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Home > Site Information
Riverdale
Operable Units:
OU 6
OU 4
Operable Unit 6
HAFB Project Manager: Alan Jones
June 2011 Update
OU-6 is defined by two trichloroethene (TCE)-contaminated groundwater plumes. The smaller west plume is wholly contained on-base. The larger east plume underlies on-base areas and extends north to off-base areas under Riverdale.
The west plume is shrinking. Hill is monitoring the plume to ensure that concentrations continue in a downward trend. Sampling results show the off-base portions of the east plume also appear to be shrinking and contaminant concentrations are declining with the help of two groundwater extraction systems which have collectively removed close to 90 pounds of TCE since 1996.
For the last several months, Hill AFB has conducted system optimization tests to determine if more efficient cleanup can be achieved in certain areas. At OU-6, the test involved turning off one of two sets of groundwater extraction wells. Initial results show contaminant cleanup levels are being maintained.
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Site Overview
Operable Unit 6 (OU-6) includes contamination on the northern portion of Hill Air Force Base (AFB) and in the community of Riverdale. OU-6 consists of:
- 1,1-dichloroethene (DCE)-contaminated soil and trichloroethene (TCE)-contaminated shallow groundwater on base near the 2000 Building Area of Hill AFB.
- TCE-contaminated shallow groundwater beneath Building 1917.
- TCE-contaminated shallow groundwater located beneath the Craigdale subdivision in Riverdale City.
- TCE-contaminated spring water located in the Craigdale subdivision.
In 1988, TCE was discovered in the groundwater during water sampling in the Craigdale subdivision in Riverdale City. Several investigations have been conducted since 1988 to determine the extent of the contamination. Two contaminated groundwater plumes were identified: the east plume, which originates in the missile training area and extends off base, and the west plume, which is much smaller and does not extend outside the base boundary.
From the 1940s through the 1970s, hazardous wastes at Hill were disposed of in both designated and undesignated waste disposal sites, such as chemical disposal pits, landfills, and waste disposal ponds. At OU-6, however, there are no known waste disposal sites and no formal records were kept to indicate the possible disposal of hazardous waste at this site. The east plume may originate from small amounts of waste solvents disposed of in various outdoor areas between buildings, a leaky underground storage tank that contained solvents, and possibly from former dry cleaning operations.
The source of the west plume is believed to be the Bldg. 1915 Sump Leach Field site.
Although buildings in OU-6 have been used for various purposes since the 1940s, it is believed that the use of operations-related solvents and fuel began in the 1960s when several buildings were modified to support the Bomarc missile testing and maintenance activities. Areas of investigation in OU-6 include the waste asphalt pile, Bldg. 1946 Evaporation Pond, and the Bldg. 1915 Sump Leach Field site.
The primary contaminants at OU-6 are cleaning and degreasing solvents, the most widespread of which is TCE. A list of the primary chemicals of concern in OU-6 is included in the table.
As a result of the 2003 Five-Year Review recommendations for OU-6, additional investigations were conducted in 2006 through 2009 to better understand the extent of the groundwater contamination in the Craigdale subdivision. Investigation results indicated that the Riverdale portion of the east plume was shaped differently than previously depicted, and that instead of two small separate arms running off-base, it is actually one single larger plume.
The cleanup remedies for the contamination at OU-6 were formalized in the OU-6 Record of Decision (ROD) that was signed in 1997 by the Air Force, the Utah Department of Environmental Quality (UDEQ) and the Environmental Protection Agency (EPA).
Health Risk Issues
The OU-6 contamination has not affected local drinking water supplies because of the subsurface geology at the site. The drinking water for Riverdale 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 Riverdale homes 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 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.
Cleanup Remedies
Several cleanup remedies have been put in place to address both soil and groundwater contamination on Hill and in Riverdale:
- A pump-and-treat system has been installed on base to prevent additional contamination from leaving the base. This system consists of six extraction wells and an air stripper. The extracted water is treated on base and reinjected into the ground.
- Six groundwater extraction wells have been installed in Riverdale along 1150 West and 1200 West in the Craigdale subdivision. Extracted water from these wells was initially sent to an air stripper for treatment, but concentrations have declined to the point that the air stripper is no longer required and the water is now discharged to the storm drain. The air stripper system has been “mothballed,” but if concentrations increase, the system can be turned on again.
- Access to 1,1-DCE-contaminated soil on Hill is restricted through controls that require permits before building or working in the area.
- TCE groundwater contamination beneath Building 1917 is being allowed to naturally attenuate after extensive soil and water testing was completed.
- A treatment system was installed at a Riverdale spring to treat TCE. Because TCE levels are now below action levels, the system is currently turned off. The spring is monitored regularly so, if necessary, the system may be turned on again.
- Permanent monitoring wells were installed in the Craigdale subdivision, which will allow the Air Force to continually monitor the groundwater contamination.
The Air Force will continue to monitor contamination levels at OU-6.
What’s Ahead
Since 2000, the Air Force has tested 76 homes in the OU-6 area for chemical vapors. The Air Force will continue monitoring and testing indoor air in Riverdale. 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 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 4
Please refer to the South Weber area for details on Operable Unit 4.
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