Applying a GIS Based DRASTIC Model in the Assessment of Risk to Groundwater Resources Within the Underlying Aquifers in Part of Lagos State, Nigeria

Lagos, Nigeria is one of the megacities in Africa with an estimated annual growth rate of 8%, and its socio-economic growth has continually encouraged influx of people into the city. Hence, this has increased pressure on the underlying water resources with implications for public health and ecological related issues. In this study, anthropogenic induced risk posed to water resources within the underlying aquifers in part of Lagos State, Nigeria, was evaluated using qualitative DRASTIC modelling approach. Geo-hydrological data were acquired for site characterization and setting up of the model. The geology indicated multi-layered aquifer horizons, with extensive lateral lithological variations. The topography ranges between 1.5 and 60 m above mean sea level (amsl). The depth to groundwater is shallow, typically less than 1 m in the south, and a maximum depth of 20 m below ground level, in the north. A vulnerability map was developed from the cumulated sum of the DRASTIC index values. Anthropogenic activities within the study area were super-imposed on the vulnerability map to generate a risk map. The results show that 1% of the study area is designated as High-Vulnerability-High-Risk area. Also, High-Vulnerability-Medium-Risk and Medium-Vulnerability-High-Risk constitute 3% each of the area, while 16% and 25% were designated as Medium-Vulnerability-Medium-Risk and Low-Vulnerability-Medium-Risk, respectively. Although, based on the DRASTIC input parameters, approximately 75% of the area is designated as high and medium vulnerability, however absence of risk activities suggests no risk is posed to the underlying groundwater resource in approximately 52% of this delineated subarea.