Application of time-lapse ERT to assess heterogeneity in soil moisture dynamics and contaminant plume and associated geotechnical and environmental risks

In modern times, time-lapse electrical resistivity tomography (TL-ERT) has become a powerful investigative tool that can be used in conjunction with regular point-based procedures to track seasonal soil moisture content (SMC) at engineering sites and to monitor the effects of induced biodegradation of contaminant plumes/effluents when released into the environment. These applications are crucial for mitigating the potential risk of geotechnical instabilities and protecting the environment, human health, and ecosystems from the risks associated with contaminants when released into the surface. TL-ERT data sets from two locations in Effurun Delta State, southern Nigeria, were employed in monitoring seasonal variations in SMC at an engineering site and the effect of biodegradation of contaminant plumes on the environment in an automobile workshop. In total, six (6) 2D ERT and four (4) 2D ERT profiles acquired during two time seasons in an engineering site and automobile workshop, respectively, were used to perform simultaneous inversion. The 2D ERT data acquired at the engineering site in the rainy and dry seasons characterized the hydrogeological behavior of the subsurface, with high and low water content (with a resistivity range of 200–356 Ωm) observed in both seasons. The geology of the study site is predominantly sand with low water-retaining capacity. The fine/clayey sand layers with fine particles that retain more water can initiate geotechnical instabilities, making the overlying materials susceptible to failure. The percentage differences in electrical conductivity (EC) between the monitoring periods showed little or no significant change in the EC of subsurface water. Also, the results of 2D ERT data sets acquired between September 2023 and September 2024 at the auto mechanic workshop showed increasing evidence of biodegradation of high-ER contaminants indiscriminately spilled on the surface into low-resistivity/conductive plumes with an ER range of 10-31.6 Ωm and percentage conductivity differences in the order of 50 to 100%. The percentage differences in ER show that the low ER/conductive zones had spread within the subsurface to a depth of about 17.1 m, and this implies that overtime the automobile workshop generates a toxic blend of contaminants which seeps into the ground/soil from the surface. This complex dynamic process can linger for longer periods, compromising the integrity of the environment and particularly the soil, surface water, and groundwater quality, posing a danger to water resources and environmental and human health. Overall, the accuracy of TL-ERT (a nondestructive geophysical technique) in predicting fluctuations in SMC and observing the effects of biodegradation in contaminant plumes was investigated, and these models determined at small scales remain valid at larger scales.