Hydrogeochemistry of the Kasane thermal spring, Botswana
Mukwati, Boitumelo Tebuho
PublisherUniversity of Botswana, www.ub.bw
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The hydrogeochemistry of the Kasane thermal spring in Kasane was conducted with the aim of characterising it, determining the suitability of its water for drinking purposes and the influence of mineralogical composition of the rocks and precipitation on the water quality. Kasane township is situated in the Northern most part of Botswana in the Chobe District. It is a few kilometres from the Chobe River's confluence with the Zambezi River; where the four countries of Botswana, Zambia, Zimbabwe, and Namibia meet. Sixteen water samples (one from the spring, twelve groundwater samples, one river and two sewage samples), forty-nine sediment samples and three rock samples were collected to achieve this objective. The water and sediment samples were analysed for both major cations (Na+, K+, Ca2+, and Mg2+), major anions (CO32-, HCO3-, Cl- , SO42-, NO3-, F-, PO43-, NO2- and Br-) and trace elements (As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Pb, Ni and Zn) at the geochemistry laboratories of the University of Botswana and at the Department of Water Affairs. The physico-chemical parameters such as pH, EC and TDS were measured in-situ. Petrographic analysis of the rock thin sections was also carried out to identify the mineralogical composition of the rocks of the studied area. The geology of the area is generally Karoo volcanic. Thin sections analyses revealed that the rock is basalt made up of plagioclase, clinopyroxene and opaque minerals with othopyroxene and amphibole in trace amounts. In the analysed water es generally based on the mean values of the chemical parameters, the cations were in the order of abundance as Na+> Ca2+>Mg2+>K+>Li+ and the anions were in the order of abundance as Cl->SO42->HCO3-> PO43->Br- > CO32- > NO3- > F->NO2-. In the analysed sediment samples the cations and anions are in the order of Na+ > Ca2+ >K+ > Mg2+ > Li+ and SO42- > Cl- >NO3- > Fl->Br->NO2- > PO43-, respectively. The spring and the groundwater from the pits north of the spring are slightly alkaline to alkaline, brackish to saline and hard to very hard whereas the groundwater west of the springs, river water and the sewage water are acidic to slightly alkaline (with the exception of sample W11, which is alkaline), fresh (with the exception of sample W12, which is brackish) and soft to hard. 75 % of the analysed groundwater samples from the pits west of the thermal spring show some similarities in the trend of concentration of the different constituents of the groundwater with the river water, suggesting that the groundwater is recent or derived from precipitation. A slight increment of values in the analysed major cations, anions, and trace elements in the groundwater of this area is due to the issolution of different minerals in the rocks of the study area. The remaining 25 per cent the groundwater samples from this part of the study area suggest that precipitation is not the sole source of groundwater and it could be derived from both precipitation and fluids of deep origin. The thermal spring and the groundwater north of the thermal springs are characterized by higher mineralization, and this could be attributed to the more supplement of the deep fluids that have deep origin. This implies that the source of the spring and the groundwater north of the thermal spring have a regional origin. The thermal spring water and all groundwater in the studied area are not recommended for drinking purposes.