Formatted Title
Bedrock Trenching as an Effective Remedial Approach for Containment of Contaminated Groundwater
Background/Objectives
Historical operations at an industrial facility impacted the quality of shallow karstic bedrock groundwater through releases of PCB-containing liquids. Investigations and remedial activities demonstrated groundwater containing dissolved PCBs migrated offsite to a network of springs and seeps along a nearby creek. A conceptual site model (CSM) was developed based on the geologic and hydrogeological information collected and supported a corrective measures study for groundwater containment. That study identified the remedial objective for groundwater, evaluated remedial options, and selected a preferred corrective measure. The remedial objective was to prevent the flow of PCB-impacted groundwater beyond the property boundary. The preferred corrective measure, a downgradient groundwater collection system consisting of a 750-foot-long bedrock trench, was designed and installed to meet the remedial objective.
Approach/Activities
A robust CSM was developed by employing data gathered from a variety of activities, including extensive drilling and coring to help characterize bedrock properties and topography; an aerial thermal study followed by ground reconnaissance to identify springs and seeps; multiple rounds of dye tracing; hydraulic head monitoring in wells; PCB monitoring of groundwater, spring water, and surface water; geophysical surveys; and interim remedial measures. The CSM showed shallow groundwater movement in an area of interest at the site was controlled by a bedrock valley, which forms a tributary to a nearby creek and directs groundwater offsite. The bedrock valley is oriented along the dominant joint set in the region and is interpreted to be the result of preferential karst weathering. Most groundwater in the shallow bedrock moves through a network of integrated solution porosity formed along bedding planes and steeply dipping joints. Bedrock becomes increasingly competent with depth. To prevent the offsite movement of groundwater in the area, the trench was installed across and perpendicular to the valley and seated in the underlying competent bedrock.
A performance monitoring program was designed and implemented to assess the horizontal and vertical capture of groundwater by the trench. The program included multiple rounds of dye tracing, hydraulic head monitoring in wells and the trench, and monitoring groundwater and surface water for PCBs. Because of the complexities of the karst setting, multiple lines of evidence were used to assess whether the remedial objective was met.
Results/Lessons Learned
The performance monitoring confirmed the groundwater flow pattern to be consistent with the CSM and demonstrated the trench was effective in preventing offsite migration of groundwater across the karstic bedrock valley. No tracer migrated beyond the trench or to monitored seeps, springs, or the nearby creek. The results demonstrate that bedrock trenches in karst can be an effective means of controlling groundwater and contaminant movement in these extremely complex aquifers. Measuring the performance of a remedy that involves controlling groundwater movement in karst requires an approach that differs from what is typically employed at non-karst remedial sites. Specifically, in addition to the typical approach (e.g., using data from monitoring wells to infer groundwater flow directions and monitor remedy performance), conducting dye tracing constitutes an important, complementary line of evidence necessary to validate the CSM and assess the effectiveness of groundwater remedies implemented in karst terranes.