Coastal shallow groundwater in Togo: vulnerability assessment using the DRASTIC method with EC validation

In South Togo, the main water resource is the continental terminal (CT) aquifer, part of the coastal sedimentary basin (CSB). However, previous studies have evidenced increasing pollution due to the continuous development of anthropic activities resulting from population growth during the last decades. In this respect, the major challenge is to identify and assess the main factors affecting groundwater quality. This study aimed to determine the intrinsic vulnerability of the CT aquifer using the DRASTIC method in a context of poor hydrogeological knowledge. In this context, and in the almost general absence of raw data, the methodology adopted is based above all on the determination and calculation of DRASTIC parameters from the combination of other data to produce a vulnerability map. The systematic measurement of water electrical conductivity (EC) is was then used to validate the outputs of the DRASTIC model. The resulting map shows that 59.6% of the surface water area is moderately to highly vulnerable. Comparison with the EC spatial distribution of values revealed that EC values lower than 700 mu S cm-1 mainly coincide with low vulnerability zones, whereas higher conductivity values (exceeding 700 mu S cm-1) correspond to areas with moderate to high vulnerability. High EC values were mainly identified in wells located in large cities or related to intense anthropogenic activities. The relationship between the mineralization and vulnerability of the CT aquifer shows that the DRASTIC method, combined with the reliable, easy-to-obtain and almost cost-free EC parameter, represents a robust decision support tool for groundwater protection and management.