Assessing the influence of a dam reservoir on groundwater quality using geochemical and stable isotopes techniques

Abstract

Stable Isotope (δ2H and δ18O) technique is employed to investigate the Dam water-groundwater interaction on the Bui hydropower project in Ghana, to ascertain; if surface water from the dam recharges the groundwater or if there is balance flow relationship between the two systems. The hydrochemical results showed that the pH of the groundwater (5.51–7.18) was slightly acidic to basic, predominantly due to lithology. The pH of the surface water (6.13–6.98) suggests decomposition of submerged organic matter at the impoundment of the dam. The hydrochemical facies showed signatures that result from groundwater interaction with soluble aquifer materials (feldspars, micas, and quartz). Stable isotope in the study area showed surface water is more enriched (δ18O = −2.46‰; δ2H −21.48‰) than rainwater (δ18O = −3.36‰; δ2H = −17.83‰), which is slightly enriched than groundwater (δ18O = −3.58‰; δ2H = −17.48‰). The stable isotope (δ2H-δ18O) biplot indicates that no interaction occurred between the groundwater and the Black Volta River; the groundwater in the catchment is of meteoric origin. D-excess confirms mineralization due to water-rock interaction while the biplot of Cl− versus δ18O delineated three (3) groups of water i.e., water being bridged by rocks; groundwater recharged by rainfall with little or no evaporation and evaporated groundwater. Residence time influences groundwater's ionic configuration which imparts aesthetic characteristics over time. The study confirms the feasibility of complimenting geochemistry with the isotope technique to investigate flow relationships between dam water and groundwater systems.