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. We assessed environmental tracers in groundwater in twocontrasting basins in Namibia; the Kuiseb Basin, which is a predominantlydry area and the Cuvelai-Etosha Basin, which is prone to alternating floodsand droughts. We aimed to determine why the quality of groundwater wasdifferent in these two basins which occur in an arid environment. Weanalysed groundwater and surface water for the stable isotope ratios ofhydrogen ( δ2 H) and oxygen ( δ18 O) by cavityring-down spectroscopy and metals by inductively coupled plasma massspectrometry. The δ2 H and δ18 O of surface water inthe Cuvelai-Etosha Basin plot on an evaporation trend below the globalmeteoric water line (GMWL) and the local meteoric water line (LMWL). The δ2 H and δ18 O of some groundwater samples in theCuvelai-Etosha Basin also plot on the evaporation trend, indicating rechargeby evaporated rain or evaporated surface water. In contrast, the δ2 H and δ18 O of groundwater samples in the Kuiseb Basinplot mostly along the GMWL and the LMWL, indicating direct recharge fromunevaporated rain or unevaporated surface water. Fifty percent ofgroundwater samples in the Cuvelai-Etosha Basin was potable (salinity< 1 ppt) compared to 79 % in the Kuiseb Basin. The high salinityin the groundwater of the Cuvelai-Etosha Basin does not appear to be causedby evaporation of water (evapo-concentration) on surface prior togroundwater recharge, but rather by the weathering of the Kalaharisediments. The low salinity in the Kuiseb Basin derives from rapid rechargeof groundwater by unevaporated rain and limited weathering of thecrystalline rocks. The order of abundance of cations in the Kuiseb Basin isNa > K > Ca > Mg vs. Na > Mg > Ca > Kfor the Cuvelai-Etosha Basin. For metals inthe Kuiseb Basin the order of abundance is Fe > Al > V > As > Znvs. Al > Fe > V> As > Znfor the Cuvelai-Etosha Basin. The relativeabundance of cations and metals are attributed to the differences in geologyof the basins and the extent of water-rock interaction. Our results showthat the quality of groundwater in Cuvelai-Etosha Basin and Kuiseb Basinwhich vary in the extent of aridity, is controlled by the extent ofwater-rock interaction at the surface and in the groundwater aquifer.

Isotopic composition and elemental concentrations in groundwater in the Kuiseb Basin and the Cuvelai-Etosha Basin, Namibia