Drinking water quality in Los Banos is regulated by the California State Water Resources Control Board Division of Drinking Water. The Division of Drinking Water is required to set and enforce drinking water standards that are at least as stringent as those set by the U.S. EPA, but may also set more stringent standards that apply only to California.

Prior to October 1, 2024, total chromium in drinking water was regulated to a concentration of 100 ppb at the federal level and to a more stringent 50 ppb level in California.  Total chromium includes both hexavalent and trivalent chromium, also known as chromium-3 and chromium-6, respectively.

The City of Los Banos was in compliance with the previous 50 ppb California regulation at all times. However, the City fell out of compliance after the lower 10 ppb limit that was adopted on October 1, 2024.

Water systems were given time to comply with the new regulation before notices of violation could be issued. The amount of time granted depends on the size of the system. The California State Water Resources Control Board Division of Drinking Water gave the City of Los Banos until October 1, 2026, to become compliant.

It is worth noting that the City’s drinking water complies with federal standards for chromium-6. Many other states limit total chromium levels to 50 ppb – and in many cases to 100 ppb – and do not specifically regulate the hexavalent form.

The World Health Organization (WHO) also recommends a guideline value of 50 ppb for total chromium in drinking water.

The City of Los Banos relies on guidance from state and federal public health agencies regarding the potential health effects of hexavalent chromium. California has established a Public Health Goal (PHG) of 0.02 parts per billion (ppb). The PHG represents a level in drinking water associated with no more than a one-in-one-million estimated lifetime cancer risk for an adult who drinks approximately two liters (about half a gallon) of water each day for 70 years.

California drinking water regulations require that the City notify all consumers of the hexavalent chromium limit exceedance four times per year and that the notification include the following health effects language: “Some people who drink water containing hexavalent chromium in excess of the MCL over many years may have an increased risk of getting cancer.”

Further questions regarding health effects should be addressed to:

Merced District Engineer
California State Water Resources Control Board
Division of Drinking Water
265 West Bullard Avenue, Suite 101
Fresno, CA 93704
(559) 447- 3316

Specific medial concerns should be discussed with a licensed medical professional.

Hexavalent chromium in Los Banos’ groundwater appears to be naturally occurring.

Geological conditions on the west side of the Central Valley, near the Coastal Mountain Range, can cause naturally occurring chromium in rocks and minerals to enter groundwater as water moves underground. Los Banos is located within this area, and hexavalent chromium levels above the State drinking water limit have been detected along the Coastal Range from Los Banos to at least as far north as Patterson.

The City hired a consultant to study hexavalent chromium levels in and around Los Banos and determine whether there were signs of man-made contamination. The study found strong evidence that the hexavalent chromium in Los Banos’ water is naturally occurring and no evidence of man-made contamination. There is also no record of an industrial discharge of Chromium-6 that would affect the City’s groundwater wells. The City remains committed to pursuing any scientific evidence of man-made contamination if it is identified.

Los Banos is not the only community affected. When the State regulation took effect, it was estimated that at least 233 California water systems would be impacted. Other areas significantly affected by what appears to be naturally occurring hexavalent chromium include the Coachella Valley and areas around Woodland and Davis.

This differs from areas with known man-made hexavalent chromium contamination, including the Hinkley and Topock PG&E gas compressor station sites in Southern California and the San Fernando Valley Superfund Site north of Los Angeles.

Los Banos faces a particularly significant challenge because all of the City’s groundwater wells exceed the new State limit. Many other affected water systems have alternative drinking water sources that do not exceed the limit. Communities affected by man-made contamination may also be able to seek compensation from the responsible polluter. Because the hexavalent chromium in Los Banos appears to be naturally occurring, that option is not available to the City.

There is no evidence that hexavalent chromium was ever used as an active or inactive ingredient in agricultural pesticides.

Chromium is used as a component of certain wood preservatives and pesticides, including chromated copper arsenate (CCA) and acid copper chromate (ACC). However, these products are applied to lumber to protect the wood from insect damage; they are not applied to agricultural fields or crops.

Based on the best science available to the City, including the City’s study of hexavalent chromium in the Los Banos area, there is also no evidence that fertilizer use has caused or contributed to the hexavalent chromium found in Los Banos’ groundwater. The available evidence indicates that the hexavalent chromium is naturally occurring.

The City has been proactively responding to the new regulation since the Notice of Proposed Rulemaking was published on June 16, 2023.

Before that date, the City did not know what the new limit would be or whether the City’s water system would comply with it.

The City retained an engineering consultant to evaluate alternatives for bringing the water system into compliance with the new standard and to assist with preparing a Compliance Plan.

The City has also completed investigations into the source of the hexavalent chromium and the viability of constructing new wells. An extensive pilot study of a potential water treatment process is also planned. In addition, the City is investigating potential funding sources to help offset the high costs associated with complying with the new regulation.

The City and its consultants have evaluated several potential solutions to the problem, including:

1. Constructing new water supply wells that produce water that is under the hexavalent chromium limit;
2. Obtaining rights to surface water and constructing a surface water treatment plant; and
3. Treating the City’s existing water supply wells to remove hexavalent chromium.

Based on the results of completed studies, the City believes treating its existing wells to remove hexavalent chromium is the most cost-effective and timely solution for bringing the water system into compliance with the new regulation.

An important element of the City’s response is to seek out and obtain any available funding assistance.

Planning-level estimates indicate that constructing treatment plants to remove hexavalent chromium would cost approximately $2.7 million to $4.8 million per well. The estimated cost to construct treatment plants at all 13 of the City’s wells is approximately $42 million, with estimated annual operating costs of $1.25 million.

The City’s current annual water fund is less than $4 million.

Because of the significant financial challenge created by the new regulation, the City is applying for Safe Drinking Water State Revolving Fund (SRF) grants and loans and seeking other grant and low-interest loan funding from state and federal sources. These funds will help pay for engineering studies, testing, design, and construction of the required water system improvements. However, there are no federal or state programs to assist with operating costs once the facilities are constructed.

Based on the costs to construct and operate new treatment facilities and the limitations on available state and federal funding, the City will need to conduct a rate study and consider limited rate increases, taking into account the City’s median household income.

The City submitted a Hexavalent Chromium Compliance Plan to the State Water Resources Control Board Division of Drinking Water in September 2025. The State approved the plan on October 1, 2025. The plan is structured to balance the need to bring the City’s water system into compliance with the City’s fiscal responsibility to minimize rate impacts to water users.

The approved Compliance Plan includes three primary phases:

1. Phase 1: Testing and Engineering: Test the proposed treatment process to confirm that it is effective, reliable, and cost-effective, and complete the necessary engineering work.
2. Phase 2: Design: Design treatment facilities for the City’s wells.
3. Phase 3: Construction and Startup: Construct and place the treatment facilities into operation.

Completing these steps will take time, particularly because all 13 of the City’s wells exceed the new hexavalent chromium limit and treatment is needed for the City’s entire water supply. Under the State-approved Compliance Plan, the City is working toward completing construction and achieving compliance with the new regulation by December 2030.

Current Status – September 2026

The City is currently in Phase 1. A small-scale pilot study using reduction-coagulation-filtration (RCF) treatment has been completed, and the City will be constructing a full-scale treatment plant at Well 9.

The full-scale facility will allow the City to address remaining issues identified during the pilot study, determine which optional features should be included in the remaining treatment plants, provide operator training, and establish actual construction and operating costs. More precise cost information will also help the City more effectively pursue available funding assistance.

Residential point-of-use (POU) and point-of-entry (POE) devices can be purchased by individuals to reduce exposure to contaminants in their household water. POU devices are much more common than POE devices for hexavalent chromium treatment.

A list of certified residential POU and POE devices capable of treating hexavalent chromium is available on the State Water Board’s website, here: https://www.waterboards.ca.gov/drinking_water/certlic/device/watertreatmentdevices.html?form=MG0AV3 .

Check with the manufacturer and the POU device’s hexavalent chromium reduction claims as percent removal may vary by device. Because bottled water is not tested for hexavalent chromium, it is not a recommended means of decreasing exposure to hexavalent chromium.