What is a Maximum Contaminant Level (MCL)?

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If you have looked at your local area's water quality report, you may have noticed something known as the "maximum contaminant level" or "MCL" for the trace contaminants in your water. 

This guide will look at the definition of "maximum contaminant level", why MCLs are enforced, and a timeline of which MCLs were introduced when. 

Maximum Contaminant Level: A Definition 

Maximum contaminant levels are drinking water standards set by the EPA. All contaminants that have been deemed unsafe by the EPA have their own maximum contaminant levels, or MCLs.

A contaminant's “maximum contaminant level” is the highest quantity of the contaminant that should be found in drinking water. This maximum limit is typically expressed as a concentration in micrograms or milligrams per liter of water, depending on whether the contaminant is a solid or a liquid. All public water systems must adhere to the EPA's maximum contaminant levels when treating water for public consumption. 

MCLs were designed to ensure that water has no safety risks, as most MCLs are set to between 10 and 1,000 lower than the amount that would potentially cause health problems.

A History of Maximum Contaminant Levels

The EPA has been regulating contaminants in drinking water since 1975. According to a timeline produced by the EPA, the first set of contaminants to be regulated included arsenic, chromium, fluoride, lead, gross alpha and gross beta, nitrate, mercury, two types of radium, and turbidity. 

Over the years, more contaminants were regulated by the EPA, including TTHMs, nitrite, asbestos, viruses, nickel, and HPC bacteria. There were revisions made to contaminants including fluoride, total coliform, turbidity, lead, nickel, and arsenic. 

The most recent EPA regulations include setting MCLs for PFOA and PFOS, chlorine, chloramine, and bromate, and revising MCLs for E.Coli, lead, copper, and Cryptosporidium. The EPA has now regulated more than 90 drinking water contaminants. 

RELATED: What is Turbidity?

How Does the EPA Establish a Maximum Contaminant Level?

The EPA establishes maximum contaminant levels by following a number of steps: 

1. Before enforcing MCLs, EPA carries out thorough research into a contaminant of concern, examining data to determine the potential health risks and dangers of the contaminant. 
2. Next, the EPA creates a maximum contaminant level goal (MCLG) for a contaminant. This non-enforceable public health goal describes the measurement of a contaminant that could be present without causing adverse health effects. 
3. The EPA then enforces a maximum contaminant level for the same contaminant. The MCL is very similar, if not higher than, the MCLG. There are several factors affecting the EPA’s decision on an MCL, including how easy the contaminant is to measure in small quantities, the potential for the contaminant to occur in public water systems, and whether the public health benefits of a lower MCL are outweighed by the costs of treatment. 
4. Some contaminants have a treatment technique (TT) rather than an MCL. The EPA enforces TTs to set out the procedure that public drinking water suppliers must follow to reduce or remove a specific contaminant. 

The EPA continues to establish new maximum contaminant levels today, and has a Contaminant Candidate List that highlights several contaminants that are becoming increasingly detected in public water. 

Do All Drinking Water Contaminants Have A MCL?

There are hundreds of trace contaminants in drinking water, and not all are regulated by the EPA. Some contaminants show no evidence of being unsafe for drinking in trace amounts, and some contaminants are solely aesthetic (affecting water’s taste, appearance or odor). 

In this case, the EPA will not introduce legally enforceable limits on the presence of these contaminants, but the EPA still recommends that public water systems do not exceed certain levels of these contaminants. These recommendations are known as secondary standards, or National Secondary Drinking Water Regulations (NSDWRs).

Non-EPA Maximum Contaminant Levels

The EPA produces the bulk of MCLs, but in some cases, states issue their own “maximum contaminant levels” according to state law or the federal Safe Drinking Water Act (SDWA). In this case, the state may either: 

• Issue a MCL that is lower than the EPA’s MCL, meaning that drinking water systems in that state must ensure lower levels of a certain contaminant than the EPA enforces
• Issue a MCL that the EPA has not regulated under federal law, so drinking water systems in that state must reduce or remove this contaminant according to state law

How Does the EPA Ensure MCLs are Met?

It can take up to three years for a primary drinking water standard to go into effect after it has been finalized. Once a MCL is in place, all water suppliers must meet this requirement. 

Public water treatment facilities must monitor, sample, and treat their water to ensure compliance with MCLs. The EPA reviews and evaluates results of water samples to confirm that a facility is adequately treating public water.

When setting new MCLs, the EPA must consider affordable methods of contaminant removal for drinking water systems of all sizes. This ensures that even small suppliers can achieve compliance with a regulation within their budget. 

Can a Maximum Contaminant Level be Changed?

Once a maximum contaminant level is enforced, it does not have to remain the same forever. The EPA reviews all of its MCLs after every six years. 

During this review, the EPA looks at new information and data that may indicate that a contaminant poses more of a threat than it did six years previously. Some of the major elements of the review are reviewing health effects information, analyzing contaminant occurrence and exposure, evaluating analytical method improvements, and conducting a treatment methods feasibility review. 

According to the SDWA, any revisions made in this review should increase or maintain protection to public health. So, while a MCL could become tighter, it could not become more lenient.