Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5e. Advances in the Application of Geologic Interpretation to Remediation

Formatted Title:

Evaluation of Chlorinated Solvent Source Mass for the Baghurst Drive Superfund Site Thermal Remedial Design

Background/Objectives:

Background/Objectives. The Baghurst Drive Superfund Site (Baghurst), comprised of one wooded and one farmed property (Farm), is located in a rural setting in Upper Salford Township, Montgomery County, Pennsylvania. Unpermitted waste disposal containing degreasers, principally 1,1,1-trichloroethane (1,1,1-TCA), its anaerobic degradation products and the stabilizer 1,4-dioxane (constituents of concern, or COCs) impacted groundwater in the Brunswick Formation, a fractured, leaky-confined groundwater system and important sole source aquifer in the region. Private drinking water wells south of the Farm were impacted, requiring an alternative drinking water source, and completion of a Remedial Investigation (RI) by the United State Environmental Protection Agency (EPA), culminating in May 2020. The RI identified a suspected source area (SA) to groundwater impact through interviews, a passive soil gas survey (PSG), and one rock core located in the PSG anomaly (Anomaly), that was sampled and tested using a methanol/microwave assisted extraction (MAE) technique that boiled the sample prior to analysis. The Anomaly area is 19,000 square feet (ft²), containing 1,1,1-TCA at concentrations up to 10,138 nanograms, which extends west over the edge of a slope to the Perkiomen Creek, before declining to nondetect near the top of the slope. Rock samples from the core in the central portion of the Anomaly identified 1,1,1-TCA at 15.5 milligrams per kilogram (mg/kg), which declined to nondetect at groundwater [30 feet below grade (fbg)]. EPA completed a Record of Decision (ROD) in May 2022, which selected thermal remediation for the SA. AECOM Technical Services, Inc. (AECOM) was awarded the Remedial Design (RD) for the SA in September 2022, which authorized a Supplemental Investigation (SI) of the SA. AECOM designed the SI to 1) thoroughly investigate the Anomaly to determine the bounds of thermal treatment, 2) understand the occurrence and quality of groundwater for potential dewatering and treatment, and 3) test the application of the MAE extraction technique relative to conventional analytical methods.

Approach/Activities:

Approach/Activities. AECOM prepared a SI workplan (SIW) for the SA for EPA approval, which was implemented in October 2023. The SIW identified 11 locations around the Anomaly for collection of soil samples and completion of rock core (HQ) to depths of 35 to 50 fbg to evaluate the SI objectives. Soil samples above bedrock (10 fbg) and rock samples from natural fracture faces and bulk rock matrix, at a collection frequency of one sample per 1.0 to 2.0 feet, for a total of 213 samples, were crushed in the field, preserved with methanol, and submitted for analysis of COCs using the same RI MAE technique with 8260 and 8270 test methods. Coreholes were to be completed with monitoring wells to first groundwater to evaluate groundwater quality and serve as observation wells for a dewatering pumping test. Lastly, the utility of the MAE technique for representation of source mass in soil and rock media will be evaluated through contrast with 24 split samples from three cores analyzed using conventional low-temperature extraction.

Results/Lessons Learned:

Results/Lessons Learned. The combination of coring, sample selection from facture faces, the MAE technique and real-time learning from rapid return (48-hour) of select sample results during field work proved an efficient and accurate means of investigation. The SI demonstrated the Anomaly principally reflects sorbed and likely separate phase mass in the vadose zone bedrock matrix, particularly the shallow (10 – 25 fbg) rock adjacent to weathered fractures, validating, and necessitating use of the Anomaly footprint as the thermal treatment area. Concentrations of 1,1,1-TCA peak on the order of 30 to 150 mg/kg in the center-part of the Anomaly, at depths of 12 to 17 fbg, the transition zone from weathered to more competent argillite, diminish to 5.0 to 35 mg/kg on the fringes and decline to nondetect outside the Anomaly. No COCs were detected below groundwater (30 fbg), a relationship consistent with the wetting nature of chlorinated compounds versus water. A working conceptual site model (CSM) envisions direct introduction of waste fluids via trenches near or on weathered bedrock (little to no soil impact), with waste fluids dispersing laterally westward in pulses near the base of the weathered zone(12 to 17 fbg) via a combination of southwestern dipping bedding plane partings and near vertical conjugate fractures (north-south and northwest-southeast), which have opened on the western fringes of the Anomaly in response to slope development to the westward Perkiomen Creek, draining fluids deeper to groundwater. Seven monitoring wells were installed to first water, will be sampled in November 2023, and will serve the pumping test in December 2023. Theoretical porewater concentrations of 1,1,1-TCA calculated from the peak rock matrix COC concentrations and bedrock physical property samples are on the order of 150 milligrams per liter (mg/L) to greater than COC solubilities. Historical maximum plume concentrations of 1,1,1-TCA determined from monitoring wells hydraulically down gradient of the SA have been on the order of 60 to 70 mg/L. The effectiveness of the MAE technique will be evaluated with the split samples of rock from the SA SI submitted for conventional extraction and analysis and the results of laboratory quality control samples.

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Kevin Mudrick