ASSESSMENT OF THE EFFECTS OF DUMP SITE ON GROUNDWATER QUALITY IN BARIKIN-SALE, MINNA NORTH-CENTRAL, NIGERIA

Abstract

Discharge of leachate from refuse dumpsites is a source of groundwater pollution within its immediate environment of location. Geophysical investigation of an uncontrolled open solid waste dumpsite located at Barikin-Sale area of Minna was carried out for possible contamination of groundwater. The investigation was aimed at delineating groundwater contamination due to leachate percolation thereby assessing the quality of groundwater from hand dug wells and boreholes within the dumpsite and the surrounding environment. A total of ten (10) Vertical Electrical Sounding (VES) points with maximum current electrode spacing of 20 m and two (2) 2D Subsurface Electrical Imaging were investigated within and outside the dumpsite to assess leachate migration in the area. Schlumberger configuration was used for the VES while the Wenner configuration was used for the subsurface imaging. The VES data were analysed and a maximum of three (3) geoelectric sections were identified; the top soil, weathered basement and the fractured/fresh basement. The obtained apparent resistivity for those layers were between 19.4 Ωm and 122.6 Ωm for the first layer, 18.2 Ωm and 33.4 Ωm for the second layer and 93.5 Ωm and 166.4 Ωm for the third layer respectively. The range of thickness for the first layer is 0.9 m and 1.8 m and for the second layer are 4.0 m and 5.3 m respectively. The 2D resistivity data were processed and inverted using the RES2DINV software. The inverse resistivity models of the subsurface from the 2D subsurface electrical imaging revealed low resistivity value <20 Ωm which is taken to be leachate derived from decomposed waste while these wastes that cannot decompose are occurring as isolated parts with slightly higher resistivity value of >20 Ωm. The areas with the highest resistivity value of >100 Ωm were further interpreted to be chemical weathering product of crystalline bedrock considered to be regolith. Sieve analysis of sixteen (16) soil samples from four (4) pits was carried out; the result shows that the soils are fine to medium sand and the hydraulic conductivity was computed from it using the HydrogeoSieveXL software. From the estimated hydraulic conductivity (K), the values range between 1.97 x 10-4 and 2.52 x 10-5 m/s. This further clarifies the geophysical investigation results. The physicochemical analyses of ten (10) water samples from hand dug wells and boreholes were carried out. The results show that the median and mean concentration of conductivity at 1290 µs/cm and 1616 µs/cm, TDS at 864.5 mg/l and 1078.7 mg/l and nitrite at 0.039 mg/l and 0.4929 mg/l all exceed the permissible limits of 1000 µs/cm, 500 mg/l and 0.2 mg/l of NSDWQ (2007) and WHO (2010) indicating groundwater contamination. Although, at some locations the analysed parameters exceed the permissible limits but their median and mean concentrations are generally inside the permissible limits. From the results, it could be concluded that leachates are concentrated within the lower part of the dumpsite; therefore the surrounding environment groundwater sources are vulnerable to leachate contamination from the dumpsite.