Groundwater vulnerability assessment in shallow aquifers using geoelectric and hydrogeologic parameters at Odigbo, Southwestern Nigeria

摘要

Geophysical and hydrogeologic measurements were conducted at Odigbo, southwestern Nigeria, aimed at characterizing the unsaturated zone (vadose zone) and use these characteristic parameters to assess the vulnerability of the aquifers within the phreatic zone to contamination from surface sources. Thevadose zone thickness, ranging between 2.3m and 9.6m across the area, is generally far below the DRASTIC index rating concerning the classification of depth to groundwater thickness rating, aptly suggesting that the aquifers in the area are significantly vulnerable to contaminants deriving from surface or anthropogenic activities. Based on the geoelectric parameters of the near surface (first and second) geoelectric layers, areas of low interpreted resistivities (1-100 Ohm-m) in the north central to south central portions constitute about 20% and 16% in the first and second layers respectively. Areas of high interpreted resistivities (> 300 Ohm-m), occurring in the north-west, south-west, north-east and southern segments constitute about 21% and 52% in the first and second geoelectric layers respectively. Aquifers within zones overlain by geoelectrically pervious materials (with resistivity values ranging from 101 to 299Ohm-m), and constituting about 45.5% of the study area, are considered vulnerable to environmental risks, while zones capped by geoelectrically impervious materials, or protective aquitard (about 54.5% of the area), are inferred less vulnerable. Since anthropogenic activities pose potentially great threat to groundwater quality, coupled with the growing demands for groundwater for water supplies, vulnerability assessment thus become imperative to enable the delineation of areas of high or low aquifer sensitivity in any grade of planning and management of groundwater resources.