Groundwater first started to be used as a source of water supply for the Dar-es-Salaam City in 1943. However, many of the existing boreholes in Dar-es-Salaam were drilled in 1997 after Tanzania experienced a severe drought; since then groundwater use in the city shows an increasing trend and currently over 50 percent of residents in Dar-es-Salaam City relies on groundwater supply. Today, there are more than 10000 boreholes for domestic, water supply and industrial purposes. In the coastal aquifer of Dar-es-Salaam, groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. The results show that the origin of salinity in the area is strongly influenced by: groundwater ascending from deep marine Miocene Spatangid Shales through faults, saltwater intrusion on the border of the Indian Ocean, and throughout, there is some salinity within the Quaternary aquifer, especially in intercalated deltaic clays in the fluviatile deposits, showing some marine influences. Finally, also anthropogenic pollution may increase the groundwater salinity.