Understanding recharge mechanisms and surface water contribution to groundwater in granitic aquifers, Ghana: Insights from stable isotopes of δ2H and δ18O

In hydrological studies, the use of stable isotopes (δ18O and δ2H) of water has proven to be a powerful tool to study groundwater recharge and mixing dynamics, which are critical to the long-term management of groundwater resources. In this study, we analyzed the stable isotopes of rain (event basis), groundwater (boreholes and hand-dug wells) and surface water in the White Volta River basin (Ghana). The aim is to understand the groundwater recharge processes, quantify the contribution of surface water to groundwater and assess the feasibility of artificial recharge in an agricultural catchment area. The results show that the event-based δ18O showed a weak dependence on air temperature (T) and rainfall (P) amount, while the monthly weighted average showed a much stronger correlation. The δ18O and δ2H show that rainfall is the main source of recharge and occurs in events greater than 10 mm. Overall, most of the groundwater falls between the June, July and August rains, suggesting that these months represent the recharge period. The groundwater isotopic variability is mainly influenced by local recharge and mixing with surface water. Low δ18O and high d-excess were found in most of the groundwater samples in the basin, indicating recent recharge conditions. The isotope mixing model shows that the surface water contribution to groundwater recharge is extremely variable, ranging from 1.47 ± 0.09% to 51.69 ± 6.04% suggesting the potential for artificial recharge. To summarize our results on the dynamics of groundwater recharge in the catchment area, we have provided a conceptual framework of our results. •We examined the interactions between rainwater, groundwater and surface water in granitic aquifers.•Rainfall is the main source of recharge and occurs in rainfall events above 10 mm.•Over 80% of the total monthly rainfall amounts of June, July and August contribute to groundwater recharge.•A local meteoric water line was developed; δ 2H = 7.22(±0.25) ∗ δ 18O + 8.66(±0.99) ‰.•A conceptual framework depicting the recharge dynamics of the granitic aquifers is presented.