Leachate delineation and aquifer vulnerability assessment using geo-electric imaging in a major dumpsite around Calabar Flank, Southern Nigeria.

Abstract

A near-surface resistivity investigation was undertaken to investigate soil contamination and aquifer vulnerability due to leachate percolation in a major dumpsite in Calabar. The study was undertaken using 2-D electrical resistivity tomography (ERT) and vertical electrical sounding. Ten traverse lines were created within and outside the dumpsite and geo-electric measurement was conducted using dipole-dipole and Schlumberger electrode configurations. Interpretation of the measured data was carried out using DIPROWIN and WinResist program. 2-D resistivity structures were produced, showing four resistivity layers. Conductive layers with a resistivity of less than 14Ωm were delineated as leachate infiltrated and soil-contaminated zones. The study has shown that leachate has spread across all the survey lines except the two control stations outside the dumpsite. The investigation has further revealed that leachate has percolated the subsurface up to 25m in the area. Subsequently, twenty vertical electrical soundings were carried out to evaluate aquifer protective capacity/vulnerability in the area. The interpretation shows the area is underlain by four to five geo-electric layers. Total longitudinal conductance values were calculated. Two VES points (14 and 19), representing 10% of the sounding points, showed a weak protective capacity rating; eight VES points (1, 7, 9, 11, 12, 13, and 18), representing 35% of the soundings, showed a moderate protective capacity rating; ten VES points (2, 3, 4, 5, 6, 8, 10, 15, 16, 17, and 20), representing 55% of the total soundings, showed a good protective capacity rating. Three (3) hand-dug wells were sampled with the view to obtaining the groundwater and leachate flow direction within the dump vicinity. An electric water level recorder was used to determine the static water level (SWL) of the three wells, and the leachate flow direction was determined to be north-easterly.