Spatio-temporal monitoring of leachates dispersion beneath a solid wastes dump in basement complex of southwestern Nigeria

Abstract

For real-time monitoring and modeling of leachates dispersion beneath Arada wastes dump, Ogbomoso geologically located within Southwestern Nigeria basement complex, an integrated technique involving time-lapse Electrical Resistivity Tomography (ERT), hydrogeology, and geotechnic was used. ERT was performed using Wenner electrodes configuration in a Northeast-Southwest azimuth on the wastes dump monthly from March to September, and twice in March and September, at the control site, during the wet season. The Total Dissolved Solids (TDS), Electrical Conductivity (EC) and static water level of two monitoring wells on the dumpsite were measured during every ERT surveying. Twenty four soil samples were taken at 0.5 m intervals from 2.0 to 3.0 m deep bores on the anomalously low resistivity spots discovered on the subsurface resistivity distribution of the wastes dump, and sieve analysis and permeability tests were performed in the laboratory on these samples. Using the approach of grain-size analysis, the hydraulic conductivity and porosity were calculated empirically. On the wastes dump and control site, the resistivities of spatially distributed lenses of material ranged within 1.45–37.3 Ωm and 22.5–52.6 Ωm, respectively, suggesting leachates and clay anomalies. The wastes dump subsurface might be regarded as a porous medium since its porosity ranged between 38.2 and 44.2%, and the permeability range of (5.4–9.6) x 10−5 m/s showed that water could freely drain from it. Water samples taken from the monitoring wells on the waste dump had TDS and EC that ranged from 520 to 970 ppm and 1.05 to 2.37 mS/cm, respectively, but decreased in the southwest direction of groundwater flow, indicating leachates dispersion. The hydraulic conductivity ranged within 0.00285–0.176 m/s but varied anomalously and spatially indicating heterogeneous hydraulic conductivity field capable of initiating local variation in velocity that caused leachate to disperse longitudinally and transversely. The rate of longitudinal and transverse leachates dispersion as revealed by subsurface resistivity distribution images ranged within 0.000–0.250 m/day and 0.001–0.045 m/day, respectively. The former rate was 5.6 folds the latter rate and hence fell within the stipulated 5–100 for dispersion process that conformed to natural consequence of dispersivity. The conductivity-time curves (CTCs) of the leachates anomalies' steepness and doubly-peaked properties indicated Fickian dispersion, while different proportion times; t-1.588 and t-2.478, which are not equal to t-0.5, showed that the CTCs described non-Fickian dispersion at a significantly later time.