Geological Behavior (GBR)

The usefulness of groundwater vulnerability mapping cannot be overemphasized in planning, policy formulation and decision-making for groundwater management and protection. The present study employed geographic information system based overlay and index methods (DRASTIC, DRASTIC-LU, GOD and AVI models) in assessing and mapping groundwater vulnerability zones in Ile Oluji area of Ondo State, Southwestern Nigeria. The models’ parameters were prepared using hydrogeological (well/borehole) data, geophysical data, and satellite imageries. The weightage of different parameters was done using analytical hierarchy process. The DRASTIC map classified the area into the low vulnerability (30 % of the area), moderate (4%), high-vulnerability zone (60 %), and very high vulnerability zone (6 %). The DRASTIC-LU distinguished the area into high – very high groundwater vulnerability zone (90 %), while low – moderately low area has 10 % aerial extent. The GOD and AVI models, categorized the vulnerable areas into quite low (<0.03) to low (0.03 – 0.3); and high (0.98 – 1.85) to extremely high (<0.98) vulnerability zones respectively. As a result, the DRASTIC, DRASTIC-LU, and AVI models all showed predominate high vulnerability zone, which was also confirmed by the nitrate map. The index values in the DRASTIC and DRASTIC-LU models revealed significant overlap. Therefore, in the research region, slope, hydraulic conductivity, net recharge, soil medium, and depth to water level are the factors that have the greatest influence on groundwater quality. Due to the significant number of highly vulnerable places, the aquifers must be protected immediately.