ASSESSMENT OF CONTAMINATION EFFECT OF LIVESTOCK WASTE ON SURFACE AND GROUNDWATER USING INTEGRATED GEOPHYSICAL AND GEOCHEMICAL METHODS

Abstract

ABSTRACT Cattle confinement causes significant amount of animal wastes in the surface and subsoil, contributing an enormous amount of pollutant into the soil and nearby aquifer units. In order to evaluate the degree of pollution, the extent of migration of leachate formed from animal wastes and its impact on shallow aquifer unit should be appraised. In view of this, electrical resistivity survey (1D vertical electrical sounding (VES) and 2D electrical resistivity Tomography (ERT)) and geochemical methods were utilized on the study site and nearby water sources respectively. Apparent resistivity data were measured along sixteen profiles using Schlumberger and Wenner configuration while soil samples were collected from laid transverses at depths of 0-30 cm and 30-60cm respectively. Geochemical assessment of surface and groundwater samples were carried out according to standard laboratory practices while hydrochemical facies of the sampled water was evaluated using Piper Trilinear software. The VES, 2D and 3D resistivity data were processed and inverted using WINRESIST, RES2DINV and RES3DINV software respectively. The ranges of resistivity and thickness of the Topsoil, Silt Fine Sand layer and Clay layers according to VES are (3.2-54.80 Ωm, 0.4-3.4 m), (5 - 75Ωm, 0.2 -19.1 m), (4.6 -188.7 Ωm, 2.7 -12.7 m). The inverse resisting model section of the subsurface from 2D and 3D imaging revealed low resistivity value less than 10 Ωm suspected to be leachate infiltration while 3D inverse section allowed delineation of animal waste plume, weathered layer and seepage path. The extent of migration was up to a depth of about 9m and more pronounced in the south eastern part of the study site. Hence, possible contamination of shallow groundwater system as livestock market ages. All the collected soil sample along the traverse lines belong to sandy loam, moderately acidic with nitrate and moisture content value ranging from 9.87-135.39 mg/kg and 0.238 to 0.324 cm3/cm3, respectively, At selected soil depths. The result of the physicochemical analysis revealed that 77% of water samples to be within the permissible limits of WHO/NSDWQ for drinking purpose besides their suitability for irrigation use. Interpretation of the piper diagram showed NaHCo3 to be the dominant facies in the area while alkaline earth metal (Ca2+ x mg2+) and weak acid (HCo3-, Co3-) are dominant cation and anions in the area. Surface and groundwater in the study area are of fresh and alkaline water type.