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Home > Site Information
Clinton
Operable Units:
OU 5
Operable Unit 5
HAFB Project Manager: Mark Roginske
Site Background
Operable Unit 5 (OU-5) is located along the northwestern boundary of Hill Air Force Base (AFB) and includes three areas of contamination:
- A groundwater contamination plume that originates from the Tooele Army Rail Shop (TARS) on Hill AFB and extends beneath the cities of Clinton and Sunset (TARS plume).
- A groundwater contamination plume that originates from the Zone 16 Complex on Hill AFB and extends beneath the cities of Clinton, Sunset and a small area of Roy (Zone 16 plume).
- An arsenic-contaminated soil area located on Hill AFB in the TARS area.
TARS Area
The TARS area is a locomotive service and maintenance facility that began operation in the 1940s. Several chemicals used in the industrial processes during the operation of this facility were likely discharged with wastewater directly into the ground. The contaminants have migrated from their point of release down through the soil to the shallow groundwater system. The TARS plume is approximately 5,400 feet in length, extending from its source area in the TARS area to approximately 890 West in Clinton.
Investigations in the TARS area also identified an area of soil approximately 0.4 acres in size that is contaminated with arsenic.
Zone 16
The Zone 16 Complex is a series of buildings on Hill AFB currently used for munitions and missile testing. The area was originally used for small arms repair (in the 1940s) and later became a loading and assembly line plant which manufactured and maintained munitions and missile components. Waste solvents from this plant were piped to a nearby flash pond for disposal, which may be a source for the Zone 16 plume. A former wastewater treatment plant located near the Zone 16 Complex may also be a source of contamination for this plume. The Zone 16 plume is approximately 9,400 feet in length, extending from the Zone 16 Complex to approximately 1220 West in Clinton. A small area of Roy is also affected.
The primary contaminants of concern in both the TARS and Zone 16 plumes are chemical cleaning and degreasing solvents used for decades in the industrial processes during the operation of the above facilities. The most widespread of these contaminants is Trichloroethene (TCE). A list of the primary chemicals found in OU- 5 is included in the table.
The cleanup remedies for the three areas of contamination at OU-5 were formalized in the OU-5 Record of Decision (ROD) that was signed in 2006 by the Air Force, the Utah Department of Environmental Quality (UDEQ) and the Environmental Protection Agency (EPA).
Health Risk Issues
The OU-5 contamination has not affected local drinking water supplies because of the subsurface geology at the site. The drinking water for Sunset, Clinton and Roy comes from deep aquifers and mountain reservoirs, not from the contaminated shallow groundwater aquifer. In addition, the deep and shallow aquifers are separated by a thick clay layer that prevents contamination from moving downward.
Chemical vapors, however, have been found in some homes in Sunset, Clinton and Roy above the area of contamination. Hill has implemented a program for testing homes located above areas of groundwater contamination for chemical vapors, and Hill will install vapor mitigation systems where vapors are detected above action levels. Indoor air action levels were established in cooperation with the UDEQ and the EPA.
Studies conducted by Hill AFB, in conjunction with Utah State University, determined that contamination is not affecting fruits and vegetables grown in areas of groundwater contamination.
The arsenic found in the TARS area does not pose any significant health risks to Hill employees working in the area, according to the Baseline Risk Assessment for OU-5.
Cleanup Remedies
The Air Force has installed three cleanup remedies as early actions in Sunset and Clinton, two of which are still operating:
- Aeration Curtain -- In operation since 1997, this air sparge system along Main Street in Sunset consists of a 30-foot-deep gravel-filled trench that extends 400 feet across part of the TARS plume. Piping laid along the bottom of the trench pumps air into the groundwater. The bubbles capture contaminants and remove them from the water as they move up to the ground surface. The system has had a positive impact on cleanup of TCE in the groundwater and prevents higher concentrations of TCE from continuing to migrate into Sunset. A Soil Vapor Extraction System was installed with the Aeration Curtain to capture any vapors coming from the process. Because TCE vapors were never detected, this system was shut off in 2000 with concurrence of the EPA and the UDEQ.
- Groundwater Extraction System -- This series of five groundwater extraction wells in Sunset was installed in 1997 to capture the 100 parts per billion (ppb) and greater TCE portion of the contamination found beyond the Aeration Curtain in Sunset. Because it was no longer having a beneficial effect on the plume due to low pumping rates, this system was shut down in March 2004.
- Groundwater Extraction Trench -- This system is located on the border of Sunset and Clinton Cities and has been operating since 2003. The system includes a 35-foot-deep, gravel-filled extraction trench, and a slurry wall that extends 600 feet across most of the TARS plume. The pumping system captures and removes groundwater contaminated with TCE and minimizes further migration of the plume into Clinton. Water from the system is treated by the North Davis County Sewer District.
The Air Force will continue to monitor contamination levels at OU-5.
What’s Ahead
A permeable reactive barrier (PRB) consisting of eight boreholes filled with crushed bomb iron was installed in July 2009 on Hill AFB property. This PRB will test the effectiveness of crushed iron from demilitarized practice bombs in treating the groundwater contaminant trichloroethene (TCE). While initial results indicate that the crushed bomb iron is treating TCE, Hill AFB will continue monitoring to determine if the iron is suitable to be used in a full-scale PRB.
Since 2000, the Air Force has tested 530 homes in the OU-5 area for chemical vapors. The Air Force will continue monitoring and testing indoor air in Sunset, Clinton and Roy. The Air Force has made a long-term commitment with residents to monitor and remove vapors from their homes.
The Air Force will complete a Five-Year Review in 2013 as required by environmental cleanup laws. This review is done to verify that treatment systems and ongoing studies are protecting human health and the environment and are meeting cleanup goals. The public has the opportunity to participate in this review.
Hill AFB environmental cleanup documents are available for viewing by the public at the Hill Information Repository located at the Stewart Library on the Weber State University (WSU) campus in Ogden and at WSU’s Davis Campus Library in Layton. For hours of operation at the libraries, call the Stewart Library at (801) 626-6403 or the WSU Davis Library at (801) 395-3472.
Current as of July 2010
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