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Abstract
Abstract. We assessed environmental tracers in groundwater in two contrasting basins in Namibia; the Kuiseb Basin, which is a predominantly dry area and the Cuvelai-Etosha Basin, which is prone to alternating floods and droughts. We aimed to determine why the quality of groundwater was different in these two basins which occur in an arid environment. We analysed groundwater and surface water for the stable isotope ratios of hydrogen (δ2H) and oxygen (δ18O) by cavity ring-down spectroscopy and metals by inductively coupled plasma mass spectrometry. The δ2H and δ18O of surface water in the Cuvelai-Etosha Basin plot on an evaporation trend below the global meteoric water line (GMWL) and the local meteoric water line (LMWL). The δ2H and δ18O of some groundwater samples in the Cuvelai-Etosha Basin also plot on the evaporation trend, indicating recharge by evaporated rain or evaporated surface water. In contrast, the δ2H and δ18O of groundwater samples in the Kuiseb Basin plot mostly along the GMWL and the LMWL, indicating direct recharge from unevaporated rain or unevaporated surface water. Fifty percent of groundwater samples in the Cuvelai-Etosha Basin was potable (salinity < 1 ppt) compared to 79 % in the Kuiseb Basin. The high salinity in the groundwater of the Cuvelai-Etosha Basin does not appear to be caused by evaporation of water (evapo-concentration) on surface prior to groundwater recharge, but rather by the weathering of the Kalahari sediments. The low salinity in the Kuiseb Basin derives from rapid recharge of groundwater by unevaporated rain and limited weathering of the crystalline rocks. The order of abundance of cations in the Kuiseb Basin is Na > K > Ca > Mg vs. Na > Mg > Ca > K for the Cuvelai-Etosha Basin. For metals in the Kuiseb Basin the order of abundance is Fe > Al > V > As > Zn vs. Al > Fe > V> As > Zn for the Cuvelai-Etosha Basin. The relative abundance of cations and metals are attributed to the differences in geology of the basins and the extent of water-rock interaction. Our results show that the quality of groundwater in Cuvelai-Etosha Basin and Kuiseb Basin which vary in the extent of aridity, is controlled by the extent of water-rock interaction at the surface and in the groundwater aquifer.
Highlights
Our results show that in the groundwater in the Cuvelai-Etosha Basin, high salinity does not necessarily correspond with high δ18O (Fig. 4); Even groundwater with moderate salinity in the Kuiseb Basin is not associated with higher δ18O either
The groundwater in the Cuvelai-Etosha Basin has poor water quality evidenced by higher groundwater salinity classified as brackish to saline than groundwater in the Kuiseb Basin
There are higher levels of evaporation observed in more enriched stable isotopes of hydrogen and oxygen in groundwater in the Cuvelai-Etosha Basin compared to the Kuiseb Basin
Summary
Kgabi et al.: Isotopes and elements in groundwater. Water scarcity is one of the key limiting factors to sustainable economic, industrial, social and environmental development. Quality is a limiting factor for the sustainable use of water for domestic, agricultural and industry activities. Salinization which impairs the use of water is a global problem. Salinization is more severe in arid regions, where groundwater is the primary source of water (IAEA, 2006)
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