The EPA identifies new, potentially harmful unregulated contaminants for monitoring as part of their Unregulated Contaminant Monitoring Regulations (UCMR). In the program, researchers test public drinking water systems for up to 30 contaminants. These tests establish whether contaminants are present and determine their concentrations. Testing data is put into a national EPA UMCR database. The EPA is now on its fourth UMCR testing cycle.
The contaminant concentration data from the UMCR helps the EPA select broadly present contaminants that may have health effects for inclusion in a Contaminant Candidate List (CCL). The EPA uses a public interest, scientific and medical studies to evaluate CCL contaminants. The EPA is now in its fourth CCL process.
The EPA evaluates the most significant CCL contaminants and emerging contaminants of suspected high risk for broad presence and risk to human health. These go through an EPA regulatory determination process. Contaminants approved during the regulatory determination are placed on the regulated contaminant listing. Precise testing standards for laboratories are specified and maximum exposure limits for public water supplies to meet are established. Exposure limits are set by epidemiological studies and correlations to potential impact on human health as part of the regulatory determination process.
There are many aspects to the UCMR, CCL, and the regulatory determination process that this short summary cannot touch on. The EPA website is a great and user-friendly source for more details.