Time-lapse dielectric properties monitoring of the flow cell during DNAPL contamination and remediation processes by full-waveform inversion of GPR data using particle swarm optimization: A laboratory study

Abstract

This study presents the monitoring of the contamination and remediation processes of an aquifer system using the ground-penetrating radar (GPR) method. To achieve this, periodic GPR measurements were performed on a flow cell, which was established to simulate aquifer contamination and remediation scenarios. The exterior of the flow cell was made of plexiglas, and a small hole was created at the top center to inject Dense Non-aqueous Phase Liquid (DNAPL) contamination. To study contaminant distribution and remediation processes, DNAPL Trichloroethylene (TCE) was used as the groundwater contaminant. Methyl-beta-cyclodextrin (MCD) was used as remediation agent. The interior of the flow cell was packed with two different sand grain size distributions: low permeability sand along the bottom and a higher permeability sand along the top. Common offset GPR measurements were performed along the top plane of the flow cell. The GPR measurements were conducted periodically over a total duration of 66 days, encompassing the phases prior to injecting DNAPL TCE, after injection of DNAPL TCE, and during the remediation process using MCD. Time-lapse GPR data were analyzed using 1D and 2D plots. In addition, to evaluate and monitor the contamination and remediation processes within the flow cell, central traces of the time-lapse GPR dataset were inverted by full-waveform inversion to estimate the dielectric properties of the layers and the depth by Particle Swarm Optimization (PSO). To utilize PSO, a forward model calculation using 1D finite difference in time domain adapted to PSO algorithm to generate theoretical GPR traces to compare the theoretical model GPR traces to the measured GPR traces as a “goodness of fit” metric. The electrical conductivity, relative dielectric permittivity, relative magnetic permeability, and depth of the layer were estimated by PSO of the GPR data. The results of the inversion process indicated a significant change in the estimated electrical conductivity values for the post TCE DNAPL contamination stage and the remediation stage. In addition, to support the inversion results, contaminant mass removal was quantified using moment analysis from dissolved-phase contaminant concentrations collected from the flow cell over time. According to the mass recovery calculation by moment analysis, which was collected via an outlet from the flow cell, approximately 38 % of the injected DNAPL TCE mass was removed by enhanced dissolution from the representative aquifer domain of the flow cell during the remediation stage of the experiment.