Estimation of depth to fresh-salt water interface and its implications for sustainable groundwater resource management: a case study of the Coastal strip of Dar es Salaam, Tanzania

Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting over-exploitation is entailing the risk of seawater intrusion. This problem has been systematically investigated for the first time, and appears to be wide-spread. Determining the depth of the fresh-salt water interface was deemed crucial in the view of sustainable exploitation of the Dar es Salaam quaternary coastal aquifer system. Vertical electrical soundings (VES) and horizontal resistivity profiles have shown a decrease of resistivity with depth and toward the coastline. Cross sections have shown the presence of salt water in the aquifer near the coastline, and the boundary between fresh and saline waters is mapped. The surface prospection resistivity data showed comparable results as reflected in hydrochemical/lithological cross sections and resistivity well logs. Salt water was found mostly in the area within 2 km of the coastline, and the depth to the interface in this zone is ranging from 1.3 to 20 m. Away from the coastline, the interface (if present) is at greater depth. VES executed at Kurasini (4 km from the sea) showed the fresh-salt water interface at a greater depth (43 m)