What is the EPA Process to Regulate a New Drinking Water Contaminant?

Federally enforceable water standards trace back to the Safe Drinking Water Act (SDWA), which was originally passed in 1974. The SDWA gave the EPA the authority to set science-based standards (called National Primary Drinking Water Regulations (NPDWRs) that apply to public water systems. 

An NPDWR can take two enforceable forms. For many contaminants, the EPA sets a Maximum Contaminant Level (MCL), which is a numeric concentration limit in drinking water. When SDWA constraints make a numeric limit impractical as the enforceable control point, the EPA can instead require a Treatment Technique (TT).  A TT is an enforceable set of treatment and operational requirements that utilities must implement to reduce health risk, even if compliance is not defined solely by a single concentration number. 

Two high-profile examples illustrate why this distinction matters in practice. The EPA regulates lead primarily through a treatment technique for corrosion control under the Lead and Copper Rule, rather than as a classic entry-point MCL. EPA’s Surface Water Treatment Rules similarly rely on treatment technique requirements, including filtration and disinfection, to control microbial risk. 

Since the SDWA was originally passed, two major Congressional amendments (1986 and 1996) fundamentally reshaped the regulatory process. 

1986 Amendments to the SDWA 

Congress amended the SDWA in 1986 and required the EPA to regulate 83 specific contaminants by 1989 and 25 additional contaminants every three years after that. The amendments also mandated disinfection and treatment requirements, and restricted lead in new plumbing materials. 

While this amendment increased the speed of regulation, which was the intent, it also prioritized volume over scientific rigor.  

1996 Amendments to the SDWA 

The 1996 amendments overhauled the entire water quality regulatory process. Congress replaced the contaminant quotas with a risk-based, prioritized system built around several new requirements that continue to serve as the structure today: 

• Contaminant Candidate Lists (CCLs): The EPA must publish a list of unregulated contaminants that may need regulation, updated every five years. 
  
• Regulatory Determinations: The EPA must evaluate at least five CCL contaminants every five years and decide whether to regulate them. 
  
• Unregulated Contaminant Monitoring Rule (UCMR): The EPA can require water systems to monitor unregulated contaminants to build a national occurrence database. 
  
• Health Risk Reduction and Cost Analysis (HRRCA): The EPA must publish a Health Risk Reduction and Cost Analysis (HRRCA) and make a formal determination about whether the benefits of the proposed level justify the costs. Where the benefits of a feasible MCL do not justify the costs, the SDWA allows the EPA (in limited circumstances and scope) to set an alternative level that maximizes health risk reduction benefits at a cost justified by the benefits. 
  
• Consumer Confidence Reports (CCRs): Community water systems must send annual water quality reports to their customers. 
  
• Six-Year Review: The SDWA requires the EPA to review each NPDWR at least once every six years and revise it, as appropriate. Any revision must maintain or strengthen public health protection. 

This framework was more defensible than the 1986 approach. However, it came with a tradeoff: Newly proposed contaminants had to clear scientific, legal, and economic hurdles before municipalities were required to comply with the new standard, making the process comparably longer. 

Upstream of SDWA: How Chemicals Enter the Market 

Two federal statutes strongly influence which manmade chemicals and pesticides can reach drinking water sources: TSCA (industrial chemicals) and FIFRA (pesticides). These programs operate upstream of SDWA and do not automatically trigger drinking water standards, but they shape the contamination landscape SDWA must address. 

Toxic Substances Control Act (TSCA) 

The Toxic Substances Control Act (TSCA), originally enacted in 1976 and substantially updated by the Lautenberg Chemical Safety for the 21st Century Act in 2016, governs industrial chemicals in commerce. In theory, the TSCA should prevent harmful chemicals from being manufactured or used in ways that contaminate drinking water sources. 

In practice, the TSCA has operated with limited pre-market testing requirements. The 2016 reforms strengthened the law by requiring the EPA to evaluate existing chemicals against a risk-based safety standard and to review new chemicals before they enter the market. But TSCA regulation does not automatically trigger a drinking water standard. The two regulatory tracks (chemical safety under the TSCA and drinking water standards under the SDWA) operate independently. 

Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) 

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) governs pesticide registration and use. Several pesticides have SDWA standards (atrazine, glyphosate, and others), but FIFRA registration does not ensure a pesticide won't contaminate drinking water at levels exceeding health-based thresholds. A pesticide can be approved for use under FIFRA while still posing a drinking water contamination risk that the SDWA must address downstream. 

How EPA Evaluates New Contaminants for Potential Regulation In Drinking Water 

The EPA takes several considerations into account when determining whether a contaminant requires regulation. 

The Contaminant Candidate List (CCL) 

The formal path toward a drinking water standard begins with the Contaminant Candidate List (CCL). Under the 1996 amendments to the SDWA, the EPA must publish a list of unregulated contaminants that are known or anticipated to occur in public water systems and are of high enough concern that they may require regulation.  For each CCL, the EPA identifies a list of potential contaminants, screens them using available health and occurrence data, and selects a final list for further evaluation. 

The EPA has published five CCLs to date: 

CCL	Published	Contaminants Listed
CCL 1	1998	60
CCL 2	2005	51
CCL 3	2009	116
CCL 4	2016	109
CCL 5	2022	81

 

Note: Totals include chemicals/chemical groups plus microbial contaminants

Unregulated Contaminant Monitoring Rule (UCMR) 

The Unregulated Contaminant Monitoring Rule (UCMR) is how the EPA determines how widespread a contaminant is found in water supplies. 

Under UCMR 1-4, the EPA selected up to 30 contaminants per cycle and requires large water systems (serving over 10,000 people) to test for them.   

In UCMR 5, all community water systems and non-transient non-community systems serving 3,300 or more people, plus a nationally representative sample of smaller systems, must: 

• Monitor during a single 12-month timeframe within the three-year national monitoring window (2023–2025).   
• Generally collect two or four sample events per location over a 12 month period, depending on source type. 
  
  

UCMR	Monitoring Period	Notes/Scope
UCMR 1	2001-2005	24 contaminants (baseline monitoring cycle)
UCMR 2	2008-2010	25 contaminants
UCMR 3	2013-2015	30 contaminants
UCMR 4	2018-2020	30 chemical contaminants
UCMR 5	2023-2025	30 chemical contaminants

 

UCMR data is the primary national evidence base for regulatory decisions. Without it, the EPA can't demonstrate that a contaminant occurs at levels and frequencies of public health concern, which is one of the three statutory criteria for regulation. 

IRIS and ATSDR 

While UCMR monitoring answers the question "How widespread is this contaminant?", a different set of scientific assessments answers "How harmful is it?" 

The EPA's Integrated Risk Information System (IRIS) 

The EPA’s Integrated Risk Information System (IRIS) program, housed within the Office of Research and Development, develops human health assessments that the EPA and other agencies use as a key source of toxicity information when building risk assessments, including for drinking water decisions. 

• Reference Dose (RfD): For contaminants that cause noncancer effects, the RfD is an estimate of the daily oral exposure that is likely to be without appreciable risk of adverse health effects over a lifetime. This is calculated from the best available study data, with uncertainty factors applied to account for the gaps between laboratory studies and real-world human exposure. 
  
  
• Cancer Classification and Slope Factors: For potential carcinogens, IRIS classifies the contaminant using EPA's standardized descriptors ("Carcinogenic to Humans," "Likely to Be Carcinogenic to Humans," etc.) and derives an oral slope factor that quantifies the cancer risk per unit of exposure. 

The IRIS assessment process involves systematic literature review, internal and interagency review, public comment, and external peer review. It is thorough. It is also slow. The program has historically been criticized for a significant backlog, and many CCL contaminants lack completed IRIS assessments, which can delay regulatory action by years. 

The Agency for Toxic Substances and Disease Registry (ATSDR) 

The Agency for Toxic Substances and Disease Registry (ATSDR), part of the Department of Health and Human Services, provides a parallel scientific input through its Toxicological Profiles and Minimal Risk Levels (MRLs). These assessments are developed independently of the EPA and evaluate the same scientific literature, sometimes reaching different conclusions about safe exposure levels. 

It's important to understand that MRLs are screening values, not regulatory standards. The ATSDR has no authority to set or enforce drinking water limits. But their assessments contribute to the weight of scientific evidence that the EPA considers in the decision of whether or not to regulate a contaminant. 

Regulatory Determination 

Before the EPA can propose a new drinking water standard, the SDWA requires the agency to make what’s called a formal regulatory determination. This is the statutory “gate” for new contaminants. The EPA must conclude that: (1) the contaminant may have an adverse effect on human health, (2) it occurs in public water systems (or is likely to occur) at levels of public health concern, and (3) national regulation would present a meaningful opportunity to reduce health risk. 

This step is where nearly all contaminants that have been included on historical CCLs have stopped in the regulatory process. Positive determinations are rare for new contaminants, and even if a positive determination is made, contaminants still need to undergo the rulemaking process before they become an enforceable standard. For example, the EPA made a regulatory determination to regulate perchlorate in 2011, later tried to withdraw that determination in 2020, and it has remained contested for years. 

MCLG Determination 

The Maximum Contaminant Level Goal (MCLG) is the non-enforceable, health-based target for which there is no known or expected risk to health for a given contaminant. The EPA bases MCLGs entirely on toxicology and sensitive-population considerations do not take cost or feasibility into account. 

For non-carcinogens (and carcinogens with a threshold), the EPA typically derives an MCLG using a Reference Dose (RfD), body weight, drinking water intake, and a Relative Source Contribution (RSC) to account for exposure from sources other than drinking water.  

For carcinogens with a linear (no-threshold) dose-response relationship, the EPA may set the MCLG at zero, which means the EPA determined there is no exposure level that can be described as risk-free, so the enforceable MCL becomes a feasibility and implementation question.