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Engineers report groundwater pilot cut contaminants near Fort Hall Mine landfill; recommend follow-up injections and $300,000 phase‑2 work
Summary
CDM Smith told the county board that an in‑situ slurry injection pilot reduced concentrations of chlorinated ethanes near the Fort Hall Mine landfill and recommended converting pilot boreholes to injection wells and performing additional injections and monitoring this fiscal year.
CDM Smith briefed the Bannock County Board of Commissioners on June 12 about results from a pilot in situ remediation test at the Fort Hall Mine landfill and recommended a phase‑2 campaign of follow-up injections and monitoring to sustain contaminant degradation.
Adrienne Mead and Tamsen Macbeth of CDM Smith described a pilot in which crews injected a slurry containing zero‑valent iron (ZVI) and carbon into low‑permeability sediments at the foot of Fort Hall Canyon to establish a permeable reactive barrier. Macbeth summarized the contaminant context: “Primary contaminants of concern are chlorinated ethanes,” and the firm reported it had injected roughly “380,000 pounds of 0 valent iron and carbon” during the pilot.
The firm reported “dramatic” concentration declines within the emplacement zone and measurable reductions in contaminant flux through the barrier; some rebound occurred outside the immediate injection footprint. The pilot also coincided with sustained reductions at a previously troublesome monitoring well on the property boundary (well 118), which regulators had cited as a driver for remediation.
CDM Smith recommended a two‑phase follow-up: convert boreholes used for the slurry test to functioning injection wells (one completed well, one packer‑injection in bedrock) and perform periodic aqueous re‑injections to sustain the reactive barrier. The consultants proposed submitting a work‑plan addendum to Idaho Department of Environmental Quality (DEQ) at the end of the month and beginning field work in August, with a phase‑2 budget of about $300,000. CDM Smith said that amount could be covered by reallocating items within the project’s existing adaptive management contract rather than a new appropriation.
Commissioners asked technical follow‑ups about slurry behavior, durability of the permeable reactive barrier, and whether the injection would require repeated treatments. CDM Smith answered that the slurry is viscous during emplacement, fractures low‑permeability material to place amendments, and leaves ZVI and carbon that enable both abiotic and biological dechlorination; periodic re‑injection will be needed while landfill source leaching continues. The consultants also emphasized parallel source‑control work (stormwater and infiltration management) to reduce ongoing leaching from the unlined Cell 1 area.
No formal vote was recorded on the phase‑2 plan at the meeting; county staff said CDM Smith and the county would continue to coordinate with DEQ and manage expenditures through the project’s adaptive‑management task structure.
Provenance: the pilot description and next‑step recommendation appear in the consultant presentation and Q&A (transcript excerpt: “we injected about 380,000 pounds of 0 valent iron and carbon” and later “we would like to start the field work in August and complete the injections this fiscal year”).
