What is an MCL?

The Environmental Protection Agency (EPA) reviews health effects data gathered for a particular set of contaminants found in drinking water and then sets a maximum contaminant level goal (MCLG). The MCLG is the maximum level of a contaminant at which no adverse health effects are anticipated, allowing for increased public safety.

As MCLGs only consider public health and not available technology or effectiveness, they are not enforceable. The EPA uses the MCLG to determine the maximum contaminant level (MCL), an enforceable standard that dictates the maximum level allowed of a contaminant in a water source that is delivered to a public water system.

The MCL is set as close to the MCLG as possible, taking into consideration costs and feasible treatment techniques to remediate water supply contamination.

How are Standards Enforced?

Standards are considered active three years after they are finalized, at which point they are enforceable. Entities may receive exemptions to allow for extra compliance time or for small systems. Otherwise, MCL standards apply to all public water systems (PWSs) that provide water for human consumption through a structured conveyance to at least 15 service connections or 25 individuals.

PWSs that are not in compliance and do not have an exemption may be subject to fines, legal fees, or additional EPA monitoring.

The EPA reviews all existing MCL standards every six years, evaluating new data, information, and technology to determine whether regulatory revisions are needed to maintain or increase public health.

What is a Recent Example of an MCL?

The EPA published a proposal in March 2023 to create a National Primary Drinking Water Regulation (NPDWR) with an MCL standard for specific per-and polyfluoroalkyl substances (PFAS) found in the water supply.

PFAS are man-made chemicals that have been used at industrial sites, as well as in common household items and cleaning products, for decades. PFAS have been found in the air, soil, and groundwater, affecting the water supply and potentially affecting human health.

The proposed MCL standard would limit contamination to an enforceable concentration of four parts per trillion (ppt) for two PFAS compounds—perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)—the lowest concentration that laboratories can reliably measure to demonstrate compliance. However, the MCLG is set at zero parts per trillion.

If enacted, public water systems would be required to monitor for these PFAS compounds and notify the public of their levels. If the MCL exceeded the proposed standard, entities would be required to reduce PFAS levels through a variety of available methods.

Read more about upcoming PFAS regulations here.

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