Formatted Title
Characterization of a PCE Plume within a Point Bar Accretion Complex by Application of HRSC Technologies and ESS Principles
Background/Objectives
The site consists of a tetrachloroethene (PCE) plume in groundwater resulting from release(es) from a former drycleaner which operated from 1969 to 1997. The site is located in a small town along the Ohio River in West Viriginia and consists of a mixed-use residential/commercial/industrial area. PCE has been detected in downgradient municipal water supply wells at concentrations exceeding regulatory limits periodically since 2010. A municipal water treatment system was installed in 2020 to mitigate the risk to the public drinking water supply; however, vapor intrusion remains a potential concern for the site. The groundwater plume was determined to extend beneath a relatively densely-populated area of town, and the potential for vapor intrusion is exacerbated by the high transmissivity of the unconsolidated overburden consisting of fill material underlain by alluvial sediments. Ongoing Remedial Investigation (RI) activities have been conducted systematically to: confirm the source of contamination; to horizontally and vertically delineate the contaminant source area; to characterize migration pathways, including potential discharge to the Ohio River; and to assess potential vapor intrusion risk. Due to the fluvial depositional environment for site overburden, application of environmental sequence stratigraphy (ESS) principles is warranted during development of the conceptual site model (CSM) to refine the understanding of groundwater flow and contaminant migration within a heterogeneous clastic aquifer.
Approach/Activities
A town-wide near-surface passive soil gas survey was conducted to confirm the source of contamination at the former drycleaner. Based on the results, high-resolution site characterization (HRSC) technology consisting of membrane-interface probe/hydraulic profiling tool (MIP/HPT) direct imaging was used to delineate the contaminant source area and to characterize hydrogeologic properties of the overburden profile. Ongoing/planned activities include correlation of HPT data to co-located downhole lithology logs for development of a high-resolution site stratigraphy model based on the HRSC data and applied ESS principles. A three-dimensional CSM will be developed for identification of preferential migration pathways and contaminant storage zones, for determining placement of groundwater monitoring wells, for assessing areas with potential vapor intrusion risk, and for targeting future remediation efforts.
Results/Lessons Learned
Results to date include findings of the source area investigation/delineation activities. Based on results of the town-wide passive soil gas survey, delineation efforts were focused on the suspected source area near the former drycleaners building. Based on the MIP results, contaminant distribution was determined to extend in some areas counter to the predominant groundwater flow direction which is toward the river. HPT results indicate heterogeneous lithologic conditions underlying the contaminant source area and the site as a whole, with evidence of low permeability sills that influence contaminant migration pathways and constitute secondary sources due to contaminant sorption and back-diffusion. Additional results anticipated to be available at the time of presentation include the developed CSM, characterization of the contaminant plume in groundwater, and findings of vapor intrusion investigation. A summary of results and value-added will be presented for each phase of the investigation. Lessons learned, including technological and practical limitations will be presented.