Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 368

greater than those equivalent to the 1 in 1,000,000 excess lifetime cancer risk. 1,4-Dioxane was
specii cally identii ed as potentially exceeding the recommended risk levels at the nearest private
well in less than i ve years (USEPA, 1987). USEPA analyzed the cumulative excess cancer risk from
all carcinogenic compounds emanating from the site, including benzene, methylene chloride,
chloroform, perchloroethylene, 1,2-dichloroethane, 1,1,2-trichloroethane, 1,1-dichloroethylene
(DCE), trichloroethylene, 1,4-dioxane, and vinyl chloride. To prevent exposure, the airport well and
the nearest operational downgradient private supply well were monitored for volatile organic
contaminants (USEPA, 1987).

8.1.3 T REATMENT
A remedial investigation and feasibility study concluded that pump and treat prior to discharge to
the local POTWs would achieve City of Seymour pretreatment requirements (total toxic organics
< 2.13 ppm). * Pretreatment consisted of mixed media i ltration (a sand i lter to remove iron to

prevent formation of iron oxide in the air stripper), followed by an air stripper and carbon adsorp-
tion. During remedial investigations, USEPA discovered that bacteria were naturally aiding in soil
remediation, which allowed construction of a smaller groundwater treatment facility than originally
planned, leading to substantial savings (USEPA, 2007c). USEPA capitalized on contaminant biode-
gradability by applying nutrients to contaminated soil to stimulate biodegradation (U.S. Congress,
1988). Soil-vapor extraction (SVE) and a multimedia cap over the most contaminated soils were
also included in the remedy. SVE was preferred over soil excavation because of the dangers of
worker exposure associated with excavating soil with high concentrations of volatile compounds.
The selected remedial alternative was projected to cost $18 million (U.S. Congress, 1988).
The discharge of treated groundwater to the POTW was projected at 150 gpm, which was
approximately 20% of the excess capacity of the Seymour POTW and approximately 15% of the
average daily wastewater l ow. An evaluation showed that there would be no detectable levels of
contaminants in the discharge of the POTW to the South Fork of the White River (USEPA, 1987).
The SVE system removed the bulk of the contaminant mass during its i rst year of operation;
more than 20,000 pounds of VOCs were vented. The system reached asymptotic levels after approx-
imately 34,000 pounds were removed, and operations were discontinued. About 85% of the total
mass of VOCs extracted by the SVE system over a four-year period consisted of methyl chloroform
(31.8%), trichloroethylene (23.2%), perchloroethylene (9.7%), cis -1,2-DCE (8.7%), 1,1,2-trichloro-
1,2,2-tril uoroethane (Freon 113) (7.0%), and toluene (4.8%).
After 12 years of operating the groundwater pump and treatment system, more than 90% of the
contamination had been removed from the groundwater (USEPA, 2002). USEPA concluded that the
plume was stable and shrinking, because the areal extent of the 1-4 dioxane and THF plumes had
decreased when compared to data from four years earlier. The pump and treat system was shut down
in October 2001 because it was deemed no longer efi cient in removing the lower levels of groundwa-
ter contaminants that were still above cleanup standards. The approval for the shutdown required (1)
increased groundwater monitoring and (2) a contingency to restart the groundwater treatment system
if the groundwater plume expanded from its position at system shutdown (USEPA, 2002). The
groundwater treatment was originally estimated to take 28-42 years to complete groundwater reme-
diation, but system shutdown was approved within less than 20 years (U.S. Congress, 1988).
The i ve-year review report of the groundwater monitoring network found that the plume remained
stable, but two additional monitoring wells were needed to sample for 1,4-dioxane (USEPA, 2007b).
The overall extent and concentration distribution of 1,4-dioxane in the shallow aquifer has not
changed signii cantly since 2001. 1,4-Dioxane concentrations in one deep-aquifer monitoring well
near the capped area showed a signii cant increase to 690 μg/L as of the i ve-year review; however,
other monitoring wells continued to show low or nondetectable levels of 1,4-dioxane.

* Most total toxic organics analyses are not performed in a manner that is likely to detect 1,4-dioxane.