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Abstract: 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 (18O, 2H), and radiogenic isotopes (3H, 14C). 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 different ages. This study has made significant
progress in understanding this system and is an initial step towards the sustainable exploitation
of resources