Physico-chemical study of groundwater in the Northeast of Kara region (Togo)

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Groundwater is a major source of drinking water for people in the Kara region. This study aims at improving the knowledge of groundwater and to determine the processes responsible for groundwater mineralization in this region using hydro-chemical methods and multivariate statistical analysis. 47 water samples taken from the wells in the crystalline basement aquifer were analyzed. Chemical analyses were made on these samples according AFNOR methods to determine hydro-chemical characteristics. The hydro-chemical study showed that the waters in the region have an average temperature of 28.7 °C. They are acid to slightly basic, with a pH ranging between 5.7 and 7.87, but for the most part neutral. The waters are weakly to moderately mineralized, which confirms the well-known chemical characteristic of basement groundwater. However, high conductivity values were found in the waters in some localities. The highest value was measured at Broukou (1083 μS.cm-1). Groundwaters in this region belong to a variety of hydro-chemical facies in relation to the type of geological reservoir. Four hydro-chemical facies were identified. The most dominant is the calcic and magnesian bicarbonated facies (85%) followed by the sodic-potassic bicarbonated facies (8.5%). The sodic-potassic chlorinated facies (2.1%), calcic and chloride (2.1%) and calcic-magnesian sulfated facies are poorly represented in these waters. Major ions which are the cause of the waters mineralization come from rock weathering, hydrolysis of silicated minerals such as anorthite in plagioclase and the decomposition of ferromagnesian minerals such as biotite and amphibole. The contribution of soil leaching and pollution due to anthropic activities is not negligible.Keywords: Groundwater, Kara region, hydrochemistry, weathering, hydrolysis, basement complex.