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Home > Site Information
Sunset
Operable Units:
OU 5
OU 9
OU 10
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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Operable Unit 9
HAFB Project Manager: Shannon Smith
Site Overview
Operable Unit 9 (OU-9) includes several sites on Hill AFB that, though unrelated to one another in location or in history, are currently being investigated and managed under one operable unit. Each site in OU-9 is unique, and both contaminated soil and groundwater are addressed as part of this operable unit. A list of the primary chemicals of concern for OU-9 is included in the table.
While most of the contamination is localized near the source areas on base, a very small area of Sunset City is affected by groundwater contamination originating from OU-9, which heads west off the base, past Interstate 15, and under an apartment complex area in Sunset.
The majority of the sites at OU-9 have been or are currently being remediated. The following sections provide a brief description of each site.
- Pond 1
Located along the southern base boundary, Pond 1 receives storm water from the southern end of the flightline and industrial area. Contaminated soil was discovered at Pond 1 in 1997 and was found to contain fuel-type compounds, metals and PCBs. Four areas in Pond 1 were found to contain contaminants that exceeded residential risk-based screening levels. As a result, an Engineering Evaluation/Cost Analysis (EE/CA) was conducted in 2001 and implemented in 2003 to consolidate the contaminated soil in a “landfill” on site.
Status: Cleanup complete in 2003
- Pond 3
Pond 3, located along the southern base boundary, has been used as a storm water retention pond since 1957. Currently, it is designated as a wildlife habitat area. Formerly included in Operable Unit 3, the investigation at Pond 3 was reopened because an area of soil contamination was found. An EE/CA was implemented in 2003 to remove the contamination, and the cleanup is complete.
Status: Site closed in 2003
- Pond 7 Area
TCE-contaminated groundwater was discovered in the Pond 7 Area during a basewide investigation in 1993. Data from the investigation revealed that the levels of contamination posed no risk to human health and the environment. A proposal to close-out the site with no further action was completed in 2004.
Status: Site Closed in 2004
- Transformer Storage Area
During a site investigation, PCB-contaminated soil was discovered in the transformer storage area. This area was investigated to determine the extent of the contamination. An early cleanup action was implemented to remove the contaminated soil.
Status: Site closed in 1999
- Building 786
Located along the eastern base boundary, Bldg. 786 was used to store pesticides and herbicides from 1984 to 1997. The building has since been demolished and sampling indicated the site does not pose a threat to human health or the environment.
Status: Site closed in 2002
Sites Under Investigation
Three sites within OU-9 are still under investigation. The following sections provide a brief description of these sites.
- 800/900 Warehouse Area
Industrial buildings in the 800/900 Warehouse Area are believed to be the possible source areas of the contaminated groundwater, which consists of carbontetrachloride (CTCL). Several monitoring wells have been installed to monitor the contaminated groundwater.
Status: Feasibility Study
- 1100 Area
The 1100 Area includes TCE-contaminated groundwater, both on Hill and off the base, along the western base boundary. Solvents used in this area led to the contamination of the shallow groundwater near Sunset City. Recent groundwater sampling shows that the contamination is shrinking and does not extend into Sunset City. The feasibility study will examine this more closely.
Status: Feasibility Study
- Golf Course
Solvents and other chemicals used in the golf course maintenance shop, located on the southeastern edge of the base, resulted in the contamination of groundwater in the area.
Status: Feasibility Study
Health Risk Issues
The OU-9 contamination has not affected local drinking water supplies because of the subsurface geology at the site. The drinking water for Hill and Sunset 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.
A few apartments above the area of groundwater contamination in Sunset have had the indoor air tested for chemical vapors. Chemical vapors have not been found in any of the tests.
Cleanup Actions
Two separate EE/CAs were conducted to look at alternatives to deal with the contaminated soil at Pond 1 and Pond 3. In the summer of 2003, the arsenic-contaminated soil at Pond 3 was removed and transported to an off-base facility. Soil samples were collected to ensure all of the contaminated soil was removed, and cleanup has been completed at this site.
Construction of the EE/CA alternative for Pond 1 was completed in the summer of 2003. The contaminated soil was consolidated and then covered with an eight-foot soil cover. The pond was reconfigured to prevent storm water from coming into contact with the contaminants. A more efficient system was also installed to control the flow of water into and out of the pond.
The Air Force will continue to monitor contamination levels at OU-9.
What’s Ahead
Hill returned OU-9 to the Feasibility Study phase of cleanup after the EPA expressed concerns about the initially proposed Record of Decision (ROD) for OU-9 in 2007. Hill is collecting data to determine the best remedies to treat the contamination in this area where there are numerous sites. Hill AFB plans to provide a new Proposed Plan in late 2010 and sign the Record of Decision in mid-2011.
Since 2000, the Air Force has tested four homes in the OU-9 area for chemical vapors. The Air Force will continue monitoring and testing indoor air in Sunset. 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. Sites with remedies in place will be evaluated 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 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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Operable Unit 10
HAFB Project Manager: Robert Wallace
Site Overview
Operable Unit 10 (OU-10) deals with groundwater contamination near Hill Air Force Base’s West Gate and in the communities of Sunset and Clearfield. The contamination found in Clearfield ranges from eight feet to 290 feet underground and is primarily composed of trichloroethene (TCE), a degreasing solvent used in aircraft and other maintenance.
In 2005, an investigation uncovered a deeper TCE plume in Sunset that appears to be related to the shallower TCE plume in Clearfield. This deeper contamination is found from 170 to 290 feet below the surface.
Hill AFB is currently investigating the plumes to determine the stability of the contamination. There is no threat of exposure to TCE by coming in contact with the groundwater or by breathing chemical vapors because of the depth of the contamination. Hill will continue to monitor the plumes.
Investigations to date have determined the most likely source of the contamination was the use of chlorinated solvents for vehicle and munitions maintenance from the end of World War II to the 1960s. Although records were not kept at the time, it is likely these solvents were released to the ground or into drains in the 1200 area of Hill AFB.
Another possible source of the shallow plume contamination is the oil/water separator previously located at Bldg. 1244. Hill removed the oil/water separator in 2003.
Tetrachloroethene (PCE), another chemical used for degreasing, and originally included as part of indoor air sampling was removed from the sampling list in October 2006 due to a lack of evidence that it was in the groundwater outside of the base. During additional investigations in 2007 and 2008, however, Hill identified a relatively narrow plume of PCE contamination. As a result, PCE was added back to the sampling list in a small area of Clearfield. Hill retrieved PCE results from the lab for air samples collected between 2006 and 2009 and provided these results to residents. The results do not show any consistent levels of PCE detections above the action level set by the Environmental Protection Agency (EPA) and the Utah Department of Environmental Quality (UDEQ).
This site is currently under investigation, and information will be updated as it becomes available.
Health Risk Issues
The OU-10 contamination has not affected local drinking water supplies because of the subsurface geology at the site. The drinking water for Clearfield and Sunset 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 have been found in some homes in Clearfield above the area of contamination. Chemicals in the shallow groundwater can evaporate and enter homes through the basement or floor slab. Hill has implemented a program for testing the indoor air for chemical vapors of homes located above areas of groundwater contamination, 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.
Testing of fruits and vegetables in several local cities, including Sunset and Clearfield, showed no traces of the chemicals most frequently found in the shallow groundwater.
Cleanup Remedies
The site is currently being investigated to determine how stable the contamination is. No cleanup remedies are in place at this time.
Hill AFB is currently conducting a treatability study to determine if phytoremediation, which uses the root systems of plants to collect and treat groundwater contaminants, is a feasible remedy option.
What’s Ahead
Since 2000, the Air Force has tested 172 homes in the OU-10 area for chemical vapors. The Air Force will continue monitoring and testing indoor air in Clearfield and Sunset. The Air Force has made a long-term commitment with these residents to monitor and remove vapors from their homes.
The Air Force is continuing to gather data to better understand site characteristics. Potential cleanup actions are being looked at and the Air Force will make a proposal to clean up the contamination once the potential cleanup actions are evaluated. The public will have the opportunity to participate in the Proposed Plan.
Hill 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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