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SUNSET, Davis County — Environmental engineers from Hill Air Force Base say it will cost $2 million to clean up dangerous contamination in soil and groundwater to make way for a planned new I-15 interchange in Davis County.
The engineers outlined the high-priority project at a public meeting Thursday.
According to a site investigation report, engineers at Hill bored 12 test holes into the soil around a site at the northwest border of Hill known as the Tooele Army Rail Shop and "discovered a high concentration source area" of soil contaminated by trichloroethylene, or TCE, migrating into nearby groundwater, with a footprint of about a third of an acre.
World War II-era rail shop site source of contamination
In preparation for the new I-15 interchange and the construction of another vehicle access gate onto Hill, the rail shop — a multi-building complex — was torn down in late 2021 and early 2022.
The shop had been overhauling, repairing and maintaining railway equipment since World War II as part of a $24.5 million effort to relocate rail servicing capabilities to Anniston Army Depot in Alabama.
The most concerning contamination reading at the site came from the northern portion of former Building 1723, where a concentration of 180,000 mcg/kg of TCE was found at a depth of 16 feet.
According to Environmental Protection Agency guidelines, that concentration is 30 times higher than the amount that may cause health problems for exposed workers, and 9.4 times higher than the threshold amount for cancer risk.
Groundwater samples in the area yielded a maximum concentration of TCE at 3,100 mcg/L, "orders of magnitude above the EPA-established maximum contaminant level," which is 5 mcg/L for drinking water.
The project team said that the contaminated water is not currently being used as drinking water, and so there are no immediate human health risks from ingestion. But Sunset in Davis County is about 700 feet away from the highly contaminated area, and the potential of TCE finding its way under houses where vapors can infiltrate indoor air presents "an imminent and substantial endangerment to public health" if not addressed, according to the project report.
Concerns about construction workers are another significant driver of the project. Hill engineers hope to remove contamination before the I-15 project requires trenching and excavating, possibly exposing workers to harmful vapors.
What is TCE?
Trichloroethylene is an unstable colorless liquid that evaporates quickly into the air. It is not flammable, and it has a sweet odor, according to the Center for Disease Control and Prevention. It is primarily used as a degreaser to remove oils, fats and waxes from metal parts.
A toxicity report from the CDC says the chemical evaporates into the atmosphere when on water and soil surfaces, where it breaks down relatively quickly. The problem is, TCE can penetrate the soil and move easily, and is slowly degraded once in the soil and groundwater.
There are a few common ways for TCE to enter a person's body, according to the report. Most commonly, it is inhaled as a vapor or ingested in contaminated water. The CDC says: "People who are overexposed to moderate amounts of trichloroethylene may experience headaches, dizziness, and sleepiness; large amounts of trichloroethylene may cause coma and even death." There is strong evidence that trichloroethylene can cause kidney cancer.
As the chemical moves through soil and groundwater, it can migrate into air spaces beneath buildings to enter the indoor air, similar to radon, through a process called vapor intrusion. Across the county, the EPA tests thousands of drinking water supply sources annually, and has found 4.5- to- 18% have measurable levels of TCE.
TCE removal
Decades ago, before stringent regulations were in place, the rail shop had chemical waste pipes that dumped refuse into a nearby pit. It was most likely historic leakage from a waste pipe under the building in question that resulted in this newly discovered contamination, environmental engineer Lindsay Burt said. The groundwater and soil around the rail shop has been monitored since the early '90s, with concentrations of TCE steadily declining since 2006.
An aeration curtain — which consists of a deep trench filled with gravel that pumps air into the groundwater to allow TCE to vaporize and bubble out of the ground — was installed around 25 years ago along Sunset's main street, Burt said. Further down at the Clinton border, another trench collects groundwater and sends it to the North Davis Sewer District wastewater facility for testing and treatment.
Hill has started the process of replacing the aeration curtain, which is at the end of its lifespan.
There is also an ongoing community outreach program in place, which involves testing the households within a certain range of the known plume. Hill Air Force Base will test the houses of all those who qualify for TCE vapors, and if found, will install mitigation equipment free of charge.
Time-critical project
Because the area is an EPA Superfund site, the cleanup requires Hill to follow an extensive process under Time-Critical Removal Action guidelines.
In the final action memorandum, Hill engineers outline their plans to install a soil vapor extraction system, which consists of seven wells attached to a "geomembrane liner" and 20-horsepower blower, extracting the TCE in gas form from the highly contaminated area.
The goal will be to reduce the TCE in the soil to less than 6,000 parts per million, but the system will not shut down if that level is reached; instead, it will remain operational to extract as much of the compound as possible before the construction of the new interchange begins, Burt says. The EPA and Utah Department of Environmental Quality are also involved as stakeholders providing oversight.
Hill Air Force Base — even after giving around 40 acres over to the state of Utah for its I-15 project scheduled to break ground in February 2025 — will be responsible for testing and mitigating the soil contamination.
The proposed removal action will leave contamination in place above residential levels, so land use controls will prevent the affected area from being developed with offices, residential homes, schools or parks. Another effort is underway to remove arsenic-contaminated ground in the same area, estimated to be completed by the end of the summer.
Because of a protective layer of clay between the contaminated shallow groundwater and the area's much-deeper drinking water aquifer, there is no danger of contamination at this time. Hill is working to bring the shallow groundwater up to federal drinking water standards, but it could take decades.
