Groundwater age dating using multi-environmental tracers (SF6, CFC-11, CFC-12, δ18O, and δD) to investigate groundwater residence times and recharge processes in Northeastern Ghana

Abstract

The granitic aquifer of the area under investigation serves as an important source of water supply for the inhabitants. However, information about groundwater renewability and recharge process has not been defined. Multi-tracers were then applied to determine the residence times and recharge patterns of aquifer systems. In this study, sulphur hexafluoride (SF6) and chlorofluorocarbons (CFCs) were used to date shallow groundwater in Ghana for the first time. The tracer concentrations were measured in 38 groundwater wells from 38 sampling sites in Northeastern Ghana. A lump parameter model (LPM) describing the different age distributions reveals that the groundwater residence times are less than 50 years. The SF6 ages obtained were much younger than CFCs, an indication of terrigenous source enrichment. The young groundwater ages inferred from the dating process indicate modern recharge and, this collaborates with the results of the stable isotopes, in which there is a direct link between the isotopic composition of the rainfall and the groundwater. The results also indicate that the weathered and fractured granitic aquifer in the area enhances rapid and diffuse recharge after precipitation. In addition, a 3D numerical groundwater model has been developed to simulate groundwater flow under steady-state and obtain information on the key aquifer properties such as hydraulic conductivity and recharge rates. A comparison of the simulated and observed heads suggests a good match, and the developed model can satisfactorily describe groundwater flow in the area. A groundwater flow system has been delineated based on the multi-tracers (SF6, CFC-11, CFC-12, δ18O) and the numerical groundwater model. The findings of this study will be valuable for water managers and stakeholders to obtain relevant hydrogeological information for improved aquifer resource planning.