Water treatment for known contaminants is constantly evolving as more effective tools, testing, and technology make new methods of treatment available. But treating contaminants that are unknown or little known is a different story. Chemicals that were not previously detected in water or were detected at much lower levels are called emerging contaminants. Examples of emerging contaminants include personal care products, pharmaceuticals, and endocrine-disrupting compounds.

Just as water treatment evolves, the regulations governing clean water also evolve. The U.S. Environmental Protection Agency (EPA) has established a process to identify and regulate emerging contaminants in community and public water systems.

A History Lesson

The Safe Drinking Water Act (SDWA) was signed into law in 1974 by President Gerald Ford and went into effect in 1977. The initial SDWA focused on meeting health standards through more sampling and testing of community and public water systems. The regulations set standards for microbiological contaminants, 10 organic chemicals, six organic pesticides, turbidity, and radiological contamination.

Currently, the EPA regulates over 90 contaminants, including microorganisms (plus turbidity), disinfection byproducts, disinfectants, inorganic and organic contaminants, and radionuclides.

The EPA monitors all large public water systems (PWSs) serving more than 10,000 people, all small PWSs serving between 3,300 and 10,000 people, and a representative sample of small PWSs serving fewer than 3,300 people.

The Road Map

The process of identifying emerging contaminants, regulating them, and then notifying the public has distinct steps:

  • Create the contaminant candidate list.
  • Collect and analyze data under the Unregulated Contaminant Monitoring Rule.
  • Establish the maximum contaminant level goal and the maximum contaminant level.
  • Enforce timely compliance from PWSs.
  • Keep consumers notified of noncompliance through the Public Notification Rule, including consumer confidence reports.

Contaminant Candidate List (CCL)

The drinking water CCL is a list of contaminants that are currently not regulated but are known or anticipated to occur in water systems. The CCL is published every five years. The SDWA instructs the EPA to add to the list contaminants that present the largest public health concern. Starting with CCL 3 in 2008, the EPA has accepted public input regarding contaminants to place on a CCL.

Previous CCLs included pesticides, chemicals used in commerce, waterborne pathogens, pharmaceuticals, and biological toxins.

CCL 1 (1998)CCL 2 (2005)CCL 3 (2009)CCL 4 (2016)CCL 5 (2022)
50 chemicals or chemical groups42 chemicals or chemical groups104 chemicals or chemical groups97 chemicals or chemical groups66 chemicals or chemical groups
10 microbiological contaminants9 microbiological contaminants12 microbiological contaminants12 microbiological contaminants12 microbiological contaminants

Right now, the EPA is in the process of drafting CCL 6.  From February to April 2023, the EPA accepted nominations of chemicals, microbes, or other substances to the CCL 6. Now the nominations and other data are being analyzed for inclusion in the CCL 6 draft.

Once the final CCL is compiled, the EPA must decide whether or not to regulate at least five contaminants from the list in a separate process called regulatory determination. Put simply, the regulatory determination is a decision regarding whether the EPA should regulate a specific contaminant. This decision is guided by information about the contaminant and whether it meets three criteria listed in the SDWA:

  • The contaminant has an adverse impact on health.
  • The contaminant occurs or could occur in levels that cause a health concern.
  • The regulation of the contaminant would reduce the health concern for people served by the water system.

Unregulated Contaminant Monitoring Rule (UCMR)

The UCMR program was established in amendments to the SDWA in 1996 and 2018. The program monitors priority unregulated contaminants every five years, monitors PWSs, and stores results in the National Contaminant Occurrence Database. Occurrence data are collected through the UCMR to support the determination to regulate particular contaminants in the interest of protecting public health.

The EPA uses a multistep prioritization process to evaluate candidate UCMR contaminants, starting with identifying those contaminants not monitored under prior UCMR cycles; those that might occur in drinking water; and those expected to have a completed, validated drinking-water method in time for the rule proposal.

Next, it considers the availability of health assessments or other health-effects information (e.g., critical health endpoints suggesting carcinogenicity), public interest (e.g., PFAS), active use (e.g., pesticides that are registered for use), and availability of occurrence data.

“UCMR data represent one of the primary sources of national occurrence data in drinking water that EPA uses to inform regulatory and other risk management decisions for drinking water contaminants,” the EPA wrote on its website’s “Learn About the Unregulated Contaminant Monitoring Rule” page. “This data will ensure science-based decision-making and help prioritize the protection of disadvantaged communities.”

Right now, the EPA is monitoring data for 30 emerging contaminants in UCMR 5 and expects completion of data reporting in 2026.

Maximum Contaminant Level (MCL)

Once the data is collected under the UCMR, the EPA makes a preliminary determination that is published and open for public comments. If the EPA makes a final determination to regulate a contaminant, the process of determining an MCL begins.

The first step is for the EPA to establish a maximum contaminant level goal (MCLG), which is the maximum level of a contaminant in drinking water at which no known or anticipated health effects would occur, allowing for a margin of safety. MCLGs only consider public health and are not enforceable. Sometimes MCLGs are set at levels that water systems cannot meet due to technological limitations.

Once the MCLG is set, the EPA works on determining an enforceable standard, which is the MCL. The MCL is set as close to the MCLG as possible.

In cases when there is no reliable method that is economically and technically feasible to measure a contaminant at concentrations to indicate there is not a public health concern, the EPA instead sets a treatment technique. According to the EPA, a treatment technique is an enforceable procedure or level of technological performance that public water systems must follow to ensure control of a contaminant.

As a part of the analysis, the SDWA also requires the EPA to prepare a health-risk reduction and cost analysis in support of National Primary Drinking Water Regulations.

The EPA must analyze the quantifiable and nonquantifiable benefits that are likely to occur as the result of compliance with the proposed standard. It must also analyze certain increased costs that will result from the proposed drinking-water standard, such as an increased health risk to the general population that may occur because of the new MCL.

Complying with New Standards

Standards go into effect three years after they are finalized, and PWSs must come into compliance. If capital improvements are needed, an extra two years may be allowed. States may also grant an exception for PWSs to seek other compliance options or financial assistance.

Even with the water testing and monitoring of 90 contaminants, water quality might change. To keep consumers educated about problems with their drinking water, the Public Notification Rule was revised in 2000. Customers must be notified if the water does not meet drinking water standards, if the water system fails to test its water, or if the system has been granted a variance or an exemption. Strict requirements are set by the EPA on the form, manner, content, and frequency of notices.
There are three tiers of notifications, including immediate notice, notice as soon as possible, and annual notice.

Annual notices give water suppliers the chance to consolidate notices and include them with annual water-quality reports, also known as consumer confidence reports (CCRs). These reports must be sent by water suppliers to consumers by July 1. Federal and state regulations guide what information must be in a CCR, such as source, any regulated contaminants found in the water, water system actions to restore safe water, and more.

Participating in the Process

Starting with CCL 3, the rule-making process has included the opportunity for public comment. That dictate came from The National Drinking Water Advisory Council and National Academy of Sciences, which felt the public could provide information that would help identify unregulated contaminants that may require a national drinking-water regulation and should be considered for the CCL.

To participate in the CCL 6 process, the EPA’s preferred method is to submit contaminant nomination comments, identified by docket number EPA-HQ-OW-2022-0946, at https://www.regulations.gov.

Additionally, the public may contact Thomas Lombardi, Office of Ground Water and Drinking Water, Standards and Risk Management Division, Environmental Protection Agency, at (202) 564-7653 or [email protected].

Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to its public docket. Do not submit to the EPA’s docket at https://www.regulations.gov any information you consider to be confidential business information, proprietary business information, or other information whose disclosure is restricted by statute.

Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include a discussion of all points one wishes to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (i.e., on the web, in the cloud, or on other file-sharing systems).

Comments may also be mailed to the U.S. Environmental Protection Agency, EPA Docket Center, Water Docket, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, D.C. 20460.

Water-treatment professionals must be aware of how drinking-water regulations are created to address emerging contaminants. New regulations and water systems that are not in compliance can open the door to a conversation with consumers about how water-treatment systems might be the right answer for their health concerns and needs.

About the author

Paul Bergeron is a freelance writer based in Herndon, Virginia.


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