Combined application of electrical resistivity and shallow groundwater sampling to assess salinity in a shallow coastal aquifer in Benin, West Africa

Motivated by prior field and numerical results suggesting possible movement of salinity towards public water supply wells serving Cotonou, Benin, this work examines the combined use of direct-push chemical sampling, electrical resistivity tomography (ERT), and vertical electrical sounding (VES) to assess the distribution of saline waters in a complex coastal aquifer. Direct-push sampling data allowed delineation of shallow salinity and provided chemical signatures useful in defining possible sources of the salinity. Complementary ERT data revealed local heterogeneity in the subsurface resistivity, largely related to variability in the depth to salinity along the survey line. Despite this local variability, interpretation of VES data using a layered model proved reasonably robust for estimating the depth of the contact between fresh and salt waters. Where field conditions allowed all three measures to be performed in close proximity, there was general agreement among the ERT, VES and direct sampling data in terms of indication of locations of shallow salinity. Combined with interpretation of the local geology, the data lead to the conclusion that the distribution of salinity is related to aquifer heterogeneity, but that the coastal region likely does not pose an immediate threat to the well field. Beyond implications for coastal Benin, these results demonstrate that combining electrical resistivity surveys, direct-push sampling, and geologic interpretation allows initial characterization of complex coastal aquifers under the constraint of limited field data.