Maximum Contaminant Levels

Overview of Primary and Secondary Contaminants

Maximum Contaminant Levels are important for all water providers and systems and community residents to know about because they generally indicate what levels of contaminants are acceptable in drinking water.

Maximum Contaminant Levels are normally only enforced for Public Water Systems, but they are still important guidance that domestic well owners and State Small Water Systems should follow to protect the health of the people using their water.

What Is a Maximum Contaminant Level?

The United States Environmental Protection Agency (US EPA) and California State Water Resources Control Board (SWB) are required by law to establish a state drinking water standard called a Maximum Contaminant Level (MCL) for every potential drinking water contaminant that is regulated by the state of California.1 

An MCL is the highest level of a contaminant that is allowed in drinking water and is often measured in the units below, which are used where really small amounts of something like a contaminant make big differences like causing worse health impacts.2

(Visual in Spanish here)

Whatever MCL the SWB sets for California must be at least as low as what the US EPA sets as the federal MCL for that contaminant, if one exists.3 Additionally, the SWB will set a “detection limit for purposes of reporting” (DLR) along with the MCL. If a laboratory tests a water sample and the test shows a contaminant concentration at or above the DLR, the water provider must report that result to the SWB. Any amount of contaminant detected below the DLR is considered not concerning and may be marked as “non detected” in any reporting. Finally, the SWB must review the MCL for each drinking water contaminant every five years.4

Primary Contaminants

Primary contaminants are contaminants that cause health issues, like cancers developed from nitrate-contaminated water or skin rashes resulting from arsenic-contaminated water. Primary drinking water MCLs that California set can be found here (English / Español). The most common contaminants in the Central Coast and Central Valley are highlighted for convenience. (Some common contaminants, such as bacteria, are not included in this document.)

Common Primary Drinking Water Contaminant Information

Arsenic
Bacteria (Fecal, E.Coli)
Chlorine
DBCP
Chrome-6
Manganese
Nitrate
Perchlorate
PFAS
Uranium
Trihalomethanes
1,2,3-TCP

How Are Primary Drinking Water MCLs Determined?

Step 1: The Safe Drinking Water Act requires the US EPA to identify and list unregulated drinking water contaminants, which can lead to the development of federal MCLs for contaminants.5 Once the US EPA sets a federal standard, California must adopt the standard in order to be able to enforce that standard against the public water systems in the state.6

Step 2: Before the SWB can set an MCL for a primary contaminant (i.e. one that causes health issues if consumed), another California state agency called the Office of Environmental Health Hazard Assessment (OEHHA) must adopt a public health goal (PHG) for that contaminant.7 The PHG represents the level of the chemical contaminant in drinking water that does not pose a significant risk to health.8 OEHHA does not consider whether it is economically or technically feasible to meet this PHG.9 (Generally, what is “feasible” lands somewhere between what is convenient to do given cost and available technology and what the most protective, attainable solution is.) PHGs are not enforceable standards, rather they are guidance for what would be ideal for human health. 

Step 3: After OEHHA sets the PHG for a contaminant, the SWB can begin the process of setting an MCL for that same contaminant. The SWB can, and often does, set stricter standards than what the US EPA sets. The SWB uses the PHG, economic considerations, and technical considerations to set the MCL.10 The MCL represents the level of a contaminant that is allowed in drinking water, taking cost and technology available to treat contamination into account. MCLs are legally enforceable, which means that certain water providers can be held legally responsible for supplying water with amounts of contaminants over their established MCLs.

For both the PHG and MCL processes, the relevant state agencies have to go through formal administrative processes, during which the public can comment on whether they agree or disagree with a certain PHG or MCL proposal.11

Secondary Contaminants

Secondary contaminants are contaminants that cause the water to look, smell, or taste different, like if the water is cloudy, brown, or stinky-smelling. They generally do not cause bad health effects, but can if consumed in a large quantity. Secondary drinking water MCLs are set purely based on the aesthetic concerns and the standards that California set can be found on the State Water Board’s website. 

How Are Secondary Drinking Water MCLs Determined?

PHGs are not set for secondary contaminants because these drinking water MCLs are set on the basis of aesthetic concerns, not health issues. The US EPA’s website has more information on considerations that determine secondary MCLs.

Why This Matters To You

For both primary and secondary contaminants, a lower MCL means drinking water needs to have lower levels of the contaminant and a higher MCL means drinking water can have more of the contaminant and still be considered safe under the law. All water providers should know whether or not they need to treat their drinking water to ensure the safety of water customers. 

This Page Last Updated: July 17, 2024