MAPPING GROUNDWATER VULNERABILITY TO CONTAMINATION STRESS USING A MODIFIED DRASTIC MODEL IN URBAN PERI-URBAN ,AKURE.

Abstract

Over the years, groundwater vulnerability assessment has proven to be reliable for delineating contaminated or potentially contamination-prone areas. As such, qualitative groundwater vulnerability mapping is widely adopted by researchers as a simple and cheap decision-making tool for protecting groundwater resources from contamination. Therefore, this study assessed groundwater vulnerability to contamination in a highly urbanised basement-complex environment in Akure, Southwestern Nigeria. The vulnerability mapping was performed in a geographic information system (GIS) environment using the original DRASTIC model and a modified form of the DRASTIC model by incorporating two exogenous factors: ‘anthropogenic impact’ (A) and ‘lineament density’ (L); due to their potentially significant impact on the overall vulnerability of groundwater in hard-rock terrains. In addition, the analytical hierarchy process (AHP) was adopted to optimise DRASTIC parameter weights and rates. The result of the original DRASTIC model classified the study area into three vulnerability classes: moderate low (31%), moderate (43%) and high (26%). However, the DRASTIC-AHP and modified DRASTIC-AHP (DRASTICAL-AHP) classified the study area into two vulnerability zones of moderate low (32%) and moderate (68%), moderate (55%) and high (45%), respectively. The integrated vulnerability maps showed that the high-risk zone is concentrated on the southwestern and northeastern part of the study area with shallow buried depth to groundwater, high lineament density and high recharge rates. The validation results obtained from Pearson’s correlation and vulnerability-EC map revealed that the modified DRASTIC-AHP (DRASTICAL-AHP) map presents a more reasonable classification than the conventional DRASTIC model. Overall, a modified GI -based DRASTAL model was proposed that could be adapted to other areas with similar geological/hydrogeological characteristics as that of the study area. Hence, findings from this study can help make informed decisions about groundwater resource management in basement complex terrains.