Assessing groundwater flow pattern in the Bara volcanic aquifer system, Republic of Djibouti

Located in the semi-arid Republic of Djibouti, the Bara area mainly relies on fractured volcanic aquifers for its water supply. Sustainable water management therefore requires a comprehensive understanding of groundwater flow pattern. This paper aims at characterizing groundwater flow inside and between the compartments of the aquifer system composed by basaltic rocks and by alluvium using geochemical and isotopic tracers, including major ion chemistry, 2H, 18O, 13C, 14C and 3H. Geochemistry shows an evolutionary trend from alluvium aquifers located in wadi-valleys and deltaic formations bordering the two sedimentary basins, with low salinity and Na-Cl-HCO3 water type, to a more saline and intermediate water type in the basalts and finally to a Na-Cl-SO4 water type in the most mineralized groundwaters. Decreasing TDIC and increasing &delta+13C-DIC reveal the absence of an active deep CO2 contribution to the groundwater system. Different recharge conditions for the alluvium and basalt aquifers are revealed by 2H and 18O, with no evidence of evaporation effects. Alluvium groundwaters are found to be modern according to radiocarbon and tritium, while basalt groundwaters are dated to the early to mid-Holocene. These features reveal a common evolutionary pattern, with a significant recharge from the alluvium, transmitted downward to the basalt aquifer through major faults in the wadi valleys or through the sediments of the alluvial fans, followed by mixing with an ancient Na-Cl water. Finally, the two basaltic aquifer compartments located below the main endorheic plain are found to form one single aquifer flowing northeastward.