The Maximum Contaminant Level (MCL) is a legally enforceable standard that sets the highest permissible concentration of a specific contaminant allowed in public drinking water systems. This measurement represents the regulatory line between safe and unsafe water as delivered to consumers. The MCL is a cornerstone of the public health framework established by the Safe Drinking Water Act (SDWA) in the United States. It ensures that contaminants, such as heavy metals, microbes, and organic chemicals, are kept below levels that could cause adverse long-term or acute health effects. By setting an enforceable limit, the MCL provides a metric for public water systems to monitor and manage water quality.
Understanding the Maximum Contaminant Level
The MCL is a specific numerical value, often expressed in parts per million (ppm), parts per billion (ppb), or milligrams per liter (mg/L), that a public water system must not exceed. This standard balances health protection and practical application. An MCL must be achievable using the best available water treatment technology, considering the cost and technological feasibility of implementation across various water systems.
This legally binding standard is distinctly different from the Maximum Contaminant Level Goal (MCLG), which is a non-enforceable public health objective. The MCLG represents the level of a contaminant at which there is no known or expected health risk, allowing for an adequate margin of safety. For known or probable human carcinogens, for example, the MCLG is often set at zero, indicating a desire for no exposure.
The enforceable MCL is set as close to the MCLG as is technically and economically possible. The MCL may be higher than the MCLG due to limitations in detection methods, lack of complete removal technology, or if treatment costs are disproportionately high compared to the public health benefit. For example, arsenic has an MCLG of zero but a legally enforceable MCL of 10 ppb, accounting for the practical realities of water treatment and analysis.
How MCLs Are Established and Regulated
The responsibility for establishing and regulating MCLs falls to the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act. The process begins with a scientific assessment of a contaminant’s potential health effects to determine the non-enforceable MCLG. The EPA must confirm that the contaminant may adversely affect human health and is likely to occur in public water systems at a level of concern.
Once the MCLG is set, the EPA establishes the enforceable MCL, which becomes part of the National Primary Drinking Water Regulations (NPDWRs). These Primary Standards are mandatory for all public water systems and set limits for substances that pose a risk to public health, such as microbes, radionuclides, and specific chemicals. The MCL measurement is designed to enforce these Primary Standards.
The EPA also issues National Secondary Drinking Water Standards, which are non-enforceable guidelines not based on health risk. These Secondary Maximum Contaminant Levels (SMCLs) address aesthetic qualities like taste, odor, and color. The legally enforced MCL measurement primarily concerns compliance with the Primary Standards, ensuring health protection is prioritized.
What Exceeding an MCL Means for Consumers
Public water systems must continuously monitor their water supply through regular sampling and testing to ensure compliance with established MCLs. The results dictate whether the system is meeting its legal obligations. Failure to meet a Primary MCL triggers a mandatory public notification process, categorized by the severity of the health risk.
Acute risks, such as certain bacteriological violations like E. coli or high nitrate levels, require a Tier 1 notification, meaning the public must be informed within 24 hours of the system learning of the violation. This immediate communication often includes precautions consumers should take, such as boiling water or using an alternative source. Most other MCL violations, which pose a risk only after long-term exposure, fall under Tier 2, requiring public notification within 30 days.
When an MCL is exceeded, the water system must take corrective action, which may involve treatment upgrades, changing the water source, or implementing other solutions to reduce the contaminant to a compliant level. Consumers can access their system’s compliance status and water quality data through the annual Consumer Confidence Report (CCR). The CCR provides a summary of detected contaminant levels and any violations.