Hydrogeophysical assessment of groundwater and saltwater–freshwater interface in a quaternary– neogene coastal aquifer, Kilwa Kisiwani island, southeast Tanzania

This study aims to identify and explore new potential sites of groundwater resources using geophysical methods, and to demarcate the depth to the saltwater-freshwater interface. Kilwa Kisiwani Island is geologically covered mostly by Neogene and Quaternary alluvial sediments, sand and gravel. The geophysical techniques employed during the research include Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT), and Transient Electromagnetics (TEM). The Island has the potential for groundwater occurrences, but the layer with fresh water is shallow and thin. Out of the 18 VES sites, 2 (11%) points have shown non-potential, but the remaining 16 (89%) of the VES sites have shown potential, and drilling has been recommended depending on the hydrogeophysical characteristics of each potential VES point. Out of the 16 TEM points, 4 (25%) have shown non-potential, but the remaining 12 (75%) have shown potential, and drilling has been recommended depending on TEM point-specific hydrogeophysical characteristics. Generally, geophysical results show the presence of freshwater lenses with a thickness range between 10 and 30 meters. The recommended drilling depth ranges between 10 and 30 meters below ground surface. The depth to saltwater-freshwater interface varies between 1 and 39 meters below ground surface.