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Hill Air Force Base cleanup team updates Sunset City on TCE plumes, new soil hotspot and indoor-air testing
Summary
Hill AFB project managers told Sunset City Council the shallow groundwater TCE plumes are shrinking in many monitoring wells, outlined near-term soil vapor extraction and replacement remedies, and urged more homeowner participation in indoor-air sampling; officials noted a newly discovered high-concentration soil hotspot under a demolished railyard building and plans to excavate arsenic for disposal.
Hill Air Force Base remediation staff told the Sunset City Council on Nov. 19 that long-running efforts to shrink trichloroethylene (TCE) plumes near the city are showing progress but that new work is needed at a recently discovered soil hotspot.
Project manager Lindsay Burt, speaking for the base’s restoration advisory board, said most monitoring wells in Sunset and adjacent Clayton show decreasing contamination trends and that the aeration curtain on Main Street — previously reported for decommissioning — remains in service while regulators evaluate an alternative remedy. "We have a document that is approved by the EPA outlining that we will decommission it," she said, but added the curtain is "still there" and "still functioning." The Air Force has spent about $184,000 on Operable Unit 5 in fiscal year 2024 and plans roughly $180,800 for 2025; total spending on OU‑5 since its establishment was given as $41,800,000.
Why it matters: TCE is a dense solvent tracked at the base; where groundwater is shallow it can volatilize and enter homes through foundation cracks, a process called vapor intrusion. Indoor-air sampling and home mitigation are the primary protections for residences in the affected area.
Indoor-air program lead Mark Rubinsky summarized the indoor-air sampling program (OU‑15) and mitigation approach. "If that happens, you can't smell TCE. You can't see it. The only way to tell if...vapor intrusion is occurring is by collecting an indoor air sample," he said. The Air Force has established risk-based action levels with the EPA and Utah Department of Environmental Quality; when samples exceed those levels and indicate a groundwater source, the Air Force installs and maintains vapor-intrusion mitigation systems (similar to radon-mitigation systems) under license agreements with homeowners.
Program scale and outreach: Rubinsky said the affected area contains 552 homes and businesses; to date crews have sampled 343 homes and found no vapor intrusion in those samples. Forty-one properties have never been sampled despite repeated outreach and 54 sampled homes require additional follow-up sampling. Twelve vapor-intrusion mitigation systems have already been installed in Sunset. Outreach methods include mailed notices, a Sunset City newsletter posting, QR-code sign-ups, and door-to-door canvassing; the sampling season runs from October through March.
Near-term remedies and a new hotspot: For the base-side source areas, the Air Force has used injections of emulsified vegetable oil to reduce source concentrations; Burt said that approach materially contracted plume areas compared with 2010 maps. On the city‑side, the existing groundwater-extraction trench remains very effective. The team reported a newly discovered high‑concentration soil hotspot beneath the former railyard building, found during demolition sampling, and plans a time-critical removal action using soil-vapor-extraction (SVE) wells and targeted excavation for arsenic. Burt said the SVE plan calls for about seven wells installed to roughly 28 feet (just above the groundwater) with piping routed to a control/treatment station; extracted vapors would be treated if they exceed state or federal air-quality thresholds.
Disposal and regulatory steps: Soil with elevated arsenic will be excavated and disposed of at Grassy Mountain Landfill (Tooele County), a regulated facility run by Clean Harbors. Burt explained the remedy-selection process will require amendments to the decision document and regulator approval (EPA and state) and could take one to two years for public review and a proposed plan.
Funding and oversight: Burt said the Air Force provides annual remediation funding and the state has contributed through the Military Installation Development Authority (MIDA), with MIDA planning roughly $1.4 million toward well management, arsenic removal and soil treatment activities associated with OU‑5.
What comes next: The Air Force will continue sampling, install the planned SVE wells under a time-critical removal action where needed, pursue a long-term replacement remedy (including injectable activated carbon barriers on the base), and publish public notices and a proposed plan for regulator and public review.
Ending: Council members thanked the RAB staff and urged continued public outreach to increase homeowner participation in indoor-air sampling.
