Theme: 10. Characterization, Fate and Transport

Topic Area: 10a. Advanced Investigation Tools and Techniques

Formatted Title:

Characterizing Shallow Groundwater and Surface Water Interaction in a Dynamic VOC Plume Using Real-Time Instrumentation

Background/Objectives:

The focus of the field investigation presented in this abstract is a volatile organic compound (VOC) groundwater plume in a heterogeneous shallow aquifer with dynamic interactions between groundwater and surface water at the site. The site is approximately 53 acres and is the former location of manufacturing and pipe coating. Waste disposed inappropriately onsite resulted in soil and groundwater contaminated by polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and VOCs, namely tetrachloroethene, trichloroethene, and vinyl chloride. Contamination exists within shallow soil and groundwater at depths predominantly from 10 to 50 feet below ground surface (bgs). The hydrogeologic framework of the site includes fluvial deposits comprised of silty sand and gravel separated into three hydrogeologic zones (shallow, intermediate, and deep). The separation of these hydrogeologic zones is not defined by the presence of aquitards and are only distinguished by subtle changes in grain size distribution, particularly the percentage of silt within the sand and gravel matrix. The first presence of a confining silt layer is observed at approximately 140 feet bgs across the site. The significant thickness of this unconfined aquifer in conjunction with less distinguished areas of reduced silt and greater permeability for plume migration presents a challenge for optimally locating permanent well screens for monitoring the groundwater plume. The relatively thick unconfined aquifer system also has a very shallow water table that interacts seasonally with surface water drainages throughout the site and discharges to a wetland area at the site’s northern, downgradient boundary. This dynamic hydrogeologic flow regime and its interaction with local surface water features presents a unique setting and complex problem for understanding the fate and transport of the large VOC plume that persists at the site.

Approach/Activities:

Following previous investigations and reporting at the site, it was determined that the potential for interactions between shallow groundwater and surface water was not well understood, and that some of the original source areas were not fully delineated. The field investigation completed in late 2022 and 2023 was completed in order to refine the groundwater flow model and conceptual site model, and to address identified data gaps at the site. The recent field investigation completed included shallow soil sampling using direct push technology, deep soil and groundwater sampling and monitoring well installation using roto-sonic technology, surface water staff gauge installations, measurements, flow monitoring and sampling, groundwater sampling in 135 new and existing wells, and long-term water level monitoring across the site. Shallow soil sampling was conducted to assess concentrations and extent of shallow soil contamination (above 30 feet bgs). Deep soil and groundwater samples were collected during the advancement of the roto-sonic drilled boreholes and evaluated for VOC concentrations in real time using a portable micro gas chromatograph - photoionization detector (FROG-5000). These real-time VOC readings with field instrumentation were used to help locate the permanent well screen location for each borehole within the thick unconfined aquifer to optimally characterize and monitor the VOC plume. Seven surface water monitoring locations were established across and adjacent to the site following reconnaissance observations and mapping. Locations were chosen based on upgradient, downgradient, or within the area of the plume, near stormwater culverts, and adjacent to wetland areas. These locations were sampled, monitored, and measured during dry and wet seasons (over the course of one year) to observe surface water and groundwater interactions and assess potential surface water contamination. Surface water was sampled for VOCs in November 2022, January 2023, and April 2023. Two staff gauges were installed to monitor surface water levels, located generally upgradient and downgradient of the site. Stream velocities were also measured at all seven surface water locations using an OTT MF Pro Flow Meter with a top-set wading rod. Stream velocities were measured during the seasonal peak for discharge. Up to 13 wells were instrumented with vented pressure transducers beginning in April 2023 to observe seasonal changes and evaluate the groundwater aquifer water levels relationship to the local surface water drains.

Results/Lessons Learned:

Real-time data acquisition using FROG VOC analysis and groundwater level and surface water gauging monitoring facilitated rapid mapping of the groundwater plume and identified areas where groundwater to surface water VOC discharge was periodically occurring. The VOC mapping confirmed that VOCs in the upper, unconfined aquifer were subject to seasonal and episodic discharge to adjacent wetlands and the stormwater system. In addition, anthropogenic pumping from sump pumping/dewatering activities for maintaining dry foundations in commercial buildings located within and adjacent to the site were identified that could potentially influence VOC plume migration and discharge throughout the monitoring period. Overall, interactions between shallow groundwater and surface water, in part, control the VOC plume in the upper aquifer. The modified remedy will need to account for these interactions. While it is likely that natural attenuation will be component of the modified remedy, seasonal and/or localized engineering controls will need to be instituted to limit the VOC discharge to the surface and contaminant migration offsite.

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Madison Gates