The MCL Review Process
Background
Health and Safety Code §116365(g) requires the Water Board, at least once every five years, to review its MCLs. In the review, the State Water Board's MCLs are to be consistent with criteria of §116365(a) and (b). Those criteria state that the MCLs cannot be less stringent than federal MCLs, and must be as close as is technically and economically feasible to the public health goals (PHGs) established by the Office of Environmental Health Hazard Assessment (OEHHA). Consistent with those criteria, the State Water Board is to amend any standard if any of the following occur: (1) Changes in technology or treatment techniques that permit a materially greater protection of public health or attainment of the PHG, or (2) New scientific evidence indicates that the substance may present a materially different risk to public health than was previously determined. Each year by March 1, the State Water Board is to identify each MCL it intends to review that year.
View the list of all regulated chemicals' MCLs and PHGs (PDF).
The Process of Review
The first step in the review process is an initial screening. The criteria for this screening include: (1) The relationship between the PHG and both federal and state MCLs (PDF); (2) any changes in technology or treatment techniques that permit a materially greater protection of public health or attainment of the public health goal; and (3) any new scientific evidence indicating that the substance might present a materially different risk to public health than was previously determined.
In addition, occurrence is assessed for each regulated contaminant in drinking water sources. The four most recent years of analytical data from the State Water Board’s Division of Drinking Water’s (DDW’s) Water Quality Monitoring (WQM) database is analyzed.
Laboratory findings are based on a standardized quantification level called the "detection level for purposes of reporting" (DLR). The DLR represents the level at which we are confident about the accuracy of the quantity of contaminant being reported by laboratories. Although any findings below DLRs are considered "non-detects" and are not technically required to be reported, some laboratories may on occasion report lower levels for chemicals. For some chemicals, the DLR affects the technical feasibility of revising the MCL, in that the limits on a chemical’s detectability by analytical laboratories also serve to limit the extent to which the MCL might be lowered.
Incorporating Racial Equity
One of DDW’s racial equity goals is to “incorporate racial equity analysis when developing MCLs using available data and as data and methods allow.”
An analysis was undertaken in 2024 to provide a framework for incorporating racial equity considerations into California’s drinking water regulations. The report, Incorporating Racial and Ethnic Considerations in the Prioritization of California’s Maximum Contaminant Levels (MCLs) for Drinking Water, found the following:
- Need for More Detailed Monitoring: Gaps in water quality data make it difficult to determine actual exposure risks across different communities.
- “The lack of sufficient monitoring results in data limitations that restrict occurrence analyses to averages of population numbers and contaminant concentrations across the entire [public water systems].”
- “Averaging of demographic characteristics is less critical in cases where all customers within the [public water system] receive identical water quality. However, some large [systems] may use different local water sources or operate multiple systems, which can lead to variations among the served population. Therefore, a more accurate assessment of these systems would require knowledge of both water quality differences (through more granular monitoring) and demographic information at a finer [system] subdivision level.”
- Racial Data Should Inform, Not Dictate, MCLs: While racial and ethnic data can help identify disparities, MCLs should continue to be set based on public health risks and feasibility rather than race-specific data.
- “Instead of integrating racial equity analysis into the regulated MCL development process, it would be more practical to incorporate racial equity considerations into the prioritization process for selecting chemicals.”
MCL Reviews from Previous Years
Historical reviews are available below:
- 2018 MCL Review
- 2017 MCL Review
- All regulated contaminants were screened. Perchlorate was considered a candidate for a potential MCL revision. No other regulated contaminant was considered for revision.
- 2016 MCL Review
- 2015 MCL Review
More Information
The following links contain related information that may be useful:
- Comparison of MCLs and PHGs
- Chemicals and Contaminants in Drinking Water
- Drinking Water-Related Statutes and Regulations
- OEHHA's PHGs and Technical Support Documents
- Information for Drinking Water Systems
- Division of Drinking Water
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Contact Us
State Water Resources Control Board
Division of Drinking Water
Regulatory Development Unit
Mailing Address:
P.O. Box 100
Sacramento, CA 95812-0100
Street Address:
1001 I Street
Sacramento, CA 95814
E-mail us at DDWRegUnit@waterboards.ca.gov.
