Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 395

Analyses of groundwater samples from FHWSA monitoring wells show presence of perchloro-
ethylene, trichloroethylene, and cis -1,2-dichloroethylene, suggesting a historical release of
perchloroethylene and reductive dechlorination occurring in the subsurface (SLAC, 2004). In
addition, detections of methyl chloroform, 1,1-dichloroethylene, and 1,1-dichloroethane in
samples further suggest abiotic and microbially mediated reductive dechlorination of methyl
chloroform (SLAC, 2004).
There are two apparent primary release areas at the FHWSA for methyl chloroform and
1,4-dioxane. SLAC staff attributes the detection of 1,4-dioxane to its use as a stabilizer for methyl
chloroform, as there are no other known signii cant uses of 1,4-dioxane at the SLAC facility (Cal
EPA, 2005). 1,4-Dioxane was i rst analyzed in groundwater extraction wells in 1997 by using EPA
Method 8270 in selected-ion mode (SIM) with a 25 μg/L reporting limit; in 1998, the reporting limit
was lowered to 10 μg/L (Sabba and Witebsky, 2003). Further 1,4-dioxane characterization in
groundwater began in 2000 using EPA 8270-SIM and a reporting limit of 1.0 μg/L (SLAC, 2006c;
Cal EPA, 2008). Although detected in soil vapor and in 30% of groundwater samples, 1,4-dioxane
has only been detected in one soil sample collected at the FHWSA, at 0.74 mg/kg (SLAC, 2004).
The distributions of 1,4-dioxane and the breakdown products of methyl chloroform at the FHWSA
are mapped in Figure 8.6.
A fate and transport study of VOCs and 1,4-dioxane in groundwater at the FHWSA was
performed by Erler & Kalinowski, Inc. (EKI), a consultant to SLAC (SLAC, 2006a). EKI modeled
the migration of 1,4-dioxane, perchloroethylene, and DCE in two separate chemical plumes using
a one-dimensional model and site-specii c data. One of the plumes is slowly migrating north toward
the accelerator subdrain, and the other plume is slowly migrating south toward San Francisquito
Creek. EKI interprets the model results to predict that a maximum concentration of 0.2 μg/L of

Sector
29

Linerar accelerator

MW-74

MW-43

MW-32

016

MW-66

761

311

Explanation

MW-31

308

MW-33

1,4-dioxane Concentrations

1000 μg/L
100 μg/L
10 μg/L
1 μg/L

1,1-DCE

Master
substation

Building 015

MW-75

MW-25

MW-50

MW-67 MW-58

MW-49

μ g/L = micrograms per liter

018

647

MW-59

MW-34

MW-68

Groundwater monitoring well

MW-60

MW- 51

MW-68 MW-69

North

0 80 feet

Stanford linear accelerator center

Former hazardous waste storage area

Scale

1,4-dioxane and 1,1-DCE groundwater concentration contours

FIGURE 8.6 Distribution of 1,4-dioxane and 1,1-dichloroethylene at the SLAC Former Hazardous Waste
Storage Area. (Adapted from SLAC, 2004, Site Characterization Report for the Former Hazardous Waste
Storage Area , Vol. 1 of 4, submitted to the Regional Water Quality Control Board, San Francisco Bay Region
(RWQCB) SLAC-I-750-3A33H-015, SLAC National Accelerator Laboratory Environmental Health and
Safety Division.)