Environmental Engineering Reference
In-Depth Information
SLAC boundary
MW-11
MW-1
Explanation
Methylchloroform
XW-1
SVE-3
MW-5
1,4-dioxane
Concentrations
1000 μg/L
100 μg/L
10 μg/L
1 μg/L
XW-2
EW-1
MW-8
XW-3
VP-1
MW-6
PZ-1
XW-4
MW-4
μg/L = micrograms per liter
MW-7
XW-5
MW-6 Groundwater monitoring well
Former solvent underground storage tank
PZ-4
PZ-2
MW-12
PZ-3
MW-2
MW-3
MW-9
North
0
100 feet
Compressed air
and HM storage
Storage shed
Scale
FIGURE 8.7 1,4-Dioxane distribution compared to methyl chloroform plume at the SLAC Former Solvent
Underground Storage Tank facility. [Adapted form SLAC, 2008, Semi-Annual Self-Monitoring Report , Winter
2008, Part 1, submitted to the Regional Water Quality Control Board, San Francisco Bay Region (RWQCB),
SLAC-I-750-2A15H-023, SLAC National Accelerator Laboratory Environmental Health and Safety Division.]
The system extracted an average of 243 gallons of groundwater per day, or 0.169 gpm, between
startup in 2001 and March 2008. 1,4-Dioxane concentrations in a single extraction well have been
as high as 3100 μg/L, where the maximum detected concentrations are as follows: methyl
chloroform—13,000 μg/L, 1,1-dichloroethane—13,000 μg/L, and 1,1-dichloroethylene—3500 μg/L
(SLAC, 2008). Combined extraction-well inl uent concentrations of 1,4-dioxane in 2006 and 2007
have ranged from 75 to 480 μg/L; the midpoint sampling location had one detection at 1.3 μg/L out
of eight quarterly measurements. Efl uent from the GAC has been consistently nondetect for
1,4-dioxane (Cal EPA, 2008). Figure 8.8 plots 1,4-dioxane concentrations in GAC inl uent and efl u-
ent from 2001 through 2008.
Individual SVE wells contain 1,4-dioxane at concentrations ranging from 3 to 267 ppbv (mea-
sured by EPA Method TO-15) and a reporting limit of 1 ppbv. The combined inl uent of the SVE
system to the GAC soil-vapor treatment unit is 0.0111 ppmv, and the soil-vapor efl uent at the GAC
treatment unit is nondetect for 1,4-dioxane at a 5 ppbv reporting limit (SLAC, 2006b, 2008). Since
2001, a total of about 2.5 pounds of 1,4-dioxane has been extracted through pump and treat; this
amount comprises less than 1% of the total mass of VOCs removed from groundwater at the FSUST
site (SLAC, 2008).
8.5.4 P OSSIBLE R EASONS FOR 1,4-D IOXANE R EMOVAL BY G RANULAR A CTIVATED C ARBON
The complete removal of 1,4-dioxane by GAC treatment vessels is not expected because of 1,4-
dioxane's low K oc . Among the several possible mechanisms by which 1,4-dioxane may be removed
by GAC, three are considered here.
The i rst possible mechanism is that 1,4-dioxane is undergoing cometabolism or another form of
biodegradation. The slow rate of l ow, large surface areas, and possible warm temperatures during
much of the year may create an environment in which passive biodegradation of 1,4-dioxane could
occur. The practice of running two carbon vessels in series can sustain a consortium of viable
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