Formatted Title
Design, Implementation, and Assessment of a Slurry Injection Pilot Study to Treat a Low Permeability Aquifer Contaminated with TCE
Background/Objectives
Fort Hall Mine Landfill (FHML) is located in southern Idaho and consists of active waste cells regulated under RCRA and a capped unlined dump (Cell 1) closed in 1993. Tetrachloroethene (PCE) and trichloroethene (TCE) have contaminated groundwater in the canyon and downgradient, impacting municipal and domestic supply wells. The groundwater extraction and air stripping treatment system has operated since 2002 with incomplete capture of contaminant mass. Groundwater concentrations of PCE and TCE exceed the maximum contaminant level (MCL) and Idaho Groundwater Rule (IDGW) primary standards. Since CDM Smith involvement, extensive site characterization, system optimization, and alternatives evaluation have been conducted. Subsurface geology consists of heterogenous unconsolidated layers and weathered and unweathered conglomeritic formation. Groundwater is sampled and analyzed three times per year. In spring 2023, a pilot study was conducted to evaluate 1. operational parameters and feasibility of slurry injections to emplace reactive amendments into the contaminated aquifer, 2. performance of slurry injections to enhance in situ remediation, and 3. longevity of organic carbon and zero-valent iron (ZVI) amendments.
Approach/Activities
Establishing location and depths of injections was an adaptive process that considered the conceptual site model, existing infrastructure and property use, monitoring requirements, geology, and downhole tooling constraints. The treatment strategy combined in situ bioremediation (ISB) and in situ chemical reduction (ISCR), which involve the addition of amendments and microorganisms to produce biogeochemical conditions in the aquifer that promote destructive abiotic and biotic processes for the removal of PCE and TCE. A custom blend of microscale ZVI was injected to achieve a 0.88% by weight (ZVI/soil) dose to the treatment zone. Pressurized slurry injections were used to emplace amendments via two pilot boreholes using a cased borehole method. Process monitoring occurred during the injection event and included downhole sonde data collection of water quality parameters, groundwater elevation, measurements, pressure monitoring, and tiltmetering to provide a line of evidence for amendment distribution. One year of performance monitoring was conducted to evaluate impacts post-injection and inform the overall site remedial strategy.
Results/Lessons Learned
Effective amendment distribution must consider a myriad of factors, including geological formation, injection method, slurry consistency and management, amendment type(s), borehole stability, and contaminant distribution. Multiple lines of evidence demonstrated successful amendment emplacement and degradation of PCE and TCE in the pilot treatment zone via abiotic and biotic degradation mechanisms. The presentation will include a summary and interpretation of chemical, geochemical, microbial, compound-specific isotope analysis, and tiltmeter data collected during and one year following injections, as well as an update to the site’s three-dimensional Leapfrog®. Model. Benefits of real-time monitoring, lessons learned from the selected delivery method, and challenges with tiltmeter analysis will be acknowledged. The pilot study results have implications for full-scale implementation and other site remedial strategies.