Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 386

assistance of the Agency for Toxic Substances Disease Registry (ATSDR) to prepare a health con-
sultation and exposure investigation report. For those with known comparison guidance concentra-
tions, all levels detected were below ATSDR's guidance concentrations (ATSDR, 2005; FDOH,
2007). 1,4-Dioxane was not detected in any of the four indoor air samples (ATSDR, 2005). *
Subsequent vapor-intrusion investigations also tested for 1,4-dioxane, including 23 soil-vapor sam-
ples and two ambient-air samples in June 2006; however, analyses did not detect 1,4-dioxane above
its 18 μg/m 3 reporting limit (5 ppbv) (Arcadis-BBL, 2007b).
Five years into the site investigation and remediation activities, in February 2005, a Site
Assessment Report Addendum presented additional groundwater i ndings, including the detection
of 1,4-dioxane in 24 upper-aquifer monitoring wells and in 16 lower-aquifer monitoring wells. Of
these results, 21 upper-aquifer monitoring wells had 1,4-dioxane greater than Florida's Groundwater
Cleanup Target Level of 3.2 μg/L, and 13 lower-aquifer monitoring wells exceeded the 3.2 μg/L
target level (TetraTech, 2005).
The discovery of 1,4-dioxane substantially increased the plume size because 1,4-dioxane had
spread much farther than the methyl chloroform † from which it originated and much farther than the
TCE and PCE that was also released from the site. Prior to the discovery of 1,4-dioxane, the plume
of chlorinated ethenes was mapped at 50 acres; following the discovery of 1,4-dioxane, the plume
was mapped at 131 acres and eventually was delineated at about 200 acres.
An independent consultant, Environmental Science & Technology (ES&T), was retained by the
local community group (but funded by the responsible party) to complete an independent sampling
study of private wells. ES&T suggested that their i ndings indicated the plume was moving more
rapidly than was i rst thought. They attributed the plume's movement to changes in hydraulic pres-
sure that occurred when Tallevast residents were connected to county water in the summer of 2004
and stopped pumping their wells. Cessation of pumping near the center of the plume, coupled with
continued pumping by surrounding residents and businesses on the periphery of the plume caused
an apparent shift in the directions and rates of plume migration (ES&T, 2005).
The responsible party's February 2005 1,4-dioxane results did not correlate with ES&T's December
2005 reported levels. ES&T's 1,4-dioxane results were obtained by EPA Method 8260 using a
reporting limit of 20 μg/L, whereas the responsible party's consultant used FDEP-approved EPA
Method 8270, which is generally regarded in the industry as a more reliable method. ES&T's EPA
8260 results suffered from quality control problems. For example, one duplicate sample was nondetect
( 20 μg/L) in the i rst analysis and 150 μg/L in the second analysis. Overall, ES&T's EPA 8260 results

showed higher concentrations of 1,4-dioxane. ES&T suggested that the differences between their
sample results and those obtained by the responsible party's consultant may have been related to how
deep samples were retrieved. ES&T noticed that samples collected from the deeper, open-cased bore
hole had higher concentrations than those from the upper, cased section of the well (ES&T, 2005). The
large differences in 1,4-dioxane results from the two sampling surveys, while collected at different times
and with different methods, led to public confusion and suspicion of the lower concentration results.
The responsible party funded a i eld study in 2006 to collect whole-volume split samples for
analysis of 1,4-dioxane by three methods at three laboratories. A new consultant, Arcadis-BBL,
collected i eld samples, including some samples spiked with standards at three different concentra-
tions. Analysis was performed by EPA Methods 8270C, 8260B, and 8270 with isotope dilution
(“8270-ID”) at three different commercial laboratories. 1,4-Dioxane concentrations in the duplicate
samples analyzed by Method 8260B were highest among the three methods; some results were more
than an order of magnitude higher than the Method 8270C results. Detailed information regarding
this laboratory study is presented in Chapter 4.

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* A common laboratory method for analysis of soil vapor and indoor air samples is EPA Method TO-15, which includes
1,4-dioxane; however, 1,4-dioxane was not the focus of the indoor air sampling.
† The methyl chloroform is virtually absent from the Tallevast plume; its abiotic and microbial degradation products,
1,1-dichloroethane and 1,1-dichloroethylene, are all that remain.