Regulation and Risk Assessment of 1,4-Dioxane - Environmental Investigation and Remediation - page 274

TABLE 6.2
1,4-Dioxane Risk Calculation Parameters for State Drinking Water Thresholds

Parameter California Colorado Connecticut

Cancer slope factor (mg/kg d) -1 0.014 0.011 0.011

Uncertainty factor See text 3000 a 3000 b

NOAEL (mg/(kg d)) See text 9.6 10

Reference dose (μg/(kg d)) See text 9.6 3.3

Body weight (kg) 70 70 70

Ingested volume (L/d) 2 2 2

Relative source contribution See text 0.2 0.2

2.5 6.7 23.1

Drinking water concentration (μg/L)

3 6.1 20

Advisory level (μg/L)

Sources: Cal EPA, 1998, 1,4-Dioxane action level. California Environmental Protection Agency, Ofi ce of Environmental
Health Hazard Assessment; CDPHE, 2004, Rebuttal testimony of Raj Goyal, Ph.D. Re: Proposed revisions to the
basic standards and methodologies for surface water, Regulation #31 (5CCR 1002-31), the basic standards for
groundwater, Regulation #41 (5CCR 1002-41), and the site-specii c water quality classii cations and standards for
groundwater, Regulation #42 (5CCR 1002-42). Colorado Department of Public Health and Environment; and
Chute, S.K. and Ginsberg, G., 2004, Comparison value determination for 1,4-dioxane in drinking water. Connecticut
Department of Public Health, http://www.ct.gov/dph/ (accessed November 11, 2006).

a 10

× for LOAEL to NOAEL; 10 × for animal to human; 10 × for human variability; 3 × for database dei ciency.

b 1000

× for animal to human; sensitive individuals; promoter; 3 × for database dei ciencies.

criteria to 77 ppb for groundwater and 1500 ppb for soils. In June 2000, the Michigan Department of
Environmental Quality (MDEQ) again modii ed its risk-based cleanup criteria to 85 ppb for ground-
water and 1700 ppb for soils. The concentration in surface water considered safe by MDEQ for
human contact and the environment is 2800 ppb. However, if that surface water is used as a source
of drinking water, the concentration considered safe is 34 ppb, assuming drinking water consump-
tion of 2 L/d and i sh consumption of 15 g/d (MDEQ, 2005a). The 34 ppb surface-water safety level
was based on the incidence of liver tumors in female mice rather than on the increase in nasal tur-
binate tumors in rats used by USEPA, because MDEQ judged that the nasal tumors were caused by
topical exposure of 1,4-dioxane to the rats' nasal cavities during drinking water consumption (MDEQ,
2003). Advisory cleanup criteria are not legally binding standards for drinking water, leaving state
regulators some discretion to consider site-specii c conditions in their implementation.
In 2004, MDEQ proposed a 35 μg/L Michigan MCL for 1,4-dioxane by using a 1 in 100,000
(10 −5 ) cancer risk factor rather than a 1 in 1,000,000 (10 −6 ) cancer risk factor. Michigan has no other
MCLs independent of USEPA's MCLs. MDEQ staff was motivated to propose an MCL because of
the large-scale impacts of 1,4-dioxane to domestic and municipal drinking water supplies serving a
large population in Washtenaw County.
MDEQ solicited comments and held a public hearing on the proposed addition of a 1,4-dioxane
MCL to the Michigan SDWA. MDEQ received comments overwhelmingly opposed to the proposed
MCL. Stakeholders objected to the (10 −5 ) cancer risk factor as the basis for the MCL; they noted that
using a 10 −6 cancer risk factor would be more protective and result in an MCL closer to 3 μg/L.
Industry and responsible party representatives favored a higher MCL or none at all.
The Michigan Manufacturers Association (MMA) issued comments objecting to the MCL and
asserting that MDEQ had not followed a proper methodology for setting a drinking water standard.
MMA objected to (1) the lack of a comprehensive effort to prioritize compounds for regulation and
(2) the lack of consideration of cost to treat 1,4-dioxane, which can be two to three times the cost of
treating more volatile solvents. MMA also opined that MDEQ had not incorporated all of the avail-
able toxicological data in its determination and these omissions led to a proposed MCL that was