Spatial targeting of groundwater vulnerability in the Wewe-Oda river watershed in Kumasi, Ghana

Abstract

It has been widely recognized that groundwater susceptibility to contamination is a function of a complex interplay of factors including land use dynamics, pollutant sources, hydrogeology, climate and biogeochemical reactions within the aquifer. Spatially targeting the risk levels and coverage of groundwater contamination is a challenging task, requiring a spatial multi-criteria and integrative approach to account for the various drivers. This study applied the original DRASTIC (DRASTIC-O) model, where its weightage was further modified using the advanced Analytic Hierarchical Process (AHP) to develop the DRASTIC-AHP approach. The groundwater modelling approach included specific vulnerability using Pollutant Potential Impact (PPI), and incorporation of land use into the DRASTIC model (DRASTIC-LU). A Composite Groundwater Vulnerability Model was developed to simultaneously integrate the DRASTIC-AHP, PPI and DRASTIC-LU, thus accounting for both natural and human sources of groundwater contamination. The approach was illustrated for the Wewe-Oda watershed, an urban catchment in the Oforikrom Municipality of the Ashanti region in Ghana. The findings show that about 34% of the catchment is at high to very high risks of groundwater contamination. The developed composite model is shown to address limitations of the DRASTIC-O model, particularly by incorporating anthropogenic sources of pollution.