The U.S. EPA’s Unregulated Contaminant Monitoring Rule (UCMR) requires all public drinking water systems serving at least 10,000 people and a random subset of smaller public systems to periodically monitor for selected contaminants of emerging concern (CECs). Some of these CECs affect source water quality and drinking water treatment needs.

The UCMR was established following a 1996 update to the federal Safe Drinking Water Act (SDWA) that required the U.S. EPA to select up to 30 unregulated contaminants to be monitored by public drinking water systems once every five years. A new list of UCMR chemicals and microbials is created for each five-year sampling period by choosing a subset of the approximately 100 chemicals listed on the U.S. EPA’s Contaminant Candidate List (CCL).  While the inclusion of a chemical on the UCMR list does lead to mandated monitoring for larger public systems, the detection of a UCMR chemical in municipal drinking water does not require any regulatory actions unless one is established by a state. It may be subject to future regulatory action by the U.S. EPA once sufficient data and risk are determined.

The CCL is a list of contaminants that are known or suspected to be present in drinking water but are not currently subject to any federal regulations. Many CCL contaminants are suspected to originate from untreated source waters and are not byproducts of drinking water treatment processes. Certain CECs present in source water may pass through drinking water treatment processes and thus be detected through the UCMR. Unlike the U.S. EPA’s process for the UCMR which includes the generation of a unique list every five years, CCL chemicals are carried over to the updated list unless a regulatory determination is made. A regulatory determination may consist of a decision that no regulatory action is necessary, or the creation of drinking water criteria that establishes a maximum contaminant level that drinking water providers would then have to comply with.

Both the CCL and the UCMR are used by the U.S. EPA to create a federal baseline for exploring the potential presence and health impacts of unregulated chemicals in drinking water, to guide federal research priorities, and to provide a basis for future regulatory actions. The UCMR focuses primarily on treated drinking water but in some instances has included source water monitoring.

The UCMR does not currently include monitoring activities for private wells or smaller public drinking water systems, which serve about 15 percent of Great Lakes basin residents in the United States. A recent amendment to the SDWA will expand the UCMR to also require monitoring for public water systems serving between 3,300 and 10,000 persons beginning in 2021. This expansion has the potential to cover an additional 9 percent of U.S. Great Lakes basin population but is dependent on the U.S. EPA administrator determining that sufficient funds are available to support increased laboratory testing. In some cases individual states or local communities have chosen to voluntarily expand on UCMR requirements by performing increased monitoring for unregulated contaminants, investigating the presence of CECs in untreated source water, and/or setting state-level standards for drinking water.

Both the CCL and the UCMR are currently on their fourth iterations: the final CCL4 and UCMR4 were published by the U.S. EPA in the winter of 2016. Sampling activities for the 30 UCMR4 chemicals began in 2018 and will be completed in 2020.