Corroborating stable isotopic data with pumping test data to investigate recharge and groundwater flow processes in a fractured rock aquifer, Rivirivi Catchment, Malawi

Abstract

Fractured rock aquifers continue to support millions of people in arid and semiarid regions. However, due to heterogeneous nature of their hydrogeology and comparative low yields, research in these formations is regarded expensive, and thus, very little is known about their dominant hydrological processes. In this study, we corroboratively used pumping test and stable isotopic data to understand groundwater recharge and flow processes in fractured hornblende-biotite-gneiss. It was established that rain is the main source of groundwater recharge. The groundwater system seems to retain the stable isotopic signatures of precipitation which has undergone evaporation before infiltration. Geochemical data also show that the groundwater system has not undergone pronounced geochemical evolution as evidenced by low total dissolved solids in both seasons. A combination of stable isotopic data and derivative plots of drawdown data helped identification of apparent recharge zones, no-flow boundaries and dominant flow types in the pumped wells with bilinear flow being the dominant type of flow in BP1 and BP2, while linear flow was dominant in BP3 and BP4. Geochemical, stable isotopic and hydraulic data further revealed hydraulic connection between two wells juxtaposed across Ntcheu Fault, indicating potential for groundwater flow across the fault and highlighting that the fault may be acting as a groundwater conduit across it. The established phenomena are important when considering groundwater development and sustainable management of the resource.