Gis-Based Integrated Approach to Groundwater Potential Assessment of Osun Drainage basin,Southwestern Nigeria.
Remote sensing-derived hydrolineament density and lineament proximity maps, drainage density and geomorphological maps, Vertical Electrical Sounding-derived overburden and aquifer thicknesses maps and extracted soil and lithological maps for Osun Drainage Basin were integrated in GIS environment. This was with a view to assessing the groundwater potential of the basin. SPOT and ASTER DEMs, Landsat8 imagery, five hundred Vertical Electrical Sounding data, hydrogeological data from seventy two boreholes and ancillary data were acquired. Lithological and soil maps were prepared from existing maps. Automated procedures were adopted to extract lineaments from enhanced images of the remotely sensed dataset. The output lineaments were sub-mapped into hydrolineaments and lineament cross points. Topographic parameters including surface curvatures, slope and drainage were derived from DEMs and used to generate 2D and 3D landform models. Vegetation Index module was employed to model the pattern of vegetation from Landsat imagery. The VES data were quantitatively interpreted using partial curve matching technique and computer assisted 1-D forward modeling. Overburden thickness, aquifer thickness, longitudinal unit conductance and hydraulic conductivity were generated from VES interpretation results. Groundwater head and basement elevation maps were prepared and used to model groundwater flow-paths. Eleven raster-based maps (lineament density, lineament proximity, drainage density, vegetation index, slope, elevation, landforms, overburden thickness, aquifer thickness, lithology, soil) were prepared and their individual influence on groundwater systematically determined. The thematic layers were subjected to Fuzzy Logic Overlay in ArcGIS environment in order to delineate the basin into various groundwater potential zones. Final groundwater potential model and borehole yields were subjected to correlation and regression analyses. Lineament orientation was predominantly N-S and E-W with minor trends consisting NE-SW, NNE-SSW and NW-SE. Drainage density (1.13 km/km2), stream frequency (0.71st/km2) and length of overland flow (0.44km) suggested moderate infiltration potential across the basin. Sixty-three percent of the VES locations had weathered layer aquifer with thicknesses ranging from 2 to 89.2 m while 31 percent of the locations constituted fractured aquifer with thicknesses varying from 4.4 to 126 m. Longitudinal unit conductance ranged from 1.28 × 10-4 to 0.202 × 101 Siemen. Conductance values were higher than 0.1 in 71 percent of the VES locations, indicating high clay content. Calculated conductivity values (0.2 × 10-5 – 1.47 × 10-3 m/s) was very low, indicating high water storativity but low transmissivity within aquifers. Groundwater flow was characterized by convergence, divergence, inflow and outflow zones; indicating extensional subsurface material and energy exchange between the basin and neighboring basins. The basin was delineated into five groundwater potential zones comprising very high, high, moderate, low and very low. Sixty one percent of the study area was rated poor, 16 percent was rated moderate and the remaining 23 percent was adjudged to have good groundwater potential. Seventy-six percent of the borehole yields correlated with the groundwater potential model with correlation coefficient of 0.872 at confidence level of 0.01. This study concluded that groundwater potential was generally low across Osun Drainage Basin. However, isolated zones of good groundwater potential were evident particularly in the upland area of the basin.