Formatted Title

Zero-Valent Iron Permeable Reactive Barrier to Remediate Volatile Organic Compounds in Groundwater

Background/Objectives

The United States Department of Energy-Savannah River Site, the United States Environmetnal Protection Agency, and the South Carolina Department of Health and Environmental Control determined it was appropriate to perform a non-time critical removal action at the P-Area Groundwater Operable Unit at the Savannah River Site to reduce the mass and downgradient transport of trichloroethylene in the P-Area Groundwater Operable Unit plume. Contaminated groundwater discharges to a nearby stream, Steel Creek, within the Savannah River Site boundaries, resulting in trichloroethylene concentrations above the maximum contaminant level. Impact to surface water is limited in areal extent and supported by recently collected characterization data.

The P-Area Groundwater Operable Unit encompasses the groundwater beneath an industrial area within Savannah River Site, P Area, where the P-Reactor once operated. Groundwater in the Upper Three Runs Aquifer of the P-Area Groundwater Operable Unit has been impacted by reactor and facility operations between 1954 and 1991, including tritium and volatile organic compounds. The P-Area surface units contributing to groundwater contamination were remediated as part of the P-Area Operable Unit in 2011. 

The nature and extent of groundwater contamination was determined using a variety of investigative approaches such as groundwater monitoring wells, direct-push technology, and surface water samples. Groundwater contamination associated with trichloroethylene is primarily exhibited in a narrow plume that extends from the source area at P-Reactor and west to Steel Creek. Maximum contaminant level exceedances in groundwater occur over an area of ~6.9 hectares for trichloroethylene with concentrations as high as 7.7 milligrams per liter. To the west of the P-Area facility area, the trichloroethylene groundwater plume is controlled by a buried geologic feature, assumed to be an old stream bed, that further narrows the groundwater plume in what has been designated as the “neck area.” This narrowing of the groundwater plume provided an ideal location for a treatment barrier. The non-time critical removal action alternative chosen was to install a zero-valent iron permeable reactive barrier within the neck area of the trichloroethylene groundwater plume, perpendicular to groundwater flow direction. The permeable reactive barrier was designed to reduce trichloroethylene groundwater concentrations by 90% with an anticipated useful life of at least 25 years. Monitoring over five years was proposed to evaluate the technology effectiveness and impact on the P-Area Groundwater Operable Unit trichloroethylene plume in the Upper Three Runs Aquifer. The removal action objective for the P-Area Groundwater Operable Unit non-time critical removal action is to protect human health and the environment by reducing the mass and downgradient transport of the trichloroethylene groundwater plume. A trichloroethylene mass flux reduction of 80% was determined to be sufficient to meet the removal action objective.

Approach/Activities

This project consisted of a field scale remediation of a trichloroethylene plume in P-Area of the Savannah River Site. Prior to installation, a pre-design investigation was performed in the neck area to confirm site lithology, hydrogeology, geochemistry, and extent of trichloroethylene contamination prior to a final design. A treatability study, conducted as part of the pre-design investigation, indicated that the subsurface and groundwater in the P-Area Groundwater Operable Unit is compatible with the zero-valent iron and will not lead to excessive buildup from mineralization/precipitation or biofouling. Probabilistic modeling was conducted using field and laboratory data to determine the expected performance of the zero-valent iron permeable reactive barrier. The model simulations indicated that a 3.81-centimeter thick barrier would provide greater than 90% reduction of trichloroethylene groundwater concentration. The thickness was increased with a safety factor of greater than 2X to account for varying trichloroethylene concentrations and for reduced performance over time. 

Construction of the zero-valent iron permeable reactive barrier construction was completed in December 2019. In order to emplace zero-valent iron in the subsurface at the designed thickness, injection wells with a specialed expansion casing system was used. Zero-valent iron was added to a hydroxypropyl guar mixture to create an injectable media that could deliver zero-valent iron to the subsurface. The guar mixture was injected through 22 injection wells, targeting the Upper Three Runs Aquifer and creating a permeable reactive barrier with a final zero-valent iron thickness between using a specialized expansion casing system. The injection wells were spaced evenly to create an 80.5 meters long permeable reactive barrier, installed in a “zig-zag” orientation to best transect the trichloroethylene plume and account for varying groundwater flow direction. The final permeable reactive barrier thickness ranged between 7.82 and 17.0 centimeters, with an average thickness of 13.0 centimeters. A total of approximately 685 metric tons of zero-valent iron were injected to complete the permeable reactive barrier. Active resistivity mapping was performed during installation to verify propagation of the zero-valent iron between adjacent injection wells and to demonstrate continuity of the permeable reactive barrier. 

The zero-valent iron grain size was designed to have a hydraulic conductivity greater than the natural subsurface, thus promoting groundwater flow through the barrier. As contaminated groundwater contacts the zero-valent iron, volatile organic compounds, including trichloroethylene, are abiotically reduced to harmless end-products such as ethylene. The performance of the zero-valent iron permeable reactive barrier is monitored using three upgradient monitoring well clusters, six downgradient monitoring well clusters, and four in-wall monitoring wells. The in-wall monitoring wells provide evidence of the technology effectiveness in acheiving immediate reduction of trichloroethylene in groundwater. Additionally, the in-wall monitoring wells allow analyses of the zero-valent iron permeable reactive barrier health. Comparison of downgradient groundwater trichloroethylene concentrations to baseline and upgradient concentrations provide data to evaluate the technoloies performance in meeting the objectives of the project.

Results/Lessons Learned

Monitoring results for the zero-valent iron permeable reactive barrier are broken down into sections for focus, which include:

• The impact the barrier has on groundwater elevations and flow directions is important to verify groundwater flow, and therefore trichloroethylene transport, through the barrier is occuring as expected. 
• The main results to demonstrate performance of the zero-valent iron permeable reactive barrier are indicators of chlorinated volatile organic compound degradation, which includes trichloroethylene and breakdown by-products such as cis-1,2-dichloroethylene, chloroethylene, chloride, and ethylene. 
• Indicators of a reducing environment demonstrate conditions that support reductive dechlorination by zero-valent iron. Such parameters include dissolved oxygen, oxidation-reduction potential, pH, and speciation of compounds present (i.e., nitrate/nitrite, sulfate/sulfite, ferric/ferrous iron). 
• Geochemical analytes that could indicate poor zero-valent iron permeable reactive barrier health (i.e., sulfate, calcium, nitrate).

Groundwater flow across the zero-valent iron permeable reactive barrier in the targeted aquifer zone is westerly towards Steel Creek, as expected, with groundwater velocity calculated as 39.50 meters per year. After two years of monitoring, it appears impact to the aquifer zone groundwater potentiometric surface is minimal. The permeable reactive barrier installation does appear to slow groundwater flow west of the barrier, as indicated by the potentiometric surface downgradient of the barrier. This impact is not expected to affect the performance of the permeable reactive barrier, as the groundwater flow direction was ultimately not impacted, and flow through the barrier in the targeted aquifer zone is supported by the 2022 data. 

Installation of the permeable reactive barrier did impact the aquifer zone underlying the targeted aquifer zone. During installation, the injected zero-valent iron penetrated the clay layer below the targeted aquifer zone in multiple injection locations. Due to the higher permeability of the zero-valent iron, this provides a conduit for groundwater flow from the upper aquifer zone to the lower aquifer zone. 

Trichloroethylene concentrations upgradient of the zero-valent iron permeable reactive barrier remained elevated after two years of monitoring, with an average concentration of 1,060 parts per billion (maximum of 2,410 parts per billion). In the four in-wall monitoring wells, there was one detection of trichloroethylene with a result of 10.4 parts per billion, demonstrating a trichloroethylene groundwater concentration reduction of greater than 99% when comparing to the average. This reduction supports a mass flux reduction greater than the removal action objective of 80%.

There are four monitoring well clusters located immediately downgradient of the zero-valent iron permeable reactive barrier. After two years of monitoring, the maximum trichloroethylene result was 72.5 parts per billion in 2022, demonstrating a 93% reduction in groundwater concentration compared to the average upgradient concentration. Further downgradient, trichloroethylene concentrations have not experienced the same level of reduction. During the pre-design investigation of the project area, high concentrations of trichloroethylene were observed in low permeability clays below the targeted aquifer zone of the Upper Three Runs Aquifer. Therefore, it was expected that back diffusion would delay the impact to downgradient trichloroethylene concentrations for an estimated 3-5 years. 

Results after two years of monitoring are prepared to discuss at this time. The third year of monitoring is complete and data is currently being prepared for reporting in an annual report to the United States Department of Energy, the United States Environmental Protection Agency, and the South Carolina Department of Health and Environmental Control. These results will be complete and available for inclusion at the time of presentation.