Groundwater vulnerability to petroleum hydrocarbons pollution of quaternary sand aquifer in Dar es Salaam, Tanzania

Abstract

Groundwater is inherently susceptible to contamination from anthropogenic activities and remediation is very expensive and often impractical. Prevention of contamination is hence critical in effective groundwater management. In this paper, an attempt has been made to assess the aquifer’s vulnerability to petroleum hydrocarbons contamination in Dar es Salaam City, Tanzania. Potential benefit from mapping vulnerability is that it aids in the prioritisation of monitoring boreholes located in high vulnerable area, as well as providing bases for developing appropriate groundwater protection strategies. The DRASTIC model coupled to ArcView-GIS and HSSM model were used in vulnerability assessment. The results from these models have clearly demonstrated that the aquifers in the Dar es Salaam City are likely to be affected from petroleum hydrocarbons, because the pollution sources are located in areas with moderate and high vulnerability classes, which covers about 80% of the Dar es Salaam City. Furthermore, the gasoline estimated arrival time is about 1 year for transport from the ground surface to a water table aquifer at a depth of 7 m. The current levels of petroleum hydrocarbons in potable water are very low (<2.5µg/l) compared to the threshold limits of the Tanzania standards of about 0.5 mg/L. However, as the gasoline takes even decade or centuries to migrate through aquifers, the concentrations are likely to increase if no preventive measures are taken.

Keywords: Aquifers, ArcView-GIS, Dar es Salaam City, DRASTIC index, HSSM, Petroleum hydrocarbons