APPLICATION OF ENTROPY MODEL TO GEOPHYSICAL AND SATELLITE DATA FOR GROUNDWATER VULNERABILITY IN A BASEMENT COMPLEX

Abstract

Groundwater vulnerability of Ogbese Area Southwestern Nigeria, has been evaluated using, the application of Entropy model to Geophysical and Satellite Data. Five (5) parameters were considered; Topsoil Resistivity (TSR), Hydraulic Conductivity (HC), Longitudinal Conductance (LC), Drainage Density (DD) and Topographic Slope (TS). Multi-criteria decision analysis technique was employed to assign weight to each of the parameters using the concept of Entropy model. In order to evaluate the groundwater vulnerability of Ogbese, twenty eight (28) Vertical Electrical Sounding (VES) points were occupied. Three (3) type-curves were delineated in the study area; the type-curves varied from three layered A and H type curves to four layered KH. Four (4) subsurface geo-electric sequences of topsoil, lateritic clay, partially weathered/fractured basement and the fresh basement were delineated in the area. With the help of Entropy technique, different weights has been assigned to all the conditioning factors; (TSR=0.25, LC=0.38, HC=0.0082, TS=0.15, DD=0.20) with the Longitudinal Conductance (LC) having the highest weight of all the five parameters. . All the physicochemical parameters of the analyzed well water samples are below the NIS maximum permissible standards for drinking water, indicating that the groundwater within the study area has not been polluted by cassava effluent or leachate from dumpsite. However the groundwater vulnerability map of the study area indicates that the northern part, central and southern parts of the study area comprises of moderate to low vulnerability, which is as a result of the clayey layer overlying the aquiferous unit. The depth extent may have been limited by relatively thick column of clay which may have acted as a barrier to further contamination.