Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 396

1,4-dioxane would reach the linear accelerator subdrain about 80 years from 2001, whereas a
maximum concentration of 1 μg/L of DCE would arrive at the subdrain about 150 years from
2001. The model suggests that perchloroethylene would never arrive at the San Francisquito creek
(685 years from 2001 if the maximum concentration is 1.6 × 10 -6 μg/L). The regulatory agency's
review of VOC and 1,4-dioxane migration at the FHWSA concluded that these compounds are not
likely to impact downgradient offsite groundwater or surface water at detectable concentrations
(Cal EPA, 2005).

8.5.3.1 Remediation at Former Hazardous Waste Storage Area
Interim remediation at FHWSA was a dual-phase soil-vapor and groundwater extraction system
that operated from December 2003 to March 2006 and removed an estimated 35.6 pounds * of VOCs
before it was integrated into the full-scale system. The full-scale remediation system at FHWSA
consists of 23 dual-phase extraction (DPE) wells, which pump groundwater and soil vapor intermit-
tently and discharge to an air stripper. Since start-up in March 2006, the FHWSA DPE system
removed about 15 lbs of VOCs. Extracted groundwater is discharged into the sanitary sewer after
treatment by an air stripper under permit from the local sewer authorities. Extracted soil vapor is
vented without treatment to the atmosphere under a permit from the Bay Area Air Quality
Management District (BAAQMD) (SLAC, 2008). Inl uent to the air stripper from the DPE system
included 1,4-dioxane ranging from 1.8 to 22 μg/L in 2006 and 2007 (Cal EPA, 2008). SLAC's loca-
tion in a hilly, windy area about a half-mile from the nearest residential area is compatible with
release of low-concentration VOCs in air-stripper emissions.

8.5.3.2 Former Solvent Underground Storage Tank Area
A 2400-gallon underground storage tank was used to store paint shop wastes from 1967 to 1978 at
the location now known as the FSUST. The tank and contaminated soil were removed in 1983, and
investigations were conducted in 1984 and 1985. Despite two major excavations performed to
remove contaminated soil, solvents at concentrations greater than 1000 mg/kg in soils were detected
in samples collected in 1996. Concentrations in soil decrease laterally over a short distance, and
highest soil concentrations generally occur in the saturated zone between 8 and 18 ft bgs but extend
as deep as 30 feet below the FSUST. In addition to chlorinated solvents and 1,4-dioxane, other
detected contaminants include bis-2-ethylhexyl phthalate, acetone, toluene, and methyl ethyl ketone
(Cal EPA, 2005; SLAC, 2008). The chemicals are migrating slowly in the groundwater, at less than
8 ft/year, and have not yet arrived at the linear accelerator subdrain located 350 ft south of the
FSUST. In 2001, SLAC installed a groundwater extraction system to hydraulically control contami-
nant migration, supplemented by i ve additional extraction wells in 2006. The extraction wells dis-
charge to a GAC treatment system (Cal EPA, 2005; SLAC, 2006b, 2008). Figure 8.7 presents a map
comparing distribution of 1,4-dioxane and methyl chloroform degradation products at the SLAC
FSUST site.
As of late 2007, the FSUST system utilizes six dual-phase (groundwater and soil vapor) and two
groundwater extraction wells, an SVE system, and two 2000-pound-capacity GAC vessels to pro-
vide hydraulic control of groundwater contamination and treatment of extracted groundwater
(SLAC, 2008). The inl uent is chlorinated prior to entering the i rst vessel to prevent the formation
of hydrogen suli de. Chlorination is expected to cause indiscriminate oxidation of organics.
Chlorine oxidation of organic compounds in the inl uent probably limits the potential for subse-
quent biodegradation of organics in the GAC; however, the GAC units were not intended to per-
form as bioreactors and have not been tested for their ability to sustain microbial degradation of
organic contaminants.

* This seemingly small quantity underscores how small a part per billion is. If the regulatory threshold for cleaning up the
VOC(s) is 5 ppb, then these 35.6 lbs (16.2 kg) would be enough to render 3 1

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4 billion liters of water contaminated to a VOC
level greater than 5 ppb; that is enough water to irrigate a square-mile more than 4 ft deep.