GEOPHYSICAL AND HYDROGEOCHEMICAL INVESTIGATION OF DUMPSITES - INDUCED GROUNDWATER CONTAMINATION IN WARRI AND ITS ENVIRONS, SOUTHERN NIGERIA

Abstract

Geophysical and hydrogeochemical investigation of dumpsites induced groundwater contamination in three dumpsites and a Control site in Warri and its environs, Southern Nigeria has been carried out. This was with a view to mapping contaminant flow and impact on groundwater in the area emanating from indiscriminate dumping of domestic and industrial wastes. Geophysical methods employed include the Electrical Resistivity, adopting the Vertical Electrical Sounding(VES) and the Dipole-dipole array techniques, and the Spontaneous Potential (SP), adopting the Gradient and the Fixed Base array techniques. A total of fifty seven (57) VES stations were occupied in the three dumpsites and the control site studied using the schlumberger configuration with electrode separation, AB/2 ranging from 40 to 225 m. Dipole-dipole profiling was carried out along two orthogonal traverse lines of length ranging from 50 to 250 m with electrode separation of 5 and 10 m. Spontaneous Potential profiling was also carried out with electrode spacing of 5 and 10 m. From each of the site, water samples were collected from boreholes, hand dug wells, and swamps, close to and far away from the dumpsites and the control site for chemical analysis and statistical evaluation using multiple regression analysis and Two - way Analysis of Variance (ANOVA). Aquifer parameter values obtained from the VES results and secondary pump test data were used to model the groundwater and contaminant flow using the Groundwater VISTAS version 4. All these measurements were carried out in both the rainy and dry seasons in order to establish the seasonal variation of the migration of leachate from the dumpsites. The geoelectric sections from the VES generated revealed three to four probable subsurface geoelectric units, which are the topsoil, clayey sand/sandy clay and sand. Around the dumpsites, the resistivity values from the VES and Dipole - dipole measurement to a depth of about 15 m varies from 18 to 400 ohm-m as against higher values of up to 2500 ohm-m outside the dumpsite area and the control site. Also, the spontaneous potential (SP) profiles show high positive potential of up to 500 mV outside the dumpsite area to as low as -450 mV around the dumpsite area using the gradient and fixed base array in both seasons when compared with the control site. The low potential and resistivity values around the dumpsite area could be the signature of the oxido-reduction phenomenon occurring at depth in the contaminated groundwater. The isoresistivity maps at depths of 1, 5 and 10 m, show values around the dumpsites varying from 10 - 400, 350 and 750 ohm-m respectively, which indicate the presence of leachate beneath the dumpsite. Hydrogeochemical analysis of water samples showed values of parameters that are higher around the dumpsites when compared to the control site, with the results for conductivity, nitrates, iron, lead and total viable bacteria count exceeding the World Health Organisation (WHO) and the Standard Organisation of Nigeria (SON) permissible limits. The results show that resistivity of groundwater is significantly related to its physico-chemical properties with a contribution of 49.2 %. Also, at P<0.05, the R2 value (0.513) show that 51.3% of the presence of Total Variable Bacteria Count is attributed to groundwater resistivity in Warri and environs. The groundwater flow direction was revealed from the radial elliptical plots and supported by the results from the groundwater/contaminant flow model from the groundwater vistas software to flow in the NE-SW direction. It was also established that the average flow velocity is 0.0178 m/d or 6.3 m/year and if advective flow is assumed, therefore the contaminant would flow at the same speed of 0.0178 m/d or 6.3 m/year. This implies that the leading edge of these contaminants will migrate a radius of 113 m, 138 m and 155 m from the edge of the dumpsites within ten (10) years in sites 1, 2 and 3 respectively. Consequently, the entire Osubi and a good portion of the west of Effurun would have been polluted, whilst Aladja, Ovwian and DSC communities would not be affected.