains Range (GMR) is presented in Figure 11. This study led to a global understanding of the functioning of this system, which will permit to work out the preliminary rules of sustainable exploitation of the system. In the volcanic part of the system, the rhyolites can constitute a privileged area to set up wellfields. On the other hand, the sedimentary aquifer must be exploited with care, taking into account the risk of seawater intrusion.
This work should be supplemented by more quantitative investigations (groundwater pattern mapping, geophysical deep reconnaissance, collection of new data such as piezometric and rain monitoring, pumping tests, modeling, etc.) allowing the development of a rational scheme for the sustainable use of this system to face the challenges of global change and of the prospects for accelerated growth in this region.