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Record of Decision System (RODS)

HILL AIR FORCE BASE

Abstract

Site Name:  HILL AIR FORCE BASE
Address:  00-ALC/EM 
City & State:  HILL AFB  UT  84056
County:  DAVIS, WEBER
 
EPA ID:  UT0571724350
EPA Region:  08
 
NPL Status:  Currently on the Final NPL
 
ROD Type:  Record of Decision
ROD ID:  EPA/ROD/R08-96/130
ROD Date:  09/30/1996
Operable Unit(s):  02
 
Media:  soil, groundwater, surface water, air
 
Contaminant:  Trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, 1,2-dichloroethene, methylene chloride, toluene, 1,1,2-trichloro trifluoroethane
 
Abstract:  Please note that the text in this document summarizes the Record of Decision for the purposes of facilitating searching and retrieving key text on the ROD. It is not the officially approved abstract drafted by the EPA Regional offices. Once EPA Headquarters receives the official abstract, this text will be replaced.

Hill Air Force Base (HAFB) is located in northern Utah, approximately 25 miles north of Salt Lake City and about five miles south of Ogden, Utah. HAFB occupies approximately 6,700 acres in Davis and Weber Counties. The base is bounded on the west by Interstate 15, on the south by State Route 193, and on the northeast by the Weber River Valley. The base is located on a prominent terrace known as the Weber Delta.

Operable Unit 2 (OU2) is located along the northern boundary of the base. Areas investigated as part of OU2 consist of Perimeter Road and two unlined trenches known together as Chemical Disposal Pit 3. The trenches are now obscured by facilities of the Source Recovery System (SRS). The SRS was installed as part of an interim remedial action to extract as much of a dense, non-aqueous phase liquid (DNAPL) as practicable. Except for the SRS and Perimeter Road, there are no other buildings or man-made structures in the on-Base portion of OU2. Traffic is sparse and the area is seldom used by HAFB for military activities.

Municipal water for South Weber is supplied by the Weber Basin Conservancy District. The district provides water from wells which tap deep aquifers believed to be unaffected by contaminants associated with OU2. Shallow groundwater is not currently used as a source of drinking water in the area, but was used for irrigation and cattle in the past.

HAFB has been a major center for missile assembly and aircraft maintenance and repair. The associated industrial processes include metal plating, degreasing, paint stripping, and painting. These processes required use of various chemicals, metal plating solutions, chlorinated and non-chlorinated solvents, degreasers, petroleum hydrocarbons, acids, and bases. HAFB records indicate that from 1967 to 1975, former Chemical Disposal Pit 3 was used for disposing unknown quantities of trichloroethylene (TCE) bottoms from solvent recovery units and sludge from vapor degreasers. In the early 1940s, an unknown volume of plating tank bottoms were disposed of at this site.

Perimeter Road provides access to most of the waste disposal areas along the northern part of the base. Most of these waste disposal areas were active in the 1960s and 1970s. Investigative activities along Perimeter Road revealed no evidence of spills or dumping except in areas already being investigated as part of this or other Operable Units on HAFB.

Prior to the investigations at OU2, HAFB took some response actions to prevent exposure to contamination. These prior actions include: providing municipal water connections to five homes known to have been affected by contamination; collecting and treating contaminated water flowing from springs and seeps, with the treated water being discharged to the original spring drainage; installing fences around springs and seeps with contaminated water to prevent livestock access; and constructing the Source Recovery System (SRS) to remove DNAPL from the area near the former Chemical Disposal Pit 3.

Based on investigative efforts, the source of contamination at OU2 is the former disposal trenches (Chemical Disposal Pit 3). Perimeter Road along the northeastern part of HAFB has been investigated and found to be free of contamination except in those areas currently being addressed as part of other OUs.

Accumulations of DNAPL occur on-base and in the shallow groundwater flow system in the vicinity of the former disposal pits. The DNAPL layer is composed primarily of a mixture of several chlorinated and non-chlorinated solvents and a lesser amount of co-solved oil and grease.

Volatile Organic Compounds (VOCs) predominate among the contaminants found in all of the media. The principal VOCs include trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,1,1-trichloroethane (TCA). Less common and widespread VOCs include 1,2 dichloroethene, methylene chloride, and toluene. VOCs are in the highest concentrations in the vicinity of the former disposal pits and decrease with distance laterally from the pit area.

A variety of pesticides were detected. Most pesticides occurred in soil media throughout the area at relatively low concentrations and are believed to be related to agricultural and pest-control application activities rather than waste disposal. Some metal concentrations are elevated above background concentrations in soils near the pits. Some of the data indicate levels of inorganic slightly above background concentrations in soils near the pits.

Current on-base land use at OU2 is restricted. The OU2 on-base area has not been used for military activity other than the documented waste disposal and is not used for any recurring HAFB function. The main activity is the SRS used to recover the DNAPL in the subsurface.

Contaminant migration in groundwater is the most significant pathway. The available information regarding operation of the waste disposal trenches indicates that spent liquid chlorinated organic solvents were poured into unlined earthen trenches. The liquid solvents infiltrated through the unsaturated soil to the water table.

Current land use in off-base areas is low-density residential development and agriculture. Off-base residents rely on municipal water for their domestic supply. Shallow groundwater is not used as a source of drinking water in the area, but has been used for irrigation and cattle watering. Alternate water supplies have been provided or treatment units installed at springs to address this pathway to prevent exposures.

Surface soils at OU2 contain elevated inorganic and pesticide constituents in concentrations greater than background. These constituents may be carried by wind-borne dust. The air pathway for VOC contaminant transport at OU2 is through direct volatilization and vapor migration from the soils to the atmosphere. Contaminants from OU2 have been found in soil gas outside the former Chemical Disposal Pit 3 area.
 
Remedy:  The selected remedy for the OU2 source area includes: an encircling vertical barrier; shallow groundwater extraction and treatment, and discharge; soil vapor extraction (SVE) to remove volatile organic compounds (VOCs) from subsurface soils; continued operation of the source recovery system (SRS) to remove dense, non-aqueous phase liquid (DNAPL) to the maximum extent practicable and for the treatment of shallow groundwater; a surface cap to prevent further degradation of groundwater; and treatability studies planned to address DNAPL contamination which include the use of surfactants and steam injection.

The selected remedy for the OU2 non-source area includes: shallow groundwater extraction, treatment, and discharge; continued collection, treatment, and discharge of contaminated water flowing from springs and seeps; and as concentrations decrease in time, it may become more cost-effective to use other on-site discharge options.

The selected remedy for both areas includes: environmental monitoring to evaluate the effectiveness of the remedy; implementing institutional controls to minimize exposure by limiting use and preventing access to contaminated water and soil.

Perimeter Road, investigated as part of OU2, has been found to be free of contamination except in those areas being addressed as part of existing OUs. No further action is needed for Perimeter Road as part of OU2.
 
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