Emerging Contaminants: What Your Community Needs to Know
- JD Solomon
- 4 minutes ago
- 6 min read
Emerging Contaminants and Contaminants of Emerging Concern sound like the same thing. Yet the terms describe slightly different things, with underlying groupings, complexities, and unknowns. The dual challenge is understanding the science and explaining the distinctions clearly to decision makers.
EPA Uses More Than One Definition
EPA does not use a single definition for Emerging Contaminants. The meaning depends on the statutory program.
Under the Clean Water Act, EPA uses the term broadly. Contaminants of Emerging Concern are chemicals or microorganisms that are detected or anticipated in rivers, lakes, and estuaries. They may pose newly identified risks to aquatic life or human health. It includes pharmaceuticals, personal care products, endocrine-disrupting compounds, flame retardants, surfactants, plasticizers, and microplastics.
The definition is intentionally wide under the Clean Water Act. EPA’s purpose is scientific evaluation and ecological risk assessment, not regulatory action.
Under the Safe Drinking Water Act, EPA does not use the term Contaminants of Emerging Concern in a formal way. Instead, EPA uses the Contaminant Candidate List, or CCL, which identifies contaminants that are known or anticipated to occur in public water systems and may require future regulation.
The CCL is a statutory tool with a defined process. It is narrower, more structured, and tied directly to regulatory decision making. A national drinking water standard is called a Maximum Contaminant Level, or MCL, and it is the enforceable limit that public water systems must meet under the Safe Drinking Water Act.
Understanding these two definitions is essential. One is a scientific inventory. The other is a regulatory screening list that may turn into a national drinking water standard.
Both lists serve different purposes and should not be used interchangeably.
Some Have Been on the List for Decades
One common misconception is that the chemicals classified as Contaminants of Emerging Concern are new. In reality, several chemicals have appeared on the EPA’s screening lists for decades.
The persistence of chemicals with the classification for decades reflects long‑standing scientific uncertainty, slow toxicological development, or the difficulty of regulating contaminants that are widespread and costly to remove.
1,4-dioxane is one of the longest-running examples. It first appeared on EPA’s early contaminant candidate lists in the late 1990s and has remained on every CCL since the program began in 1998. Its persistence on the list reflects both its common use as an industrial solvent and the technical challenges associated with treatment.
Manganese has also been present for decades. It appeared on the earliest CCLs as a contaminant of health concern and continues to appear on CCL 5. Although manganese is naturally occurring, its neurological effects at elevated levels have kept it on EPA’s radar for more than twenty years.
PFAS chemicals are newer by comparison but still have a long history. PFOA and PFOS were first nominated for the CCL in the mid‑2000s and have appeared consistently since CCL 3 in 2009. The prominence of PFAS has increased because of several high-profile releases, producers such as 3M and DuPont with deep pockets, and improved detection methods.
These three example contaminants on the CCL illustrate an important point. Emerging does not mean new. It means the science is still evolving and the regulatory process is still underway. Some contaminants remain in this category for decades because the evidence develops slowly or because the cost and feasibility of regulation require extensive evaluation.
The Numbers Are Different for a Reason
The CEC contains approximately 200 chemicals and CCL 5 contains 66 and 12 microbial contaminants. The size of each list reflects its purpose.
The CEC universe under the Clean Water Act includes hundreds of chemicals. The list is intentionally large to support research, monitoring, and method development.
The CCL universe under the Safe Drinking Water Act is much smaller. EPA evaluates these contaminants for occurrence, health effects, and the potential need for regulation. Only a subset moves forward to Regulatory Determination. The list is smaller because it is tied to rulemaking, cost analysis, and enforceable standards.
The overlap between the two lists is limited. Only a few contaminants appear prominently in both programs. This is a point that often surprises decision makers and reinforces the need for clear communication.
Where the Lists Overlap
Only two contaminant groups stand out as priorities in both the CCL and the CEC programs.
PFAS
PFAS are the strongest point of overlap. They are persistent, mobile, and widely detected in both drinking water and surface water. EPA lists several PFAS on CCL 5, including PFOA, PFOS, PFHxS, PFNA, PFBS, and GenX. These same PFAS are also central to EPA’s CEC work because of their ecological toxicity and widespread environmental presence.
Cyanotoxins
Cyanotoxins are the second major overlap. Microcystin LR and related toxins appear on CCL 5 as a chemical group of concern for drinking water. They also appear prominently in the EPA’s CEC work because harmful algal blooms affect lakes, reservoirs, and estuaries. Cyanotoxins have both human health and ecological implications, which places them at the intersection of the two programs.
Most other contaminants fall clearly into one category or the other. Pharmaceuticals, hormones, UV filters, PBDE flame retardants, and phthalates are major CECs but do not appear on the CCL. Conversely, 1,4-dioxane, nitrosamines, manganese, and molybdenum are high-priority CCL contaminants but are not major CECs.
What’s Next in Drinking Water Regulations
EPA’s CCL process identifies contaminants that may require future drinking water regulation. Within CCL 5, several contaminants stand out because of their occurrence, health effects, and regulatory momentum.
PFAS
PFAS chemicals remain the most significant group. PFOA and PFOS already have national primary drinking water standards and have compliance dates effective in 2031. PFHxS, PFNA, PFBS, and GenX are also under active evaluation.
1,4-dioxane
1,4-dioxane is a high-priority industrial solvent and byproduct. It is frequently detected in groundwater and is difficult to remove with conventional treatment. It is unlikely that EPA will develop a 1,4-dioxane MCL in drinking water in the next five years.
Cyanotoxins
Cyanotoxins remain a priority because harmful algal blooms (HABs) are increasing in frequency and severity. It’s nearly impossible to identify the causes of HABs, so don’t look for new standards on this one anytime soon.
Metals and Inorganics
Metals and inorganics such as manganese, molybdenum, and strontium appear on CCL 5 because of their occurrence and health effects. Manganese would be the first to be regulated, but it's not a carcinogen and is naturally occurring. Don’t look for a new regulatory push on this one.
Nitrosamines
Nitrosamines, especially NDMA, are potent carcinogens that form as disinfection byproducts. N-Nitrosodimethylamine (NDMA) is the most well-known and can form when certain nitrogen-containing substances react with disinfectants used in water treatment. Drinking water utilities have other regulations that monitor for this, so there is not a strong driver for a drinking water MCL.
The Role of Human Health Advisories
Human Health Advisories are an important bridge between detection and regulation. They provide non-enforceable guidance on contaminant concentrations in drinking water that are expected to pose no adverse health effects.
Advisories are issued when EPA has enough toxicological information to provide health-based recommendations but has not yet completed the full regulatory process required for a national drinking water standard, or Maximum Contaminant Level (MCL).
For technical professionals, advisories help utilities and state agencies interpret monitoring results, guide interim risk communication when contaminants are detected above advisory levels, and signal which contaminants are gaining regulatory momentum.
PFAS is the clearest example. EPA issued interim health advisories for PFOA and PFOS long before proposing enforceable MCLs.
Cyanotoxins followed a similar path, with health advisories preceding formal guidance and monitoring requirements.
Advisories do not carry the force of law, but they often shape public expectations and influence state-level actions.
Many Emerging Contaminants, So What’s Next?
Emerging Contaminants are not a single list or a single concept. For technical professionals, the value of “emerging contaminants” lies in understanding the distinctions, explaining them clearly, and focusing on the contaminants that matter most for our communities. With that said, emerging contaminants are confused and abused in the public. Remember, emerging contaminants fall into well-defined categories that support either scientific research or regulatory screening. The harsh reality is that there are not enough resources to significantly reduce the list anytime soon.
JD Solomon is the founder of JD Solomon, Inc., the creator of the FINESSE Fishbone Diagram®, and the co-creator of the SOAP criticality method©. He is the author of Communicating Reliability, Risk & Resiliency to Decision Makers: How to Get Your Boss’s Boss to Understand and Facilitating with FINESSE: A Guide to Successful Business Solutions.
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