Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 373

TABLE 8.1
Exceptional Results for Duplicate 1,4-Dioxane Analyses by USEPA 8260 SIM

Primary
Concentration (ppb)

Duplicate
Concentration (ppb)

Relative Percent
Difference Well Analyte Status

88A 1,4-Dioxane 1100 800 32 Accepted/l ag

90A 1,4-Dioxane 14,000 20,000 35 Accepted/l ag

RW-1A 1,4-Dioxane 1600 4700 98 Rejected

RW-2A 1,4-Dioxane 210 <50 157 a Rejected

a

Half the reporting limit, 25 ppb, was used to calculate the relative percent difference.

range because historical samples were often reported as nondetect at elevated reporting limits. In
the fourth-quarter 2006 monitoring report, 1,4-dioxane remained elevated at 47,000 ppb in one
extraction trench; that detection exceeded historical maximum values. In some wells, a dei nite
decreasing trend from 3000-1000 ppb is interrupted by a single detection at 6000 ppb; the fact that
the values then return to the previously dei ned trend suggests an analytical anomaly.

8.2.3 R EGULATION
The regulated party in the SSI case recommended that the Water Board delay imposing a cleanup
level for 1,4-dioxane until a health-based standard is promulgated. The basis for the request was a
citation of the paper on PBPK (physiologically based pharmaco-kinetic modeling) modeling of
1,4-dioxane toxicity and dose-response by Reitz et al. (1990), in which a 20,000 ppb cleanup level
is suggested. The Water Board denied this request and required continuous treatment to achieve
containment and attainment of California's action level of 3 ppb for 1,4-dioxane and the USEPA
Region 9 PRG of 8.8 ppb for THF. In addition, the board required a report documenting complete
investigation of the extent of 1,4-dioxane in groundwater, delineated to 3 ppb. The board also
required a feasibility study for 1,4-dioxane and THF to evaluate remedial measures that will effec-
tively address these chemicals. The primary fate of 1,4-dioxane at this site has been sewer discharge;
the treatment train was not modii ed to add treatment modules capable of 1,4-dioxane removal.
A risk assessment was developed for the site for future hypothetical exposure scenarios involving
residential land use and construction-worker inhalation. Drinking water was not among the 1,4-diox-
ane exposure scenarios evaluated because only the shallow groundwater is affected and installation
of drinking water wells is restricted by institutional controls. Site-specii c risk-based cleanup levels
(RBCLs) were based on a cumulative cancer risk of 1 × 10 −5 and a noncancer total hazard index of
1.0. The RBCLs for 1,4-dioxane ranged from 472 to 4512 ppm, and for THF, from 253 to 86,544 ppm.
On-site and off-site concentrations of 1,4-dioxane are well below the calculated RBCLs.
Risk from potential vapor intrusion to indoor air was evaluated by using the Johnson and Ettinger
model (Johnson and Ettinger, 1991). The estimated residential indoor air concentrations that could
result from underlying groundwater 1,4-dioxane concentrations averaging 32 ppm was 0.23 μg/m 3
(0.83 ppbv), under the assumption that future homes will be constructed atop the plume core. These
concentrations were determined to present no signii cant risk by the risk-assessment authors.

8.2.4 D ISCUSSION
The SSI case illustrates the repercussions of discovering a new contaminant after more than a
decade of cleanup and investigation. 1,4-Dioxane was not a listed COC for the case until its acciden-
tal discovery. The laboratory reported the discovery of 1,4-dioxane to the consultant, which proba-
bly led to an interesting conversation with the client, because 1,4-dioxane was not on the specii ed