On April 2, the EPA published its draft version of the 6th Drinking Water Contaminant Candidate List (CCL6). You wouldn’t have known it from the press release,
“EPA Takes Bold Action to Ensure Drinking Water is Safe from Microplastics, Pharmaceuticals, and Potential Hidden Contaminants. Historic Step for MAHA Movement: For the first time, EPA targets microplastics and pharmaceuticals as priority drinking water threat groups.”
But the publication of a CCL is a routine requirement of the Safe Drinking Water Act, occurring every 5 years. The press release overemphasizes the inclusion of microplastics and pharmaceuticals on the list but understates a significant and constructive development—the EPA's release of human health benchmarks for pharmaceuticals.
The Contaminant Candidate List
The Safe Drinking Water Act requires the EPA to publish a list of contaminants known as the Contaminant Candidate List (CCL), which includes:
- not subject to current drinking water regulations
- known or anticipated to occur in public drinking water systems
- may require regulation under the Safe Drinking Water Act.
The CCL6 includes 88 contaminants, 75 chemicals, and four chemical groups, including microplastics, PFAS compounds, disinfection byproducts, and pharmaceuticals. While the EPA has listed chemical groups on all previous CCLs, this is the first time microplastics and pharmaceuticals are included as a group. Many contaminants are carried over from previous lists; 46 individual contaminants from CCL5 are also present in CCL6, including methylmercury, manganese, bisphenol A, and lithium [1].
The CCL is the first step in the long regulatory process of the Safe Drinking Water Act, serving as an initial screening to identify contaminants that might need regulation. Including a contaminant on the list does not necessarily mean it will be regulated in the future, and listing chemical groups does not imply that EPA will regulate the entire group (they might regulate only one or two members).
As mandated in the Safe Drinking Water Act, the EPA evaluates whether to regulate these CCL contaminants, based on three criteria:
- The contaminant may have an adverse effect on human health
- The contaminant will occur, or there is a substantial likelihood they will occur, at levels of public health concern in drinking water
- Regulation presents the opportunity for “health risk reduction”.
If the EPA makes a positive determination on all three of the criteria, it begins the multi-step process of setting a drinking water regulation, which typically takes several years to complete.
Human Health Benchmarks
Along with the CCL6, the EPA released human health benchmarks for 374 pharmaceuticals. Human health benchmarks are “acceptable” levels of contaminants that water systems or states use to determine whether there is a health risk from unregulated contaminants in drinking water. The EPA has previously released human health benchmarks, including health advisories, for about 200 chemicals.
The importance of these benchmarks is that the EPA is unlikely to regulate pharmaceuticals in drinking water because they do not occur at levels that pose a public health concern. Pharmaceuticals have been detected at very low levels, and risk assessments have not found health risks at those levels. EPA must follow the law, regardless of how politically popular or unpopular an action might be.
However, if pharmaceuticals are detected, these benchmarks, like health advisories and human health benchmarks for pesticides, serve as useful tools for water systems and states to assess health risks. Water systems can compare the detected levels with these benchmarks and, if they are exceeded, treat the water to reduce levels below the benchmarks.
Pharmaceuticals in Water
While pharmaceuticals as a group have never appeared on a previous CCL (individual pharmaceuticals have appeared in previous CCLs), they have attracted interest from the EPA and the media for over 20 years. They were first reported by the U.S. Geological Survey (USGS) in streams across the U.S. in 1999 and 2000 and gained public attention in 2008 when the Associated Press (AP) published a three-part series reporting pharmaceuticals detected in the drinking water of 24 U.S. metropolitan areas serving about 41 million people. This story was featured on the front page of many newspapers and was a lead story on network news.
However, the detected levels are extremely low, around a nanogram per liter (ng/L), which is a billionth of a gram of substance dissolved in a liter of water. In the largest study of drinking water to date, conducted by the EPA and the USGS, the median concentration of all pharmaceuticals found in untreated water at 25 drinking water treatment plants across the U.S. was 14.2 ng/L, and it was 10.6 ng/L for treated water. The most detected pharmaceuticals in drinking water included lithium, bupropion, metoprolol, carbamazepine, and cotinine.
A 2025 study investigated pharmaceuticals and other contaminants in drinking water across Minnesota. Less than 1% of samples detected pharmaceuticals, with concentrations ranging from 0.502 to 86.2 ng/L. A study on opioids in drinking water found hydrocodone and codeine in 40% of the samples tested, at levels between 0.3 and 20 ng/L. Fentanyl was not found in any drinking water sample.
Risky business?
Risk assessments of pharmaceuticals involve studying the concentration of a pharmaceutical in water compared to an “acceptable” level, i.e., a human health benchmark. A pharmaceutical concentration below the human health benchmark indicates little risk, while a concentration above the benchmark signifies potential risk.
As I previously discussed, none of the seven human health risk assessments of pharmaceuticals in the U.S. and Canada published between 2005 and 2019 reported a potential health risk. Similarly, the 2025 Minnesota study did not find any potential health risk. The reason? It is due to the very low levels at which pharmaceuticals have been detected, which are far below human health benchmarks.
While headlines focused on pharmaceuticals and microplastics, the true significance lies buried in the Federal Register: the release of human health benchmarks for pharmaceuticals. Ironically, these benchmarks, in various forms, have been in development for over ten years. The fact that they are finally being published is the real story. These benchmarks offer concrete help to many organizations, including public and private water systems and State agencies nationwide. As is often the case, the press release overlooks the core substance of EPA’s action.
[1] Although lithium is used as a pharmaceutical, it is listed separately on CCL5 and CCL6 because it is also a naturally occurring element that has other commercial uses besides as a pharmaceutical drug.
