Formatted Title
Sustainable and Resilient Adaptive Management Strategies for Source Area Bioremediation of TCE DNAPL in Fractured Bedrock
Background/Objectives
A manufacturing facility operating since the late 1940s in central Pennsylvania experienced releases of solvents, (primarily tetra and trichloroethene), through leaking underground waste storage tanks (UST) that were installed on top of fractured bedrock. Soil samples collected in the area of the former USTs during Site decommissioning activities in the early 2000s identified PCE and TCE above the state agency medium specific concentrations for soil. Subsequent high resolution Site characterization including sorbers to refine the conceptual site model also identified elevated concentrations of TCE and degradation products in the bedrock aquifer at depths of 95 to almost 200 feet below ground surface (bgs) at concentrations exceeding 300,000 µg/L. Concerns regarding off-site migration of the solvent plume in the bedrock aquifer towards a large nearby river caused the state agency to request that an aggressive remediation strategy be implemented. A focused and semi-quantitative sustainability assessment was conducted comparing pump and treat, thermal, in situ chemical oxidation and in situ bioremediation. Phased in situ bioremediation (biostimulation and bioaugmentation) was selected as the remedy to achieve risk-based remedial goals. Identification of TCE DNAPL during bioremediation activities initiated adaptive management strategies that included transition to a solid carbon source and electron donor.
Approach/Activities
The sustainable and resilient remedial approach was initiated in March 2012 and included installation of nine injection points ranging in depth from 100 to 170 feet bgs, using a 6% emulsified vegetable oil solution to establish the anaerobic reducing treatment zone. Nine more injection points were installed targeting the source area and to create a downgradient biobarrier to depths ranging from 110 to 180 feet below ground surface in 2014, targeting the most transmissive fractures using subsurface geophysics. Supplemental injections that included laboratory cultured microcosms were implemented at 10 of the 19 injection points after review of the pilot test data and observation of concentrations of TCE exceeding 250,000 µg/L in 2014. A longer lasting locally-sourced organic carbon/electron donor in the form of hardwood mulch placed in large diameter borings (bioborings) was considered in 2019 to replace fouled injection points when DNAPL was identified in one of the pilot borings at approximately 90 feet bgs. Nine bioborings were installed in 2021 to replace several fouled injection points backfilled with hardwood mulch and gravel.
Results/Lessons Learned
Anaerobic reducing conditions were observed in adjacent shallow and intermediate monitoring wells (depth to 140 feet bgs) within 6 months of the initial substrate injections including low dissolved oxygen, negative oxidation-reduction potential and increase in daughter products ethene and ethane concentrations. Reduction of TCE has been observed in the source area of approximately 98% in the shallow well and 99% in the intermediate well over the 10-year treatment period. The initial injections mobilized potential separate phase TCE DNAPL from solution channels in the fractured rock that had not been previously identified during investigations. Adaptive management strategies were used to adjust the scope of remediation to include additional injection points, locally-sourced hardwood mulch as the carbon source and electron donor in bioborings and bioaugmentation to address and reduce both DNAPL and dissolved phase TCE. The use of the hardwood mulch placed in large-diameter bioborings as a longer lasting carbon source and electron donor allowed for maintenance of anaerobic reducing conditions without additional mobilization of the residual TCE DNAPL.