Origin of nitrate and sulfate sources in volcano-sedimentary aquifers of the East Africa Rift System: An example of the Ali-Sabieh groundwater (Republic of Djibouti)

Within the East African Rift System (EARS), the complex Ali-Sabieh aquifers system, located in the south of the Republic of Djibouti, was overexploited and subjected to anthropogenic and/or geogenic pollution with high concentrations of dissolved nitrate (up to 181 mg/l) and sulfates (up to 1540 mg/l). This study is the first undertaken on the hydrochemistry of this aquifer system, combining geochemical tools and multi-isotope - δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ15N(NO3), δ18O(NO3), δ13C(DIC), and 14C- was used to decipher the origin and fate of different nitrate and sulfate sources to groundwater. The groundwater samples of the region show a chemical evolution from fresh Ca(Na)-bicarbonate to brackish Na-Cl , mainly due to water-rock interaction. The combined chloride and water isotope data show that evaporation and transpiration are present, with the latter occurring primarily in the shallow alluvial aquifer waters. Inspection of δ15N(NO3) vs. δ18O(NO3) and NO3/Cl vs. Cl diagrams show that dissolved nitrates are primarily of anthropogenic origin. In particular, higher nitrate concentrations may be related to animal manure used as organic fertilizers during agricultural activities. Sulfates are from a natural origin related to the interaction of water with gypsum of hydrothermal or sedimentary origin. SO4/Cl ratio and isotopic composition show that dissolved sulfates in saline and ancient groundwater of the Cretaceous sandstone aquifer (between 7.4 ± 2.2 and 5.8 ± 1.4 k-years before the present) are generated by interaction with gypsum from oxidation of pre-existing (Jurassic?) sulfides. This work highlight that isotopic ratios of the two molecules -δ18O(SO4), δ34S(SO4), δ15N(NO3), δ18O(NO3)- are not sufficient for tracing the origin of nitrate and sulfates in groundwater, but that a complete hydrogeochemical study is needed. In the absence of this, the relatively high concentration of chloride and sulfates could be wrongly linked to the anthropogenic source of nitrate (manure or sewage).