Environmental Engineering Reference
In-Depth Information
laboratory animals. The RfD value is used to demarcate safe levels of exposure. The RfD is an
estimate of a daily human oral exposure that is not likely to cause an appreciable risk of deleterious
noncancer health effects during a lifetime. Uncertainty associated with the RfD may span an order
of magnitude. This section reviews the toxicological basis for promulgating legal standards or
Advisory Action Levels by the states leading the nation in regulatory response to 1,4-dioxane con-
tamination of groundwater. Not all states use a toxicological basis for setting 1,4-dioxane guidance;
Massachusetts set its guidance value at 50 ppb because of the practical quantitation limit for
1,4-dioxane analysis.
6.1.2.1 Colorado's 1,4-Dioxane Standard
The Colorado Department of Public Health and Environment (CDPHE) i nalized the i rst enforce-
able American water quality standard for 1,4-dioxane in groundwater and surface water in September
2004 with enforcement beginning in March 2005. In March 2005, the CDPHE Water Quality
Control Commission adopted Regulation 41,
Basic Standards for Groundwater
, in which the stan-
dard for 1,4-dioxane is set at 6.1 μg/L for 5 years. The commission proceeded with adopting this
5-year standard for 1,4-dioxane in order to address ongoing water quality issues while USEPA
reviews risk levels for 1,4-dioxane in the IRIS database. The 6.1 μg/L level was chosen as an interim
standard because it was the value typically used as a cleanup level in Colorado, based on USEPA's
PRG, and provides a basic level of human health protection.
The CDPHE Water Quality Control Commission's methodology for setting health-based stan-
dards yields a 3.2 μg/L standard for 1,4-dioxane. Unless USEPA provides further clarii cation on
risk levels for 1,4-dioxane, CDPHE will change the 1,4-dioxane standard for groundwater and
surface water from 6.1 to 3.2 μg/L on March 21, 2010. The CDPHE Statement of Basis and Purpose
for Regulation 41 lists criteria considered in deriving the 3.2 μg/L standard as follows:
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1,4-Dioxane is classii ed as a group B2 chemical, that is, a probable human carcinogen.
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1,4-Dioxane is a groundwater contaminant in Colorado, and treated groundwater contami-
nated with 1,4-dioxane is discharged to Colorado surface waters.
1,4-Dioxane is readily treated with advanced oxidation processes (AOP) in combination
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with ultraviolet (UV) light.
Because the adopted standard for 1,4-dioxane is aligned with existing cleanup standards,
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it will not have a major impact on treatment costs during the initial 5-year period.
1,4-Dioxane is primarily used as a solvent stabilizer, and it will most likely be found in
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areas with known chlorinated solvent contamination.
1,4-Dioxane is characterized by a high solubility, moderate vapor pressure, and low Henry's
•
law constant; therefore, it will be persistent within the aquatic environment. Available data
indicate that 1,4-dioxane will not readily degrade in the environment.
1,4-Dioxane has been found at nine Colorado sites and is suspected at 19 others.
•
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1,4-Dioxane contamination does not arise from natural sources.
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1,4-Dioxane standards are adopted to protect domestic water supply uses.
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The commission stated that although conl icting scientii c interpretations warrant further
review of 1,4-dioxane toxicity, there is no sufi cient evidence as yet to invalidate the cur-
rent USEPA IRIS value. The commission decided to err in the direction of protection of
public health by approving the 6.1 and the 3.2 μg/L standards for 1,4-dioxane, starting in
2005 and 2010, respectively (CDPHE, 2005).
In hearings held on CDPHE's 1,4-dioxane regulation, industry groups argued that Colorado
should hold off on setting a 1,4-dioxane standard until USEPA completes its review of risks for its
IRIS database. Industry representatives were concerned that USEPA may update the acceptable
exposure levels for 1,4-dioxane within 2 years (Inside EPA, 2004). Some parties argued that a
nonlinear toxicity model should be used to characterize 1,4-dioxane toxicity, rather than the linear
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