Abstract
Lagos is the world’s sixth largest city, the most populous city in Africa and the most populous city inNigeria. A total of eighteen groundwater exploitation borehole logs together with hydrogeological and geotechnical data were used for the study. The eighteen available borehole logs were categorized into seven areas spanning the shoreline to inland boundary ofLagosState. The study area has a high net recharge of1838 mm/yr and the aquifer media is sand. The intrinsic vulnerability map show areas of highest potential for groundwater pollution based on hydro-geological condition and human impacts. Seven major hydro-geological factors incorporated into DRASTIC model and the geographic information system (GIS) were used to create a groundwater vulnerability map by overlaying the available hydro-geological data. The output map shows that the southeast of the aquifer is under very high vulnerability while central parts of aquifer have high vulnerability. Other parts (north, northwest and south) of the study area have moderate vulnerability to pollution. For testing of the vulnerability assessment, groundwater quality data were collated from literature for the different vulnerability zones of the study area. The chemical analysis results show that both the southeast and northwest west parts of study area aquifer (very high and moderate vulnerability zones) have higher nitrate concentration relative to the rest of aquifer, that are located in high vulnerability zone. The validation of the DRASTIC models was accomplished through pair wise comparison of DRASTIC vulnerability maps (using Pearson’s r correlation coefficient) with a total of 14 layers representing original DRASTIC input data, Land cover (LC) features, and groundwater TDS, Cl- and NO3 data. Results from the correlation analysis indicate a significant association between high groundwater TDS, NO3 concentrations and distances from certain LC types.
Highlights
In developing countries in Africa, Asia and South America for an estimated 1300 million urban dwellers the main source of drinking water is groundwater
Seven major hydro-geological factors incorporated into DRASTIC model and the geographic information system (GIS) were used to create a groundwater vulnerability map by overlaying the available hydro-geological data
The results show that Area 1 (Eti-Osa LGA) consisting of Victoria Island, Ikoyi and Lekki suburbs, south east of the study are has the highest vulnerability index (208), and has the greatest contamination potential
Summary
In developing countries in Africa, Asia and South America for an estimated 1300 million urban dwellers the main source of drinking water is groundwater. This groundwater may be contaminated by infiltrated wastewater, because very often a sewer system is not present and households dispose of their solid and liquid waste on-site [1]. A positive correlation between the model results and field data was reported by [8,9], while others have reported little correlation [10,11] Despite these concerns DRASTIC has been applied worldwide with adaptations on the procedures to elaborate thematic maps and the use of different ratings criteria [12,13,14,15]. This paper presents a method to assess groundwater vulnerability to pollution and risk mapping with limited data based on the integration of a GIS-based DRASTIC method with groundwater quality data and the proximity analysis of groundwater pollution aggravating land cover features for Lagos, Nigeria
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