How Drinking Water Standards Are Set in California

Joe Grindstaff

Oct 2

9 min read

Water pours from a clear pitcher into a glass on a white background, with bubbles and reflections.

When people turn on the tap, they expect the water to be safe.

Delivering on that expectation requires much more than treating and testing water. Regulators must decide which contaminants should be monitored, what concentrations may create a health risk, how accurately laboratories can detect those contaminants, which treatment technologies can remove them, and what those treatments will cost communities.

Those decisions become drinking water standards.

The process is based on science, but it also involves technology, economics, public participation, and difficult questions about acceptable risk. A standard must protect public health while also being measurable, enforceable, and technically achievable across thousands of public water systems.

Understanding how California drinking water standards are established can help explain why the process may take years and why identifying a potential contaminant is only the beginning.

Who Regulates Drinking Water in California?

Public drinking water is regulated at both the federal and state levels.

The U.S. Environmental Protection Agency establishes national drinking water requirements under the Safe Drinking Water Act. These rules create minimum standards that apply to public water systems throughout the country.

California has primary responsibility for implementing and enforcing drinking water requirements within the state. The State Water Resources Control Board’s Division of Drinking Water regulates public water systems, adopts state standards, conducts inspections, reviews monitoring data, and oversees compliance.

California may adopt drinking water standards that are more protective than federal standards. It also regulates some contaminants that do not have a federal maximum contaminant level.

Another California agency, the Office of Environmental Health Hazard Assessment, evaluates the health risks associated with contaminants and develops public health goals.

These agencies have different but connected responsibilities. OEHHA evaluates the science and health risks. The State Water Board determines the enforceable drinking water standard.

What Is a Public Health Goal?

A public health goal, often abbreviated as PHG, is the concentration of a contaminant in drinking water that is not expected to create a significant health risk.

The California Office of Environmental Health Hazard Assessment develops these goals using health studies and risk assessments. The agency also considers sensitive populations, including infants, children, pregnant people, older adults, and people with serious health conditions.

Public health goals are based on health protection. They do not consider how much treatment would cost or whether every laboratory can reliably measure the contaminant at that concentration.

They are also not enforceable standards.

Instead, a public health goal provides the scientific starting point that California uses when considering a maximum contaminant level.

What Is a Maximum Contaminant Level?

A maximum contaminant level, or MCL, is an enforceable drinking water standard.

Public water systems must monitor regulated contaminants and ensure that their water complies with applicable maximum contaminant levels. When a system exceeds a standard, it may be required to notify customers, increase monitoring, change its water source, install treatment, or take other corrective action.

California law requires the State Water Board to place primary emphasis on protecting public health and establish an MCL as close to the public health goal as is technically and economically feasible.

This means the public health goal and maximum contaminant level may not be identical.

A health assessment may determine that an extremely low concentration would provide the greatest protection. However, available laboratories may not be able to consistently detect the contaminant at that level. Treatment technology may also be unable to reliably remove it to that concentration across public water systems of different sizes and designs.

The State Water Board must turn the health-based goal into a standard that water systems can measure, meet, and regulators can enforce.

How Does California Establish a Drinking Water Standard?

The State Water Board’s maximum contaminant level process involves several steps.

1. Scientists evaluate the health risk

OEHHA reviews scientific studies and develops a public health goal. This process may involve public workshops, outside scientific peer review, public comments, and revisions to the original analysis.

The goal is to determine the concentration that would not be expected to produce significant adverse health effects over a lifetime of exposure.

2. Regulators examine where the contaminant occurs

The State Water Board evaluates available water quality data to determine how frequently the contaminant appears, where it has been detected, and how many people or water systems could be affected.

A chemical found in only a few locations presents a different statewide treatment challenge than one found across hundreds of groundwater wells.

Occurrence data helps regulators estimate the potential public health benefit and cost of different regulatory options.

3. Laboratories must be able to measure it

A drinking water standard cannot be effectively enforced if laboratories cannot reliably detect the contaminant at the proposed concentration.

Regulators examine available analytical methods, laboratory capacity, accuracy, reporting limits, and monitoring costs.

Testing must produce results that are scientifically defensible and consistent enough to support regulatory decisions. As laboratory technology improves, regulators may be able to detect substances at much lower concentrations than they could in the past.

This increased sensitivity can reveal contaminants that were previously undetectable, but detection alone does not determine the level of health risk.

4. Treatment technology is evaluated

The State Water Board identifies treatment methods that can remove or reduce the contaminant.

Depending on the substance, a water system might use filtration, ion exchange, activated carbon, reverse osmosis, blending, chemical treatment, or a different water source.

Regulators must consider whether the technology works consistently under real operating conditions. A process that performs well in a laboratory or large treatment plant may be difficult for a small rural water system to operate.

Treatment may also create waste that must be managed or disposed of, consume energy, or require significant upgrades to other parts of the water system.

5. Costs and health benefits are compared

California law requires regulators to consider the cost of monitoring and treatment when adopting a maximum contaminant level.

This is one of the most difficult parts of setting a drinking water standard.

A lower standard may reduce potential health risks, but it may also require more expensive treatment. Those costs can include construction, equipment, energy, replacement filters, specialized employees, laboratory testing, waste disposal, maintenance, and long-term system upgrades.

Regulators compare different possible limits and consider the expected health-risk reduction associated with each option.

The final decision must prioritize public health while also producing an enforceable standard that is technically and economically feasible.

6. The proposal goes through public review

A proposed standard moves through California’s regulatory process. Water agencies, public health organizations, community groups, technical experts, businesses, and residents may review the proposal and submit comments.

The State Water Board considers this input before adopting the final regulation.

Public participation does not replace scientific analysis, but it can identify operational challenges, affected communities, overlooked data, and potential consequences of a proposed standard.

Does Meeting a Standard Mean the Water Has Zero Risk?

No human activity can be guaranteed to have zero risk.

Drinking water standards are designed to protect public health, often based on the potential effects of long-term exposure. However, individuals may have different levels of sensitivity based on age, health, genetics, medications, and other factors.

Some public health goals for substances associated with cancer are set at or near zero because no exposure can be described as completely risk-free. An enforceable maximum contaminant level may be higher because the contaminant cannot be reliably detected or removed to zero.

That does not mean the standard is arbitrary. It means regulators are translating a health objective into a measurable and achievable requirement.

The difference between a public health goal and an enforceable standard is one reason water quality information can be confusing. A contaminant may be detected below the legal limit, but the result may still be higher than the public health goal.

Both numbers are important, but they serve different purposes.

Why Can Drinking Water Regulations Take Years?

New drinking water regulations often require extensive scientific and technical work.

Regulators may need to:

Review health and toxicology studies
Collect water quality data from across the state
Develop reliable laboratory methods
Determine where the contaminant occurs
Test different treatment technologies
Estimate statewide compliance costs
Evaluate impacts on large and small water systems
Conduct scientific peer review
Receive and respond to public comments
Complete the formal regulatory process

Scientific understanding can also change while a standard is being developed. New studies may identify different health effects, improvements in laboratory testing may reveal lower concentrations, or new treatment technologies may become available.

The process can feel slow, especially to communities concerned about a contaminant. However, a drinking water regulation must be supported by enough evidence and technical detail to guide thousands of water systems and withstand legal and scientific review.

Why Are Small Water Systems Affected Differently?

The cost of drinking water treatment does not fall evenly across communities.

A large water agency can spread the cost of a treatment facility, laboratory program, or specialized employee across hundreds of thousands of customers. A small system may have only a few hundred customers supporting similar regulatory responsibilities.

The treatment equipment may be smaller, but the system still needs qualified operators, monitoring, maintenance, replacement parts, reporting, and emergency planning.

This can make the cost per household much higher in a small community.

Some small water systems also rely on a single groundwater source. If that well contains nitrate, arsenic, perchlorate, PFAS, or another contaminant, the system may not have another source available for blending or replacement.

California offers funding and technical assistance for small community drinking water projects. In some cases, the long-term solution may involve connecting to a larger neighboring water system or consolidating management responsibilities.

Funding construction alone may not solve the problem. Communities also need the technical, managerial, and financial capacity to operate the treatment system for decades.

Different Contaminants Create Different Challenges

There is no single treatment method that removes every drinking water contaminant.

Nitrate, for example, is a common groundwater concern in some agricultural areas.

California’s maximum contaminant level for nitrate as nitrogen is 10 milligrams per liter.

Perchlorate can affect thyroid function and has a California MCL of 6 micrograms per liter. It may be associated with industrial activities and has been detected in some Southern California groundwater basins.

Arsenic occurs naturally in some groundwater. PFAS includes a large family of manufactured chemicals that may require advanced testing and treatment. Lead often enters water through plumbing materials rather than the original water source.

Disinfectants present another challenge. Water systems use chlorine and other disinfectants to control microorganisms that could cause immediate illness. However, disinfectants can react with naturally occurring material in the water and create disinfection byproducts.

Water agencies must control those byproducts without reducing disinfection to a level that allows harmful microorganisms to survive.

Each contaminant requires regulators and water systems to evaluate its source, health effects, treatment options, monitoring methods, and potential interactions with the rest of the treatment process.

What Is California Prioritizing in 2026?

The State Water Board adopted a list of drinking water regulatory priorities for 2026.

The 2026 drinking water regulation priorities include work involving:

PFAS
N-nitrosodimethylamine, commonly called NDMA
Disinfection byproducts
Styrene
Cadmium and mercury
1,4-dioxane
Arsenic
Lead and copper requirements
Manganese
Cyanotoxins
Recycled water regulations
Laboratory and electronic reporting requirements

A priority does not necessarily mean that a final maximum contaminant level will be adopted immediately. It means the State Water Board has identified the regulation or policy as an important area for staff resources and development.

The list shows how water quality regulation continues to evolve as new health information, treatment methods, and contaminants emerge.

Why Water Quality Standards Must Continue to Evolve

Some of today’s water quality measurements and regulations are built on decades of historical data. Maintaining consistent testing methods can help scientists compare current conditions with past conditions.

However, technology does not stand still.

Modern laboratories can detect chemicals at concentrations that would have been impossible to measure in the past. Researchers continue to learn more about long-term exposure, sensitive populations, and interactions between contaminants.

California is also developing new water sources, including advanced recycled water and desalinated groundwater. These supplies require regulators to evaluate water quality in new ways while maintaining public confidence.

Drinking water standards must preserve the value of historical knowledge while incorporating better science and more accurate technology.

California law requires periodic reviews of maximum contaminant levels. These reviews give regulators an opportunity to determine whether a standard should remain in place or be updated based on new information.

Protecting Health While Maintaining Reliable Water Service

It can be uncomfortable to acknowledge that drinking water standards involve financial and technical considerations. Public health should always be the primary focus.

At the same time, treatment systems must be built, operated, tested, maintained, and paid for. A regulation that cannot be accurately measured or consistently achieved will not provide effective public health protection.

The challenge is to establish standards that reduce risk as much as reasonably possible while ensuring that communities can implement them successfully.

That requires good science, transparent decision-making, modern testing, effective treatment, financial assistance, and honest communication about both health risks and costs.

Safe drinking water does not happen automatically. It is the result of continuous testing, investment, regulation, operations, and review.

As science identifies new concerns, California must continue improving its standards while ensuring that every community, including small and disadvantaged communities, has the resources needed to meet them.