Maximum Contaminant Levels (MCLs) are the highest levels of contaminants that are allowed in drinking water. MCLs are set by the United States Environmental Protection Agency (USEPA) for public water supplies to protect human health. MCLs are designed to ensure that the concentration of contaminants in drinking water does not pose a risk to consumers.

Although there are no MCLs specifically for private wells, experts and practitioners often use the MCLs for community water supplies to determine whether the well water is safe or not. By comparing the amount of a particular substance detected in the well water with the MCL set by the USEPA, you can determine if the well water exceeds the acceptable level. (For example, if the MCL for arsenic in water is 10 parts per billion (ppb), and your well water sample tested at 20 ppb, it would indicate that your well water has arsenic levels above the National Primary Drinking Water Standard for arsenic, which means that your water is unsafe to drink.)

Primary MCLs (or Maximum Contaminant Levels): Primary MCLs are a set for contaminants that can cause health effects such as illness, disease, cancer, or other health problems. These standards are mandatory for public water systems, but they are voluntary for private water systems.

For more information, visit: EPA's National Primary Drinking Water Regulations

Secondary MCLs (or Recommended Maximum Contaminant Levels ): Secondary MCLs are created for contaminants that cause aesthetic problems such as bad taste, discoloration, or odor in water. In the past, these standards were voluntary and used mainly as guides. However, some community water systems are now required to meet some of these secondary standards.

For more information, visit:

EPA's Secondary Drinking Water Standards: Guidance for Nuisance Chemicals