Abstract:
The aquifer vulnerability to contamination risk may increase daily due to continuous
dumping of waste in an area within a typical Basement Complex terrain of Itaogbolu
in Ondo State, southwestern Nigeria. The aim of the research is to develop a
composite aquifer vulnerability map around some selected dumpsites within the area
using a modified ‘DRASTIC’ (DRASTIC-L) model. DRASTIC model accounts for
seven aquifer parameters namely, depth to water table, net recharge, aquifer media,
soil media, impact of vadose zone and hydraulic conductivity. However, the
modified DRASTIC (DRASTIC-L) incorporates land use as additional input
parameter and the analysis was performed using ArcGIS software to prepare a
vulnerability map of the study area. Data from Vertical Electrical Soundings (VES),
remote sensing and hydrogeology were used for the eight parameters. The VES
involved fifty six (56) depth soundings using Schlumberger array, with half
maximum current electrode separation (AB/2) of 65 m. The remotely sensed
parameters were extracted using LANDSAT 7 Enhanced Thematic Mapper Plus
(EMT+) and Digital Elevation Model (DEM). The DRASTIC-L Vulnerability Index
(DVI) was calculated as the sum of product of ratings and weights assigned to each
of the parameter on the scale of 1 to 10 and 1 to 5 respectively. Using the produced
vulnerability index, the area was classified into five classes; very high, high,
medium, low and very low vulnerability zones. The research revealed that, part of
the dumpsites falls within the high vulnerable zone; hence the groundwater within
the area may be under the threat of contamination from the leachate products of the
dumpsites. Using the chemical analysis of water samples within the classified zones
to validate the model, the modified DRASTIC-L shows a reliable accuracy of 76%.
