Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 376

8.3.3 D ELINEATING 1,4-D IOXANE C ONTAMINATION
Beginning in 1986, investigations by the company identii ed soil contamination on the P/GSI
property and groundwater contamination extending off the property. Groundwater concentrations
of 1,4-dioxane were as high as 221,000 μg/L. The plumes collectively encompass an area of approx-
imately 0.6 square miles, as dei ned by the Michigan drinking water action level of 85 μg/L (Brode
et al., 2005). Citizen groups dei ne the plume to a 1 μg/L contour and describe the plume of deeper
1,4-dioxane contamination as “18 million square feet and growing, three miles long and one mile
wide” (Kellogg, 2005). 1,4-Dioxane method detection levels have ranged from 100 to 1 ppb during
the course of the project (Fotouhi et al., 2006). As of 2005, groundwater was monitored routinely at
50-100 locations (Brode et al., 2005). Figure 8.4 displays the extent of the shallow and deep plumes
delineated to 85 ppb, the Michigan 1,4-Dioxane Generic Residential Cleanup Criteria [Part 201 of
Natural Resources and Environmental Protection Act (NREPA)].
Deeper contamination was discovered during an investigation in spring 2001, following a
detection of 1,4-dioxane at 2 ppb in the City of Ann Arbor's Northwest Supply well. The city
turned off the well immediately following the detection, and investigations of deep aquifer con-
tamination began. After an intensive effort to characterize deep contamination, concentrations as
high as approximately 5000 ppb were found in the deep Unit E aquifer. A clay layer separating
the two shallower aquifers from the deeper Unit E aquifer in areas to the east was found to be
absent in areas to the west of the site. The Unit E aquifer generally l ows to the east toward the
Huron River, a major regional hydraulic feature (MDEQ, 2005). State regulators have required
vigilant monitoring to ensure that spreading plumes of 1,4-dioxane do not pose a threat to private
wells outside of the city. The plume is headed in the direction of the Huron River, which is the
source of about 80% of the City of Ann Arbor's water supply; however, Ann Arbor's drinking

94

85 ppb
contour

94

Deep plume

Prohibition
zone

Shallow plume

P/GSI facility

0.5 miles

FIGURE 8.4 Distribution of 1,4-dioxane in the shallow and deep (Unit E) aquifers, P/GSI site, Ann Arbor,
Michigan (Fotouhi, F., 2006).