5 Colo. Code Regs. § 1002-31.61
The provisions of C.R.S. 25-8-202(1)(a), (b) and (2); 25-8-203; 25-8-204; and 25-8-402; provide the specific statutory authority for adoption of these regulatory amendments. The commission also adopted, in compliance with 24-4-103(4) C.R.S., the following statement of basis and purpose.
BASIS AND PURPOSE
The commission adopted a revised chronic Water Supply standard for total recoverable molybdenum of 530 µg/L. The standard was calculated using the non-cancer equation and some of the default exposure assumptions from Policy 96-2. The molybdenum Water Supply standard uses the Agency for Toxic Substances and Disease Registry's (ATSDR) minimal risk level (MRL; ATSDR's alternative to a reference dose (RfD)) of 0.06 mg/kg/day (ATSDR 2020), a relative source contribution (RSC) of 0.8, a subchronic to chronic uncertainty factor of 3, and the U.S. Environmental Protection Agency (EPA) updated exposure factors, as discussed below. Climax Molybdenum Company (Climax) proposed a Water Supply standard of 1,600 µg/L in this hearing, using an RSC of 0.8 and no subchronic to chronic uncertainty factor; however, the commission found that a standard of 1,600 µg/L would not be adequately protective of human health, given the available data and information.
In adopting the revised molybdenum standard, the commission relied on its past policy decisions and precedence documented in Commission Policy 96-2, along with the EPA Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health (2000).
As per Departmental policy, the commission relied on toxicity information from ATSDR's 2020 Toxicological Profile for Molybdenum as its source of toxicological data to derive an updated Water Supply standard for molybdenum.
The commission declined to use EPA's 1992 Integrated Risk Information System (IRIS) assessment for molybdenum, because it is based on an outdated, discredited study by Koval'skiy et al. (1961). The commission previously departed from the EPA IRIS assessment for molybdenum when it first adopted the Water Supply standard of 210 µg/L for molybdenum in 2010 based on Fungwe et al. (1990). See Section31.48(I)(H). The commission at that time acknowledged there were ongoing studies on molybdenum, and urged that the standard be reviewed and revised in the future.
Since the 2010 rulemaking, significant advances have been made in the development of molybdenum toxicological information, including three peer-reviewed and published studies in 2014 and 2019 that were conducted according to the Organization for Economic Cooperation and Development (OECD) guidelines and Good Laboratory Practice. ATSDR considered these studies in its 2020 Toxicological Profile for Molybdenum and in calculating its intermediate oral MRL of 0.06 mg/kg/day. ATSDR calculated this MRL using the no observed adverse effect level (NOAEL) of 17 mg/kg/day (based on kidney effects in rats), and by applying an uncertainty factor of 100 (10 for interspecies, 10 for intraspecies). ATSDR also applied a modifying factor of 3 to address concerns that reproductive/developmental effects may be a more sensitive endpoint than kidney effects in populations with marginal copper intakes. This resulted in a total uncertainty factor of 300.
In its testimony, Climax suggested that more recent peer-reviewed and published studies suggest a lack of support for ATSDR's application of the modifying factor of 3 and call into question the results of Fungwe et al. (1990). However, at this time, the commission did not make any modifications to ATSDR's intermediate oral MRL of 0.06 mg/kg/day. The commission may consider reviewing the molybdenum standard in the future, if EPA or ATSDR publish new assessments or toxicological profiles.
Thus, the commission used the ATSDR MRL of 0.06 mg/kg/day as the RfD-like value in calculating the revised Water Supply standard for molybdenum.
Because sufficient data to calculate a chronic MRL were not available, the ATSDR Toxicological Profile for Molybdenum developed an intermediate (i.e., subchronic) duration MRL of 0.06 mg/kg/day. However, Colorado's Water Supply standards are intended to provide adequate protection for the general population from a substance over a lifetime of exposure.
Climax proposed a chronic Water Supply standard of 1,600 µg/L in this hearing, using no subchronic to chronic uncertainty factor. Climax's position that an additional uncertainty factor to account for chronic exposure was unnecessary was primarily based on two arguments. First, Climax cited a 1997 National Toxicity Program (NTP) inhalation study in which rats were exposed to molybdenum over a two-year (chronic) period. While the rats were exposed to molybdenum in air, not water, Climax argued that the results of the NTP study could be compared to the results of the ingestion study selected by ATSDR as the critical study to derive its intermediate oral MRL and that this comparison provides sufficient evidence to understand chronic toxicity, thereby making an additional uncertainty factor accounting for chronic exposures unnecessary. However, to conclude the chronic inhalation study provides sufficient evidence to fully characterize chronic ingestion exposure would be contrary to ATSDR's conclusions. ATSDR evaluated this chronic inhalation study as part of its Toxicological Profile for Molybdenum and only used the study to assess inhalation toxicity, concluding that there was insufficient evidence to derive a chronic oral MRL. Second, Climax stated that because molybdenum is an essential element, homeostatic regulation prevents overexposure and it is therefore unnecessary to apply an additional uncertainty factor to account for chronic impacts. However, other essential elements can cause toxic impacts on humans, such as iron, selenium, and copper (e.g., see Denver Water Responsive Prehearing Statement Exhibits 2 and 3). In addition, essential elements have both a recommended daily intake to maintain health, as well as a tolerable upper limit intended to prevent adverse health effects, and this range can be narrow (e.g., National Institutes of Health; see division Responsive Prehearing Statement). Therefore, the commission concluded that homeostatic regulation of essential elements is not always a reliable process to prevent toxic effects in humans.
Therefore, because limited evidence is available to understand potential health impacts from chronic molybdenum exposure, and ATSDR found insufficient evidence to derive a chronic MRL, the commission determined that an additional uncertainty factor of 3 to account for extrapolation from a subchronic study to chronic exposure conditions was appropriate. This factor addresses the increased risk associated with lifetime of exposure to elevated molybdenum in drinking water. To calculate the revised chronic Water Supply standard, ATSDR's subchronic MRL was used as the non-carcinogenic reference dose (RfD) in Commission Policy 96-2 equation 1-1 and the additional uncertainty factor of 3 was included in the denominator of equation 1-1.
The RSC is the percentage of the total daily exposure to molybdenum contributed by drinking water. Climax presented information to support departure from the default RSC of 0.2, as provided for in Commission Policy 96-2. This information included a detailed analysis using the Exposure Decision Tree from EPA's Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health (2000), demonstrating that intake from the diet and other potential exposure pathways (air, soil, etc.) is a small percentage of the calculated MRL for molybdenum for the general public.
To reach an RSC of 0.8, the critical step in the EPA decision tree requires answering "Yes" to Step 3: "Are adequate data available to describe central tendencies and high-ends for relevant exposure sources/pathways?". Climax provided data and calculations on molybdenum concentrations in food to demonstrate that it is unlikely that a typical adult diet would contain more than 20% (320 µg/day) of the amount of theoretical molybdenum intake based on ATSDR's intermediate MRL and an additional uncertainty factor of 3 to account for chronic exposure (0.06 mg/kg/day * 80 kg = 4.8 mg/day or 4,800 µg/day ÷ 3 = 1,600 µg/day). Therefore, an RSC of 80% (0.8) is acceptable for drinking water. Inhalation, dermal contact, and soil ingestion were determined to not be significant exposure pathways for the general population.
Based on this information, the commission applied an RSC of 0.8 in calculating the revised Water Supply standard for molybdenum.
In 2015, EPA updated its exposure factors for adults. The body weight factor was increased from 70 kilograms to 80 kilograms, and the drinking water ingestion rate was increased from 2 liters per day to 2.4 liters per day. The commission applied the EPA updated exposure factors, as they rely on more recent exposure data than those used to derive the exposure factors in Commission Policy 96-2. This decision is also consistent with recent commission actions on other human health standards (e.g., 31.58(I)).
Therefore, the commission applied the average body weight of 80 kg and daily drinking water consumption rate of 2.4 liters per day in calculating the revised Water Supply standard for molybdenum.
The commission also heard evidence that there are sensitive subpopulations that may experience adverse effects from molybdenum exposure at lower levels than the general population, including individuals with kidney impairment, formula-fed infants, and others. For example, data from one study (Hosokawa and Yoshida, 1994; e.g., see Denver Water Responsive Prehearing Statement Exhibit 2) indicate that patients on kidney dialysis have substantially more molybdenum in their blood than healthy adults, likely because kidneys control molybdenum removal from the blood. Reduced kidney function is a common occurrence and the result of many factors. Also, infants fed formula made with water containing a molybdenum concentration of 530 µg/L would be ingesting about 60 to 250 times more molybdenum than is typically contained in breast milk (breast milk values from Bougle et al., 1988; e.g., see Denver Water Rebuttal Exhibit 17; infant exposure values from EPA Responsive Prehearing Statement). The lack of scientific data on how individuals with reduced kidney function and infants will respond to a large increase in molybdenum in their water supply and the lack of studies on more subtle outcomes of molybdenum toxicity, such as neurological effects, are compelling reasons to be conservative in setting a new statewide molybdenum standard.
Applying the MRL from ATSDR (2020) of 0.06 mg/kg/day, subchronic to chronic uncertainty factor of 3, RSC of 0.8, and updated exposure factors of 80 kg body weight and 2.4 L/day drinking water consumption rate, the commission calculated a revised Water Supply molybdenum standard of 530 µg/L using the following equation:
This revised standard was adopted in Section 31.16, Table III.
In adopting the revised Water Supply molybdenum standard, the commission has considered the factors enumerated in Section 25-8-204(4), C.R.S. The commission has considered evidence regarding: the need for standards to regulate molybdenum, the existing low levels of molybdenum in most of the state, and the fact that molybdenum is naturally occurring in certain areas of Colorado. The commission has also considered the technical evidence regarding treatment techniques to achieve the revised standard of 530 µg/L. Pursuant to Section 25-8-102(5), C.R.S., the commission also found that the revised standard is economically reasonable and consistent with a water quality program in which the water quality benefits of pollution control measures utilized have a reasonable relationship to the economic, environmental, energy, and public health costs and impacts of those measures. Based on the evidence presented, the commission believes that the revised standard will support the beneficial uses of State waters, including drinking water, and that the standard adopted is appropriate and scientifically supported by the record.
The commission added a new reference at Section 31.16(3)(P) for the ATSDR Toxicological Profile for Molybdenum. A notation was added to the 530 µg/L standard to refer to this profile.
The commission also adopted the revised 530 µg/L standard on one water quality segment in Regulation No. 33 (Blue River Segment 14). No other segments received this updated value at this time. Before adopting this standard on other segments, it will be necessary to conduct outreach to stakeholders that may be impacted by the change.
5 CCR 1002-31.61