Hydrochemical Analysis Of Groundwater In The Danakil Depression, Northern Ethiopia

An investigation of surface and groundwater chemistry was undertaken as part of a resource assessment of water-supply for potash solution-mining in the Danakil Depression of the Afar Rift, situated in northern Ethiopia on the N/S-trending Ethiopia-Eritrean arm of the East African Rift System. Four hydrostratigraphic units are recognised in the area: 1) Basement Tsaliet Group rocks, low yielding and with poor water quality; 2) Adigrat Formation and Antalo Group fractured sandstones and karstic limestones, high yielding with good water quality; 3) Danakil Group and Zariga Formation fine-grained sediments, forming a good regional aquifer; 4) Dogua Formation alluvial fans, forming a major, regional, primary aquifer with high yields and poor water quality (yields of >50 l/s but 3 times the salinity of sea water). In the mining concession area the only available target for groundwater exploration is the Dogua alluvial-fan system that extends off the Dogua Mountain range to the west. During 2015 and 2016, 5 production and 5 exploration/monitoring boreholes were drilled into the fans, thereafter test pumped and sampled for hydrochemical and isotope analysis. The aquifer stress test proved that there is sufficient groundwater available for the life of mining, but that the water quality is poor. The hydrochemical analysis found that in general, water quality is poor (brine with TDS values >100 000 mg/l and a density >1.05 g/cm3 ) and with temperatures of ~50°C. Water quality degrades from south to north, possibly due to recharge of fresh water from the Antalo limestones in the south and lesser quality water from the Tsaliet basement in the north. The high salinity and temperature found throughout the fans is due to hydrothermal influx along rift-boundary faults that extend downwards into the thermally active, volcanic rift zone. Prolonged pumping results in the degradation of the water quality in the southern boreholes and improvement of quality in the northern boreholes; this is likely as the groundwater heads towards equilibrium, with a similar hydrochemical signature to that of the dominant hydrothermal influx. Stable and radiogenic isotope sampling and analysis was undertaken to determine the source of the recharge, the results showing that it is likely a combination of evaporation from fresh water, lateral recharge of ancient sea water along faults from the Red Sea, and hydrothermal recharge from the sub-surface leaching of young Afar and Red Sea Basalts in the active rift zone.