Regulation and Risk Assessment of 1,4-Dioxane - Environmental Investigation and Remediation - page 275

unnecessarily low. MMA pointed out that USEPA has reopened its risk assessment because studies
showed that the IRIS analysis may dramatically overstate the cancer risk posed by 1,4-dioxane
(MMA, 2004).
MDEQ's survey of water supplies affected by 1,4-dioxane was also cited by MMA as a reason
not to establish a state MCL. The survey showed that about 1% of public water supplies in Michigan
were affected by 1,4-dioxane, and no public water supplies contained concentrations approaching
the proposed MCL of 35 μg/L (MMA, 2004). Eight sites out of about 3200 in MDEQ's state
code Part 201 site list include 1,4-dioxane as a listed contaminant; more than 100 sites include
1,1,1-trichloroethane (methyl chloroform). It is unlikely that all sites with methyl chloroform have
been tested for 1,4-dioxane.
The city of Ann Arbor issued comments to MDEQ on the proposed MCL objecting to the use of
a 10 −5 cancer risk factor and argued that using 10 −6 to obtain an MCL of 3.5 of 3 ppb is more protec-
tive. Ann Arbor's population was 114,000 in 2000; water supply is obtained from municipal wells
and the Huron River, and several of Ann Arbor's wells are in the path of 1,4-dioxane migration from
the PLS/Gelman plumes.
The city of Ann Arbor now asks developers faced with contamination cleanup to use a 10 −6
residual cancer risk for carcinogens, including 1,4-dioxane. Both the city of Ann Arbor and
Washtenaw County passed resolutions for a Best Available Control Technology (BACT) standard of
10 ppb daily maximum and 3 ppb monthly average for the remediation of 1,4 dioxane. Existing
cleanup operations at PLS/Gelman routinely reduce 1-4 dioxane concentrations to 3 ppb or less
(City of Ann Arbor, 2004).
Washtenaw County also issued comments objecting to MDEQ's proposed 35 ppb MCL. The
county contended that the effect of 1,4-dioxane exposure on susceptible subpopulations had not been
adequately addressed and that exposure from inhalation was also not given sufi cient consideration.
Washtenaw County ofi cials also objected because adopting a high MCL would require the county to
issue permits for new wells in areas where 1,4-dioxane is expected to be present below the MCL.
MDEQ withdrew the proposed 1,4-dioxane 35 ppb standard based on the objections from stake-
holders and regulated parties (TOSC, 2007). Section 8.3 presents a detailed review of the PLS/
Gelman site.
6.1.3 N EED FOR R EGULATION : O CCURRENCE OF 1,4-D IOXANE IN D RINKING W ATER
The extent of 1,4-dioxane occurrence in drinking water is unknown because most water utilities do
not analyze samples for 1,4-dioxane. Nevertheless, 1,4-dioxane has certainly been found as a drink-
ing water contaminant in many water systems. Given the small percentage of water systems that
have undergone testing for 1,4-dioxane, the number of affected systems is likely to increase as more
systems are tested. A survey of 5500 drinking water analyses in the mid-1970s revealed 15 detec-
tions of 1,4-dioxane at concentrations greater than 10 −7 g/L (i.e., greater than 100 ppb) (Donaldson,
1977). More recent detections of 1,4-dioxane in the United States, Canada, Japan, South Korea, and
the Netherlands are listed in Table 6.3 .
Table 6.3 presents known detections of 1,4-dioxane in drinking water; however, this list is by no
means inclusive of all detections, and no effort was made to determine the rate of detection by tal-
lying the frequency of nondetect results for drinking water analyses that include 1,4-dioxane. A
highly correlated association with methyl chloroform occurrence was shown in a survey of 1,4-di-
oxane detections in drinking water sources in Japan (Abe, 1999). In the United States, a survey of
analytical results in the U.S. Geological Survey's (USGS) National Water-Quality Assessment
(NAWQA) Program, including more than 5000 wells * between 1985 and 2002, indicated a 7% fre-
quency of detection for methyl chloroform (Moran et al., 2007). Although broad generalizations
* The wells in the NAWQA survey include drinking water sources, shallow urban groundwater, and groundwater underlying
agricultural areas.