Groundwater chemistry and isotopes reveal vulnerability of granitic aquifer in the White Volta River watershed,West Africa

Abstract

Granitic aquifers, which occur in crystalline Basement bedrocks, are generally fresh and serve as potable water sources in many developing countries. In this study, we examine the chemical evolution processes and water isotope geochemistry of a sparsely studied granitic aquifer in a sub-river watershed of the White Volta River basin in West Africa. Our goal is to establish the sources of water that replenishes the aquifer in a watershed dominated by irrigation farming and to determine the most vulnerable areas that may be impacted by human activities and climate change. The results show that in most samples collected from the aquifer, the level of dissolved ions permits safe agricultural use and requires minimal treatment for domestic consumption. The aquifer water evolves from recharge water of Ca-Mg-HCO3 and Ca-Na-Mg-HCO3/Cl types in the highlands of the watershed to Na-Ca-Mg-HCO3, Na-Ca-Mg-HCO3/Cl and Na-HCO3 types in the discharge areas. Groundwater recharge is diffuse and flow is continuous with little attenuation from the highlands to the lowlands. The water isotopes δ2H and δ18O show groundwater recharge comes mainly from rainfall and occurs throughout the watershed. Tritium, 3H, measurements confirm diffuse recharge and suggest the aquifer contains recharge water of ages between 1 and 40 years. The assigned Hydrogeochemical Vulnerability Index (HVI) is confirmed with nitrate and tritium data and shows that the aquifer could be negatively impacted by climate change and land-use activities. A conceptual framework for aquifer recharge, flow, and vulnerability is presented to summarize our findings.