Aquifer Vulnerability Assessment of Groundwater in a Basement Complex Geology using Fuzzy Analytical Hierarchy Process

Groundwater which is found in aquifers is one of the most reliable sources of water supply and its quantity is as important as its quality. Various factors contribute to the increase in contamination of groundwater thereby limiting its use. Basement complex have been known to be more susceptible to contamination due to the nearness of the aquifers to the surface. The study was carried out to develop an aquifer vulnerability map of the area using a fuzzy-analytical hierarchy process (FAHP). Vertical Electrical Sounding (VES) was adopted for the electrical resistivity survey using the Schlumberger array. A total of 72 parametric VES locations were occupied beside wells and boreholes in the study area. The results of the geoelectric data show a three, four and five-layered geologic subsurface essentially characterizing the study area with HA, H, and A curves observed to be dominant. From the interpreted geophysical data obtained, three parameters namely; overburden thickness, longitudinal conductance and coefficient of anisotropy including slope and lithology from remote sensing and geological datasets were employed for the aquifer vulnerability assessment. The fuzzy-analytical hierarchy process was employed in assigning weights to the various parameters implemented for this research. The aquifer vulnerability index of the study area was classified into five; very low, low, moderate, high and very high. The aquifer vulnerability map produced showed that high and very high aquifer vulnerability indices were observed to dominate the majority of the study area most especially in the migmatite gneiss, porphyritic granite and biotite hornblende granite region. The model validated using a correlation of the aquifer vulnerability index values and water quality index values via the receiver operating characteristics (ROC) curve showed 64 % accuracy. The result obtained showed that the method is effective for the assessment of aquifer vulnerability in the study area.