Monitoring carbon-based remediation of DNAPL-contaminated groundwater via spectral induced polarization.

Colloidal activated carbon (CAC) is an emerging remedial enhancement fluid that is injected into the subsurface to adsorb hazardous industrial compounds for subsequent removal. CAC-enhanced remediation relies on accurate subsurface characterization and monitoring to ensure CAC reaches intended treatment locations. The objective of this study was to assess the effectiveness of the spectral induced polarization (SIP) technique to track CAC migration within porous media and its adsorption of the chlorinated solvent, tetrachloroethylene (PCE). Dynamic column experiments were performed with cyclic injection and flow of groundwater, CAC, and PCE within porous media, and simultaneous measurements of SIP and effluent quality. Results showed an increase in both the real and imaginary conductivities of the SIP response during injection/flow of CAC within porous media. Real conductivity returned to pre-CAC levels during subsequent flushing of CAC with groundwater, which had left behind only carbon-coated soil grains; however, imaginary conductivity identified the change in polarizability due to the alterations on the grain surface. The subsequent adsorption of aqueous phase PCE did not generate a distinctive change in SIP response, mainly due to the low 50 mg/L concentrations used. Overall, this study suggests that SIP can be a valuable tool to effectively and non-invasively track the migration of injected CAC within porous media for contaminant adsorption, suggesting it can be used to enhance the implementation and management of environmental remediation programs.

Complex electrical measurements of waste rock during acid mine drainage generation and release: Kinetic column tests.

Acid mine drainage (AMD) emanating from waste rock piles (WRPs) at mining sites is a global concern. Successful rehabilitation of these sites requires effective characterization and monitoring of the waste rock during AMD generation/release. Traditional approaches involve ex-situ analysis of waste rock and porewater samples collected via corings and monitoring wells; however, this is highly disruptive, costly, and provides sparsely distributed point information across enormous volumes typical of WRPs. Geoelectrical techniques are a promising approach for non-invasive continuous imaging; however, their application has been limited to 'one-off' imaging with few studies on monitoring waste rock evolution. The objective of this study is to assess the geoelectrical signatures of changing waste rock during AMD generation/release. Field waste rock samples were extracted from three mine WRPs and first characterized for mineralogy and acid generation potential. Kinetic tests were then performed on each sample using leaching columns and humidity cells, with simultaneous measurements of effluent quality and complex electrical conductivity (real and imaginary components measure conduction and polarization, respectively). Results show that real conductivity was highly sensitive to changes associated with AMD leachate quality (e.g., 28,800 to 68 mg/L acidity) and surface of the waste material. Imaginary conductivity measurements identified changes in the waste mineralogy over time, though these signatures were not very distinct, which is likely due to low sulfide contents and limited oxidation (e.g., 0.59 wt% sulfide and 33% air saturation). This study improves our understanding of geoelectrical signatures associated with real waste rock, demonstrating the potential application of the electrical resistivity tomography and induced polarization techniques for mine waste investigations.

Spectral induced polarization signatures of smoldering remediation enhanced with colloidal activated carbon: An experimental study.

Monitoring the remediation of soil and groundwater contaminated by organic compounds remains highly challenging. Thermal treatments, such as smoldering combustion, have become established remediation techniques for destroying contaminants. Smoldering combustion can now be supported by colloidal activated carbon (CAC), with CAC being able to both adsorb contaminants and supplement the fuel source for destroying them. Despite this potential, effective performance monitoring of smoldering remediation remains limited. The objective of this study is to investigate the potential of the spectral induced polarization (SIP) geoelectrical technique to assess the performance of smoldering remediation of soils supplemented with CAC. SIP column experiments were first conducted to assess the response of SIP (i.e., real and imaginary components of the complex electrical conductivity) to varying concentrations of CAC in imitated field soils that contain, or do not contain, organic matter (OM). Results demonstrate that increasing OM and CAC contents increase both the real and imaginary conductivities, with the imaginary conductivity also showing frequency dependence. Smoldering and SIP column experiments were then conducted to assess the effectiveness of SIP for detecting changes in soils of varying OM and CAC contents that have been remediated by smoldering. Examination of the soils before and after smoldering indicates that SIP can track the evolving real conductivity and imaginary conductivity (in particular) between different soil compositions and different stages of the remedial process. High resolution scanning electron microscopy imaging was performed on all samples to validate the SIP and smoldering experiments, confirming significant reductions in carbon after smoldering. Overall, this study suggests that SIP has potential to track changes associated with the addition of remedial fluids like CAC in the subsurface, and the destruction of contaminants adsorbed to CAC by smoldering combustion.