Regulation and Risk Assessment of 1,4-Dioxane - Environmental Investigation and Remediation - page 264

and territories and uses a PRG of 6.1 μg/L. Region 3 covers the mid-Atlantic states and uses a
Risk-Based Concentration of 6.1 μg/L. Region 6—serving Arkansas, Louisiana, New Mexico,
Oklahoma, Texas, and 65 tribes—uses a Human Health Tap Water Screening Level of 6.1 μg/L.
The absence of a federal MCL for 1,4-dioxane has left the states to determine whether to promul-
gate regulatory standards or Advisory Action Levels. Only Colorado has adopted a standard;
Michigan held hearings on adopting a state MCL of 35 ppb for 1,4-dioxane, but did not adopt a
standard. It is unlikely there will be a federal MCL adopted anytime soon, as the criteria required
to justify adopting an MCL have not materialized, and it is unclear whether USEPA will i nd that
there has been enough exposure or that the health risks are sufi ciently understood. More familiar
contaminants, such as the widely used fuel oxygenate MTBE and the ubiquitous solid rocket fuel
oxidizer, perchlorate, still lack federal MCLs. Nevertheless, the author has witnessed the discovery
of 1,4-dioxane in drinking water in several communities following his dialog with state and federal
regulators working on solvent cleanups in several states. It is likely that there is additional ongoing
but undiscovered exposure to 1,4-dioxane in drinking water.
MCLs are usually preceded by Maximum Contaminant Level Goals (MCLGs), which are simi-
lar to Public Health Goals or Advisory Action Levels. MCLGs are set at the level that causes no
known adverse effects to people and incorporate an adequate margin of safety; MCLs are sub-
sequently set as close as feasible to MCLGs. The MCL may be higher than the MCLG if available
analytical methods do not quantify the contaminant in the range at which the MCLG is set. The
availability of treatment technologies and the cost of treatment may also factor in the USEPA
administrator's selection of the MCL.
The MCLG for an initiator carcinogen (i.e., a contaminant with no threshold for adverse effects)
is set to zero by default. The MCL for an initiator carcinogen is generally set between the 1 in
10,000 and the 1 in 1,000,000 increased lifetime risk. For a promoter carcinogen or for a noncar-
cinogen (a contaminant showing a threshold for adverse effects), the MCLG is based on the RfD, set
to a level below known adverse health effects (Macler, 2006).
A number of events would have to transpire before 1,4-dioxane could be regulated with a federal
MCL. First, 1,4-dioxane would have to be identii ed as a possible health concern in drinking water,
leading to its inclusion on the USEPA drinking water CCL. The CCL is compiled by a committee
of experts that reviews potential contaminants based on toxicity, potency, or severity, on prevalence
and magnitude of occurrence, and on persistence and mobility (NDWAC, 2006). USEPA weighed
inclusion of 1,4-dioxane on the CCL in 1997 but decided against adding it. In 2006, USEPA solic-
ited candidates for the third drinking water CCL, and in February 2008, 1,4-dioxane was included
on the list (USEPA, 2006b, 2008). Once listed on the CCL, health, occurrence, and exposure infor-
mation will be assembled to conduct a preliminary risk assessment. To assist with establishing the
presence of a contaminant, some contaminants under review for the CCL are included in the
UCMRs. If regulators conclude that an opportunity exists to reduce public health risks by regula-
tion, the requisite regulatory elements would have to be developed. These include health risk assess-
ments to determine potential standards and quantify the benei ts derived from regulating 1,4-dioxane.
Further, viable analytical methods would have to be available for analyzing 1,4-dioxane in drinking
water at the levels of interest for reducing risk. USEPA developed Method 522 for determination of
1,4-dioxane in drinking water in 2008. * Review of 1,4-dioxane treatment methods required to ana-
lyze costs for compliance and enforcement is also needed to adopt an MCL. The draft MCL and
other regulatory elements are then reviewed by USEPA and the President's Ofi ce of Management
and Budget (OMB). Thereafter, public comments are solicited and addressed, the i nal National
Primary Drinking Water Regulations (NPDWR) document is reviewed by USEPA and OMB again,
and then the regulation is promulgated. Implementation and enforcement would follow a schedule
outlined in the NPDWR (Macler, 2006).
* Method 522 (2008) uses solid-phase extraction and gas chromatography-mass spectrometry with selected-ion
monitoring.