Evaluating groundwater recharge processes using stable isotope signatures—the Nabogo catchment of the White Volta, Ghana

Abstract

This study evaluates the source and evolution of groundwater in parts of the Nabogo catchment of the White Volta Basin in Ghana. Porewater δ18O and δ2H signatures of vadose zone material were used together with the isotope signatures of rainwater, groundwater, and surface water in the area to suggest an evolutionary model and the mode of fluid fluxes through the unsaturated zone material in the area. This study finds that the local precipitation in the study area presents a relatively isotopically heavier signature compared to the average signature of global meteoric water. This is because the local meteoric water line (LMWL) has shallower slope, intercept, and average deuterium excess values compared to the global meteoric water line (GMWL). On the other hand, the local groundwater in the area presents relatively enriched isotopic signatures. Stable isotope profiles suggest piston flow as the main mechanism of vertical water movement in the vadose zone, indicating that direct groundwater recharge from local precipitation is principally based on this mechanism of transport. There is a progressive decline in the deuterium excess data of porewater vertically down the soil profile. This observation is consistent with an evolutionary pattern caused by fractionation processes attending evaporation of infiltrating water. Mineral-weathering processes have not been noted to have an effect on the isotopic signature of groundwater in the area. However, there appears to be a significant impact of evaporation on the total dissolved solid content of surface water in the area. In the case of groundwater, field-measured electrical conductivity (EC), total dissolved solids (TDSs), and pH are, respectively, in the ranges of 30–2407 μS/cm, 15–1196 mg/L, and 5.35–8.36. The EC, TDS, and pH are, respectively, in the ranges of 30–139 μS/cm, 15–70 mg/L, and 6.52–8.66 in surface water in the area.