Influence of Individual mechanisms and remediation concerns involved in the enhanced attenuation of toluene in a shallow fractured dolostone bedrock

Abstract:

Shallow sedimentary bedrock aquifers contaminated with light, non-aqueous phase liquids (LNAPLs) are complex hydrogeochemical subsurface systems with a wide range of natural processes involved in their attenuation. While the fracture network controls the plume migration along the groundwater flow direction by advection, transverse diffusion enhanced by sorption into the porous and impermeable matrix is responsible for the strong retardation of the plume position relative to the groundwater velocity. Prior hydrogeological characterization of a fractured dolostone aquifer contaminated more than three decades ago with toluene indicated the toluene is strongly attenuated in the shallow bedrock due to reduced flux from the source zone resulting from matrix diffusion with sorption, volatilization, phytoextraction and degradation mechanisms. Compound-specific isotope analyses indicated seasonal variability in biodegradation processes is occurring across the site due to dynamic redox conditions resulting from variable recharge conditions. Various mechanisms can be targeted to enhance the natural attenuation capacity and reduce the contaminant mass in the source zone including chemical oxidation that stimulates biotic degradation. However, the performance of enhanced attenuation approaches in fractured sedimentary rocks is unknown due to the complex combination of processes that are not simple to quantify. This study aims to systematically evaluate the individual effects of a number of mechanisms on overall system remediation, including evaluation of dynamic redox conditions, nutrient reduction, and chemical oxidation through controlled laboratory experiments in combination with high-resolution field data. A series of microcosm experiments are being performed using crushed rock cores and groundwater samples collected from the contaminated fractured dolostone aquifer. The tests will provide insight into various biogeochemical processes and their enhancement or hindrance with different remediation practices including persulfate injection, sulfate biostimulation, and low temperature heat addition using an array of multi-disciplinary measurement techniques. This presentation will provide an overview of the laboratory experiments, field activities, and key findings.