Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-31 - REGULATION NO. 31 - THE BASIC STANDARDS AND METHODOLOGIES FOR SURFACE WATER
Section 5 CCR 1002-31.58 - STATEMENT OF BASIS SPECIFIC STATUTORY AUTHORITY AND PURPOSE; APRIL 13, 2020 RULEMAKING; FINAL ACTION MAY 11, 2020; EFFECTIVE DATE JUNE 30, 2020

RDX is characterized in IRIS with the cancer descriptor "Suggestive evidence of carcinogenic potential" per EPA 2005 guidelines. This designation is comparable to the cancer group designation of "C - Possible human carcinogen" from the 1986 EPA guidelines. Per Policy 96-2: "for Group C compounds that have both carcinogenic (cancer slope) and toxic (reference dose) data the Commission decided, in accordance with their past practice, to base the standards for these compounds on the reference dose approach, but to adjust the resulting standard with an uncertainty factor of 10 to account for any unknown carcinogenic effects." However, this approach is not aligned with best science practices set forth in the CWA § 304(a) criteria development method for these types of chemicals, under which both cancer-based and non-cancer-based water quality standards would be calculated and the lower of the two standards selected for use protection. Therefore, the commission adopted the proposed calculation of the RDX Water Supply standard, which uses the lower, cancer-based water quality standard of 0.42 µg/L, based on the IRIS cancer slope factor of 0.008 per mg/kg-day. This approach follows the more protective, 304(a)-compliant approach of selecting the lower of the two calculated standards (cancer-based or non-cancer-based). Derivation of previous standards for "Group C carcinogens" has not been consistent; therefore, the division will, ongoing, follow the practices set forth in the CWA § 304(a) criteria development method for these types of chemicals. The division will also work towards bringing previous standards up-to-date, as resources to do so become available.

The Water Supply standard uses most of the default exposure assumptions from Policy 96-2, along with updated exposure factors of a mean adult (21 years and older) body weight of 80 kilograms and a drinking water ingestion rate of 2.4 liters per day, as discussed above. There are no EPA human health ambient water quality criteria (HHAWQC) available for RDX, which would help inform development of Water +Fish and Fish Ingestion standards for RDX. Furthermore, based on available physical and chemical data available for RDX, this chemical is not likely to bioaccumulate. Therefore, the commission did not adopt Water+Fish or Fish Ingestion standards for RDX at this time.

The commission adopted new Water Supply standards for 1,2,3-trimethylbenzene, CAS 526-73-8; 1,2,4-trimethylbenzene, CAS 95-63-6; and 1,3,5-trimethylbenzene, CAS 108-67-8, calculated using the non-cancer equations and most of the default exposure assumptions from Policy 96-2 in combination with the RfD of 0.01 mg/kg-day from IRIS. The Water Supply standards use updated exposure factors of a mean adult (21 years and older) body weight of 80 kilograms and a drinking water ingestion rate of 2.4 liters per day, as discussed above. The calculations resulted in Water Supply standards of 67 µg/L. The commission did not adopt Water+Fish or Fish Ingestion standards for these trimethylbenzenes because there are no EPA HHAWQC available for these chemicals. Furthermore, as documented in the 2016 IRIS assessment for these chemicals, the estimated bioconcentration factors (133-439) and high volatility of trimethylbenzenes suggest that bioaccumulation of these chemicals will not be significant.

The commission adopted revised Water Supply, Water + Fish, and Fish Ingestion standards for BaP based on updates to the EPA IRIS assessment. In addition to providing an updated cancer slope factor, the IRIS assessment identified BaP as a mutagen. Therefore, the standards adopted by the commission were calculated using age dependent factors, following EPA 2005 guidance on risk assessment for mutagenic compounds and Minnesota's Human Health-based Water Quality Standards Technical Support Document, in combination with the default Incremental Lifetime Cancer Risk of 1E-06 from Policy 96-2, the oral cancer slope factor of 1 per mg/kg-day from IRIS, and a bioaccumulation factor of 3900 L/kg from EPA's human health ambient water quality criteria. Age-bracketed upper 90th percentile, per capita, combined direct and indirect, water ingestion rates for community water sources from Table 3-13 of the 2019 revision to the Exposure Factors Handbook were used to derive the Water Supply and Water + Fish standards. Age-bracketed upper 90th percentiles for consumption of finfish and shellfish, fresh and estuarine (but not marine species), raw weight, and only the edible portion from Tables 9a (adults) and 20a (youth) of the EPA's "Estimated Fish Consumption Rates for the U.S. Population and Selected Subpopulations (NHANES 2003-2010) were used to derive the Water + Fish and Fish Ingestion standards. The mutagenicity calculations required for the PAH water quality standards require fish consumption rates to be expressed on a body weight basis. Therefore, the age-bracketed body weights from Table 8-1 of the 2011 EPA Exposure Factors Handbook were used in combination with the fish consumption rate data.

Previously, water quality standards of several related polycyclic aromatic hydrocarbons (PAHs) [benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, dibenzo(a,h)anthracene, and indeno(1,2,3-cd)pyrene] were set equal to those for BaP; therefore, the Water Supply, Water + Fish, and Fish Ingestion standards for these PAHs were also revised. Table 1 summarizes the revised standards for BaP and the other, related PAHs adopted by the commission. The commission adopted revised standards for these PAHs calculated by applying the estimated order of potential potency (EOPP) factor, for each chemical relative to BaP, presented in EPA's 1993 Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons. In this approach, the potencies of other PAHs relative to benzo(a)pyrene are determined. These EOPP factors were applied using the revised cancer slope factor for BaP from IRIS and using age dependent factors appropriate for use with mutagenic chemicals. Treatment of the related PAHs as mutagens, based on that determination for BaP, is consistent with the approach described in EPA's 1993 guidance. Footnote 13 was added to indicate that BaP and related PAH standards were calculated as mutagens. In 2010 EPA provided a draft of updated guidance, which applied new relative potency factors (RPFs). However, since the guidance was never finalized, the new RPFs are widely not used throughout EPA risk assessment framework, and are thus not used for the derivation of the revised water quality standards.

Previous to revision, the Water Supply standard for BaP adopted by the commission was a hybrid standard that ranged from the concentration protective of human-health to the drinking water maximum contaminant level (MCL). The hybrid standard approach was adopted in the 2004 rulemaking in response to ongoing debate dating back to 1989 about whether standards for parameters with MCLs should be based on the MCLs or purely health-based numbers. The arguments for MCLs focused on whether it is reasonable to require surface water remediation to a level below that required for drinking water. The arguments for health-based standards focused on maximizing human-health protection, putting the clean-up burden on pollution sources, and protection of surface water as a resource. In response, the commission adopted a hybrid standard approach that provided much of the benefits advocated for each of the above options. This hybrid approach had the intention to allow for existing contamination to be addressed at levels that are deemed acceptable according to the Safe Drinking Water Act, but allowed for the protection of surface water as a resource by implementing a more protective human-health health based standard for future contamination.

There are more appropriate alternative regulatory pathways, such as variances, through which dischargers can seek regulatory relief. Furthermore, recent litigation in Idaho has resulted from attempts to adopt water quality standards that are not fully protective of the beneficial uses. In May 2016, EPA entered into a consent decree with Northwest Environmental Advocates to reconsider EPA's 2010 approval of Idaho's human health criteria for arsenic, which were based on the MCL in drinking water. In September 2016, EPA disapproved Idaho's MCL-based criteria, citing that the criteria "are not protective of Idaho's designated uses, including primary and secondary contact recreation and domestic water supply". EPA also noted that there are significant differences between the allowable factors for developing MCLs and water quality criteria to protect designated uses under CWA section 303(c). EPA points out that MCLs are in some cases based on feasibility considerations, including the availability of technology to achieve the regulatory level and the cost of such treatment. In other cases, MCLs are based on concentrations that can be measured reliably rather than concentrations expected to be protective of human health. In contrast, water quality standards must be based on a sound scientific rationale and protect the designated use, rather than being based on available treatment technology, costs, or other feasibility considerations. In addition, water quality standards regulations at 40 CFR 131.11(a)(1) are explicit that states must adopt water quality criteria that protect designated uses.

For BaP, the Colorado Hazardous Materials and Waste Management Division (at the time of rulemaking) uses the risk-based water quality standard to derive the groundwater protection level for BaP. Furthermore, the MCL for BaP is 0.2 µg/L; the incremental lifetime cancer risk factor resulting from this concentration would be 1.21x10-5, which is more than an order of magnitude greater than the risk factor that has been considered to be the appropriate level risk by the commission in past determinations (1x10-6). Therefore, the commission adopted a risk-based Water Supply standard for BaP of 0.016 µg/L that is protective of human-health.