Regulatory Policy Implications of 1,4-Dioxane - Environmental Investigation and Remediation - page 457

The risk of continued consumption of 1,4-dioxane in drinking water stems from long regulatory
timeframes and the regulation of solvents but not solvent stabilizers. Applying the Precautionary
Principle to emerging contaminants could avert such risks.
10.2 1,4-DIOXANE AND DRINKING WATER POLICY
At this time, Colorado is the only state that regulates 1,4-dioxane with a legal standard. * Chapter 6
provides a detailed review of the basis for Colorado's MCL for 1,4-dioxane and the basis for advi-
sory guidelines adopted by California, Connecticut, and Michigan as well as an inventory of similar
action levels in use in more than 30 other states and countries (see Table 6.1). Colorado was moti-
vated to adopt an MCL because 1,4-dioxane contamination was documented at nine sites and sus-
pected at 19 others, because of its classii cation as a probable human carcinogen, and because of its
mobility, persistence, and widespread use as a solvent stabilizer for methyl chloroform. Other states
such as California have many more sites with known 1,4-dioxane releases and a relatively high inci-
dence of 1,4-dioxane in drinking water, yet California has not adopted an MCL. Whether 1,4-diox-
ane should be regulated in drinking water depends on both the resolution of outstanding questions
regarding 1,4-dioxane's carcinogenicity and toxicity and on whether there is a sufi cient threshold
of contaminated drinking water supplies to justify regulation. In May 2009, USEPA published a
draft toxicological review of 1,4-dioxane. † The report retains the original IRIS carcinogenicity clas-
sii cation: 1,4-dioxane is classii ed as likely to be carcinogenic to humans, based on evidence of liver
carcinogenicity. The draft report determined an oral CSF for 1,4-dioxane of 0.19 (mg/kg-day) -1 . ‡
Because the mode of action for liver carcinogenicity of 1,4-dioxane is not known, the CSF was
derived by linear low-dose extrapolation (USEPA, 2009).
Ironically, most drinking water utilities do not test for 1,4-dioxane because it is not a regulated
contaminant nor there is sufi cient regulatory incentive, such as a PHG or a statutory requirement to
report PHG exceedances. Consequently, drinking water utilities face the dilemma of deciding
whether to budget for discretionary testing beyond the list of analytes required for permit compli-
ance and include emerging contaminants such as 1,4-dioxane, or risk being placed in a reactive
position should 1,4-dioxane turn up in the water supply (see the examples of both proactive and
reactive discovery of 1,4-dioxane discovered in drinking water supplies listed in Table 6.3). The
decision regarding whether to test for 1,4-dioxane hinges on whether the water utility in question
embraces the precautionary principle, described further in Section 10.1.1 .
10.2.1 1,4-D IOXANE D ETECTIONS AND D RINKING W ATER T ESTING P ROGRAM P OLICY
Monitoring water supply wells and surface water is the only available defense against unknowingly
consuming or serving drinking water that contains 1,4-dioxane, as the taste and odor threshold (23
milligrams per liter [mg/L]) is substantially higher than the concentrations that are likely to appear
in sources of drinking water. Most water utilities do not routinely test for 1,4-dioxane. In California,
65 public water supply wells out of 779 sampled since 1997 had detections of 1,4-dioxane above
California's 3 μg/L notii cation level (Cal EPA, 2008). Because testing was motivated where there
was a likelihood of 1,4-dioxane detection, this high rate of detection (8.3%) is unlikely to apply to
the approximately 14,000 water supply wells operating in California, yet it is likely that there are
more yet undetected occurrences of 1,4-dioxane in California's drinking water wells located in
* As of April, 2009.
† USEPA, 2009, Draft Toxicological Review of 1,4-Dioxane US Environmental Protection Agency, Washington, D.C.
EPA/635/R-09/005, www.epa.gov/iris
‡ As noted in Chapter 5, the previous oral CSF value published on USEPA's IRIS database for 1,4-dioxane was 0.011 (mg/
kg-day)-1. The higher CSF published in the draft 2009 report suggests that lower drinking water thresholds for 1,4-diox-
ane may be appropriate if the new CSF is adopted.