Abstract
Due to complex and erratic nature of groundwater occurrences in crystalline basement terrains, groundwater development in form of boreholes/wells without the necessary pre-drilling hydrogeological investigations usually results in failure. Therefore, there is the need for adequate characterization of aquifers and delineation of groundwater potential zones in such crystalline basement setting. This study employed the integration of multi-criteria decision analysis (MCDA), remote sensing (RS) and geographical information system (GIS) techniques to delineate groundwater potential zones in crystalline basement terrain of SW-Nigeria and validation of the result with existing borehole/well yield data. The study approach involved integration of nine different thematic layers (geology, rainfall geomorphology, soil, drainage density, lineament density, landuse, slope and drainage proximity) based on weights assignment and normalization with respect to the relative contribution of the different themes to groundwater occurrence using Saaty’s analytic hierarchy approach. Following weigh normalization and ranking, the thematic maps were integrated using ArcGIS 10.0 software to generate the overall groundwater potential map for the study area. The result revealed that the study area can be categorized into three different groundwater potential zones: high, medium and low. Greater portion of the study area (84,121.8 km2) representing about 78 % of the total area, fall within the medium groundwater potential zone which are generally underlain by medium-porphyritic granite, biotite-hornblende granite and granite gneiss bedrock settings. About 18,239.7 km2 (17 %) fall under high groundwater potential zone which are characterized by weathered/fractured quartzite, quartz-schist, amphibolite schist and phyllite bedrock settings. However, areas of low groundwater potentials constitute only 3 % (3,416.54 km2) of the total study area and are mostly underlain by migmatite, banded and augen gneiss bedrock settings. Subsequent validation with boreholes/well yield data revealed a good correlation with respect to the observed groundwater potential zonation. Wells/boreholes with yields greater than >150 m3/day are generally characteristic of areas with high groundwater potential while those with yields of 75–150 and <75 m3/day are typical of areas with medium and low groundwater potentials, respectively. The validation clearly highlights the efficacy of the integrated MCDA, RS and GIS methods employed in this study as useful modern approach for proper groundwater resources evaluation; providing quick prospective guides for groundwater exploration and exploitation in such crystalline basement settings.
Highlights
The ever increasing population in the developing world, especially in the sub-Sahara Africa, coupled withAppl Water Sci (2014) 4:19–38 increasing agricultural and industrial development warrants greater demand for essential public utilities, most especially water supply for domestic and agricultural purposes
Wells/boreholes with yields greater than [150 m3/day are generally characteristic of areas with high groundwater potential while those with yields of 75–150 and \75 m3/day are typical of areas with medium and low groundwater potentials, respectively
As the GPI is the total weightage of all the factors in an area and the classification of groundwater potential zones represents the relative availability of groundwater, the greater the weight-score, the higher the groundwater potential
Summary
Appl Water Sci (2014) 4:19–38 increasing agricultural and industrial development warrants greater demand for essential public utilities, most especially water supply for domestic and agricultural purposes. The readily available alternative option to the epileptic pipe-borne water supply from reservoirs and dams is the development of groundwater system in the form of dug wells and boreholes, especially in the crystalline basement terrains, where shallow groundwater systems can be tapped with minimal cost compared to with the costlier surface water development. The occurrence and movement of groundwater in a crystalline terrain depends on the degree of weathering and extent of fracturing of the bedrock rocks, while identification of potential groundwater zones could aid in proper development and utilization of groundwater resources for eliminating water scarcity (Rao 2006). There is the need for adequate characterization of basement aquifers and delineation of groundwater potential zones
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