Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 410

identii cation of 1,4-dioxane at the site led to the supposition, and later empirical demonstration, that
the existing treatment system was not adequate to address this new-found recalcitrant contaminant.
In response to this i nding, the extraction well-i eld operation was reconi gured to containment-only
at minimal extraction rates, resulting in a reduction in 1,4-dioxane levels in the efl uent/recharge
water from approximately 10 μg/L to below 4 μg/L. Additionally, the USAF conducted a Technology
Evaluation (Earth Tech, 2004) that determined that an AOT system could treat both the VOCs and
the 1,4-dioxane for about the same annual cost as the present system (when taking into account costs
for chemicals, GAC, natural gas, evaporative water loss, electricity, and ongoing maintenance of the
20-year-old stripping towers).
After the USAF decided to install an AOT system, the EPA issued an Administrative Order to the
USAF in July 2007 under Section 1431 of the Safe Drinking Water Act (SDWA), stating that the
1,4-dioxane in the regional aquifer posed an “imminent and substantial endangerment” to the public
(USEPA, 2007d). The Administrative Order required installation of an AOT system at the AFP 44
GWTP to address 1,4-dioxane identii ed in the groundwater above a Region 9 PRG of 6.1 μg/L and
to ensure that recharge water contained less than 3 μg/L. A proposed Settlement Agreement separate
from the order requested that the USAF conduct a focused remedial investigation (FRI) of 1,4-diox-
ane in the entire plume area. The USAF believes that the SDWA Administrative Order is invalid,
because the remediation is occurring under CERCLA, and therefore other regulatory enforcement
mechanisms are not applicable, among other reasons. In spite of this disagreement, the USAF expe-
dited the design and installation of the AOT system, which was programmed for installation in the
i scal year 2009. The startup of the system is currently scheduled for May 2009. The USAF also initi-
ated a Phase I FRI, which also would address the 1,4-dioxane concerns expressed by the USEPA in
the proposed Settlement Agreement submitted in April 2007. The scope of the Phase I FRI is (1) to
evaluate the geology, hydrogeology, and water quality with data gathered from all the TIAA Superfund
Site parties, (2) to assess the groundwater model that had been used by the Environmental Protection
Agency, and (3) to provide recommendations to i ll any data gaps identii ed. The Phase I FRI was
completed in November 2008. In July 2008, while the Phase I FRI was being conducted, the EPA
issued a Special Notice Letter to the TARP parties, which required a full remedial investigation to be
performed (to include drilling, well installation, well abandonment, and other tasks) (USEPA, 2008b).
This scope includes tasks from Phase I and Phase II, which was funded by the USAF in March 2009
and is anticipated to be completed in the spring of 2010. The requirements of the Special Notice
Letter did not materially change the USAF activities that were funded and ongoing.
Most recently, in August 2008, the USEPA issued a request to the TIAA that a specii c area near
their Three-Hangars building be investigated as a potential source of 1,4-dioxane. This request was
stimulated by sampling conducted by the USGS on the Tucson Airport property that identii ed
elevated 1,4-dioxane in a location and geologic unit that could not have been related to AFP 44
contaminant migration. Ongoing and future investigations by several parties will focus on whether
additional potential sources, besides AFP 44, exist for the 1,4-dioxane in the regional aquifer.

8.7.7 A DVANCED O XIDATION T REATMENT S YSTEM U PGRADE
The USAF accelerated the design and installation of an AOT system to address both chlorinated
VOCs and 1,4-dioxane. The system selected (following an extensive review of available technolo-
gies) was APT's HiPOx™ system. The AOT system utilizes ozone and hydrogen peroxide to form
hydroxyl radicals. This application of peroxone chemistry is well documented and is further detailed
in Chapter 7. Hydroxyl radicals are second only to l uorine in their oxidizing potential. Hydroxyl
radicals react very rapidly to oxidize organic compounds into nonhazardous compounds including
carbon dioxide and water. This technology has been demonstrated to be effective on recalcitrant
compounds, including 1,4-dioxane. The oxidation process occurs in the aqueous phase and does not
increase the temperature or pressure of the water because it usually occurs at very low concentra-
tions ( < 1%). The fundamental difference between other peroxone technologies and HiPOx is in the