Everything You Need to Know About MCLs

With hundreds of possible contaminants, how do you know your water is safe? The Environmental Protection Agency (EPA) regulates the safety of our drinking water, considering the effect of each of these particular contaminants. The EPA sets an individual Maximum Containment Level (MCL) for each possible contaminant. This article will explain what MCLs are, how they are established, and what you need to know about MCLs to keep your water safe.

What are MCLs

The Safe Drinking Water Act authorized the EPA to set safety regulations for possible drinking water contaminants. These regulations revolve around National Primary Drinking Water Regulations (NPDWRs), which set rules to limit the presence of specific contaminants that may cause health problems. For contaminants that do not cause health problems but may cause cosmetic problems like tooth discoloration, or may affect the taste or smell of tap water, the EPA sets National Secondary Drinking Water Regulations (NSDWRs). MCLs are a crucial part of the primary standards. MCLs are a legal standard; they set the maximum amount of a contaminant legally allowed in public drinking water. Conversely, the maximum contaminant level goal (MCLG) is not a legally enforceable rule. The MCLG are non-enforceable goals for water safety. However, despite their non-enforceable status, MCLGs are crucial to determining the legal standards set with MCLs.

How MCLs are determined

MCLs are determined based on MCLGs. The EPA first determines a safe level for the amount of a potential contaminant in tap water. For example, with the chemical chlorite, the EPA determined at concentrations of 0.8 mg per liter, no adverse health effects were found. This level then becomes the MCLG. MCLs are then set based on the MCLG, but they also take into account practical considerations like the cost of eliminating the contaminant, as well as the limits of measuring the contaminant. For contaminants that are suspected to cause cancer, the MCLG is always set to zero. For contaminants that do not cause cancer, a reference dose, or the amount of contaminant believed to be safe, is first determined. This reference dose is then adjusted based on calculations involving expected water consumption, exposure to the contaminant from other sources, and other factors. For microbial contaminants, the MCLG is also set to zero. Because other factors are taken into account when determining the MCL, sometimes the MCL can be higher than the MCLG.

Limitations of MCLs

MCLs are intended to be as close to the MCLGs as possible, but the EPA takes several factors into account when determining the MCL. Detection limits are a big factor in determining MCL. For example, with microbial contamination, even one bacterial cell in a billion parts of water is higher than MCLG, but detecting one single bacterium in a billion is not at all feasible. For contaminants like this, instead of an MCL, the EPA instead prescribes a treatment technique, which is also a legally enforceable standard. For coliforms, like E. coli, treatment facilities must test water supplies monthly, and no more than 5% of samples can test positive for coliforms. The cost of removing a contaminant can also cause the EPA to set the MCL higher than the MCLG. Additionally, updating the MCL or adding regulations for newly identified contaminants can be a slow process. For example, the EPA set an MCL for total chromium 0.1 milligrams per liter in 1991. Since that time, many studies have pointed to the link between chromium-6 and cancer (ever heard of what happened in Hinkley, CA fifteen years ago?). Although the EPA began reviewing this evidence in 2008, an MCL specific for chromium-6 still has not been set. I suggest you re-read that last sentence a couple of times and you will know everything you need to know about MCLs and how the EPA has your back.

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