Formatted Title
PetroFix® and PFAS-Free Water Used at a UST Site to Get Closer to Closure
Background/Objectives
Two gasoline underground storage tanks (USTs) and one diesel UST were identified (closed in 1985) during due diligence activities at a former UST site in Wilmington, North Carolina in 1996. In 2000, a Corrective Action Plan (CAP) was developed with soil excavation and soil amendment placement, followed by monitored natural attenuation (MNA) as the remedy. Benzene has been the primary regulatory driver with a cleanup target of 1 microgram per liter (µg/L) in groundwater. Years of groundwater monitoring led to the need to review remedial alternatives and revise the conceptual site model (CSM). Following the use of multiple remedial alternatives, the final remedy was selected which helped bring the site closer to closure.
Approach/Activities
Ongoing MNA monitoring alone was not sufficient to achieve site closure, so the North Carolina Department of Environment and Natural Resources [(NCDENR), now North Carolina Department of Environmental Quality (NCDEQ)], requested evaluation of remedial alternatives. Initial evaluation in 2011 identified magnesium sulfate electron acceptor solution as a viable remedy for the anaerobic groundwater plume. A total of six magnesium sulfate applications were completed from 2012-2016 by direct push injection (DPI) and included the use of a magnesium hydroxide buffer for pH adjustment. Environmental molecular diagnostics (EMDs) including QuantArray®-Petro, as well as geochemical data, were used to evaluate remedial progress in the four effected monitoring wells.
In 2018-2019, the NCDEQ requested that Oxygen Release Compound (ORC®) socks be placed in the four impacted monitoring wells (MW-2, MW-15, MW-16, and MW-18) to stimulate aerobic bioremediation. This created pH in the 8-10 range in the monitoring wells and aerobic conditions localized to the wells. Use of this technology was discontinued in early 2019 due to the negative impact it had on aquifer conditions.
With the development of PetroFix® in the Fall of 2018, its use as a final remedy was evaluated. At the time, PetroFix® had not yet been used at any site in North Carolina. A remedial design was developed and implemented in late 2019. With the known impacts of per- and polyfluoroalkyl substances (PFAS) in the Wilmington, North Carolina area, the NCDEQ requested verification that PFAS-free water would be used for mixing during application therefore, a PFAS-free water source was identified and used.
An in situ PetroFix® injection was completed in October 2019 by applying 16,377 gallons of solution and electron acceptor (nitrate and sulfate) through an array of 41 DPI points within the area with highest levels of contaminant mass. Soil borings completed during the application for design verification showed good distribution of PetroFix®. The soil borings also identified areas of shallow secondary contaminant mass in unsaturated soil that was not previously known.
Wellbooms® (beeswax socks) have been used successfully at many sites to remove free floating hydrocarbons from monitoring wells by sorbing the hydrocarbons, and they also promote biodegradation of the sorbed hydrocarbons. Because additional PetroFix® applications were not practical, Wellbooms® were placed into monitoring wells MW-2, MW-15, MW-16, and MW-18 in February 2021, to help stimulate microbial activity in the wells and enhance the ongoing biodegradation.
Results/Lessons Learned
Evaluation of the magnesium sulfate application showed an initial order of magnitude decrease in mass (Total BTEX 20,026 µg/L to 2,553 µg/L) followed by a stable to slightly decreasing trend in source area monitoring well MW-15. The sulfate reducing bacteria population in MW-15 grew from non-detectable prior to the application to 1.1×105 after the application.
Use of the ORC® socks created oxidizing conditions, and contaminant concentrations in monitoring well MW-15 increased while the socks were used. With a site dominated by anaerobic and reducing conditions, selecting a remedy that works under those conditions is best. Trying to convert to aerobic conditions proved to be unsuccessful and potentially detrimental.
After using MNA and multiple technologies, the first PetroFix® application in North Carolina proved to be successful in bringing the site closer to closure. The total BTEX in MW-15 was 5.57 µg/L in 2022 and a notable correlation between xylene and benzene concentrations was observed in MW-15. Due to the shallow groundwater depth of three to four feet below ground surface (bgs), some daylighting occurred during the PetroFix® application. Daylighting was expected and groundwater data from MW-15 indicate that it may have helped remediate the secondary source in the shallow unsaturated soil by eliminating contribution to groundwater. Additional QuantArray®-Petro analysis is planned as part of the ongoing groundwater monitoring.