Topic Area: 2a. In Situ Bioremediation Applications

Formatted Title:

Evaluation of In Situ Pilot Study Impacts on a TCE Groundwater Plume: Microbial, Geochemical, and Isotopic Analyses over One Year

Background/Objectives:

The Fort Hall Mine Landfill (FHML) in southern Idaho includes active waste cells regulated under RCRA and a capped, unlined dump (Cell 1) closed in 1993. Groundwater contamination with tetrachloroethene (PCE) and trichloroethene (TCE) has impacted municipal and domestic supply wells in the canyon and downgradient. A groundwater extraction and air stripping treatment system has operated since 2002 with incomplete contaminant capture. PCE and TCE concentrations exceed the maximum contaminant levels (MCL) and Idaho Groundwater Rule (IDGW) primary standards. Since CDM Smith’s involvement, extensive site characterization, system optimization, and alternatives evaluation have been performed. The subsurface consists of heterogenous unconsolidated layers and weathered and unweathered conglomeritic formation. In spring 2023, a pilot study was conducted to evaluate the performance and longevity of slurry injections to enhance in situ remediation in a mixed aerobic/anerobic zone of the low-permeability aquifer. Contaminant, geochemical, and microbial data were analyzed to 1) determine if conditions conducive to the reductive dechlorination (RD) of TCE were enhanced and sustained, and 2) quantify key microbial populations and functional genes present and stimulated in the treatment area.

Approach/Activities:

The treatment strategy combined in situ bioremediation (ISB) and in situ chemical reduction (ISCR). Amendments and microorganisms were injected in the aquifer to enhance biogeochemical conditions that promote the destruction of PCE and TCE via abiotic and biotic processes. The injection slurry consisted of a custom blend of microscale ZVI (0.88% by weight ZVI/soil), multiple solid and liquid fermentable carbon substrates, and KB-1® microbial culture. Pressurized injections delivered the slurry into two boreholes at multiple depths using a cased borehole method. Performance monitoring was conducted over one year, with sampling at baseline (pre-injections) and 1, 3, 6, and 12 months post-injection of monitoring wells in pilot area, upgradient (background), and downgradient monitoring wells. Analyzed geochemical parameters included total organic carbon, dissolved oxygen, oxidation-reduction potential, total and dissolved metals including ferrous iron, dissolved ions, and dissolved gases (e.g., methane). QuantArray® Chlor assessed sulfate-reducing bacteria (SRB), methanogens, total eukaryotes, degradation functional genes, and key RD microbial populations, while compound-specific isotope analysis (CSIA) was performed on pilot area monitoring wells.

Katarina Savatic

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Evaluation of In Situ Pilot Study Impacts on a TCE Groundwater Plume: Microbial, Geochemical, and Isotopic Analyses over One Year