Geochemical, multi-isotopic studies and geothermal potential evaluation of the complex Djibouti volcanic aquifer (Republic of Djibouti)

The complex Djibouti volcanic aquifer system was studied to improve understanding of the recharge conditions of the Awrlofoul low-enthalpy geothermal system located in the middle of the aquifer. Forty-four thermal and non-thermal groundwater samples were analyzed to determine their major chemical compositions, trace element compositions, and multi-isotopic compositions (δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ13C(DIC), 14C, 87Sr/86Sr, δ11B, δ15N(NO3−), and δ18O(NO3−)). Statistical analysis (Hierarchical Cluster Analysis and Principal Component Analysis) of chemical composition identified three main water groups, two affected by salinization (C1 and C2) and one fresh water group useful for drinking (C3). The latter group includes thermal water from the Awrlofoul geothermal field. This separation into three different water groups is also clear on a Langelier-Ludwig plot and is confirmed by analysis of historical chemical data over the last 30 years. The main causes of salinization are contamination of the fresh groundwater either by recent seawater intrusions (C2) or mixing with Ca-Cl fossil saline water (C1). The C1 waters are also highly affected by Mg/Ca-Na clay exchange. As expected, the 11B/10B isotope ratio of the intruded salt water, both recent and fossil, was much higher than that of seawater (δ11B up to +55‰). Unexpectedly, groundwater of meteoric origin (i.e., unaffected by a seawater intrusion), also showed a δ11B higher than that of seawater (46.3‰ < δ11B < 51.3‰). That the unexpectedly high δ11B values are likely due to 10B sequestration resulting from interaction with clay and/or carbonate precipitation is demonstrated by activity diagrams and saturation indices. The C1 water group is also affected by nitrate contamination (56.8 ± 19.2 mg/l). That the nitrate contamination is likely due to manure contamination is indicated by comparing the dual isotopic composition of nitrate to the boron isotope ratios. The isotopic composition of sulfate highlighted the importance of SO2-disproportionation to the local sulfate minerals that interacted with the meteoric recharge, while the strontium isotope ratios showed the importance of the seawater-basalt interaction with the fossil saline water component. The results of the mixing analysis using chemical composition, δ13C(DIC), and 14C data by geochemical software (NetpathXL) confirmed the presence of ternary mixing with at least three sources (seawater, meteoric, and fossil) in the waters with the highest chloride concentrations. The estimation of groundwater age by 14C was complicated by overexploitation (as testified by the lumped parameters approach). However, the fossil saline water component was dated back to the Holocene Humid Period. To estimate the temperature of the Awrlofoul low-enthalpy geothermal system, a multi-method geothermometric approach was applied. Chemical (mainly SiO2) and isotope (sulfate-water oxygen fractionation) geothermometers were employed together with multiple mineral equilibria. These different geothermometric approaches estimated a temperature range of 102 °C–140 °C for the geothermal reservoir, with a mean temperature of about 110 °C. Finally, a conceptual model was proposed for the Awrlofoul low-enthalpy geothermal system on the basis of the geochemical and isotopic data of the thermal and non-thermal groundwaters combined with the geology and hydrogeology of the study area