Chromium-6 in California tap water: a 10 ug/L state MCL, phased by system size from 1 October 2026

If you are outside California, this page is the wrong one: no federal rule sets a hexavalent chromium limit, and your utility reports against EPA’s 0.1 mg/L total chromium standard instead.

Is the California chromium-6 MCL actually in force?

Yes. The rulemaking page for SWRCB-DDW-21-003, stamped page last updated 09/01/2026, prints an MCL of 0.010 mg/L, a detection limit for purposes of reporting (DLR) of 0.0001 mg/L, and the status line “This rulemaking is completed”.

The DLR is the level at or above which a laboratory result must be reported to the Board. At 0.0001 mg/L, or 0.1 ppb, it sits a hundred times below the MCL.

As of 21 September 2026, neither Board page names litigation, a stay or a suspension. An earlier attempt did fall: a 2014 hexavalent chromium MCL was invalidated in 2017, the court finding it failed to properly consider economic feasibility, which is why secondary accounts of this rule are worth distrusting.

When does your water system have to comply?

Adoption and compliance are different dates, and most confusion about this rule lives in the gap. The regulation has been effective since 1 October 2024; compliance monitoring starts later, on a schedule set by service connections counted at that effective date.

The first date belongs to the largest systems, not to mid-size ones. A system with 5,000 connections has until 1 October 2027.

Sampling was not deferred with it. The Board’s FAQ states each community and non-transient non-community system had to sample its sources by 1 April 2025, and that an initial result above 10 ppb goes to the Division of Drinking Water within 48 hours. Compliance is judged on a running annual average.

Why is the MCL 500 times the public health goal?

Because the Board must set an MCL as close to the public health goal as is technologically and economically feasible, not at it. The 2011 public health goal is 0.02 ug/L; the MCL is 10 ug/L, five hundred times higher.

The FAQ gives two reasons. Only a few laboratories have confirmed they can measure hexavalent chromium as low as 0.02 ppb, and treatment technologies have not been studied below about 1 ppb.

How common is chromium-6 in California sources?

The Board’s occurrence table covers 7,780 sources with more than a single detection between 2012 and February 2022. Read it as a distribution, not a headline.

The counties with the highest number of sources currently exceeding the MCL, per the Board’s FAQ, are San Bernardino, Los Angeles and Fresno. Detections must be reported in the annual Consumer Confidence Report, which is the free lookup before any purchase: the guide to reading a Consumer Confidence Report covers where inorganic chemicals sit in that table.

Which treatment class reduces hexavalent chromium?

Three, named as best available technology (BAT) in the Board’s FAQ: ion exchange, reduction-coagulation-filtration (RCF), and reverse osmosis. BAT means the Board expects the technology to bring the chemical below the MCL at system scale.

At the sink, two of those three survive. RCF is a plant process. That leaves reverse osmosis and strong-base anion exchange resin, which holds the chromate anion rather than exchanging hardness.

The caution here is about certification, not about any product. NSF lists “hexavalent and trivalent chromium reduction” as an optional claim under NSF/ANSI 58, and the claim is real. The pass level is not California’s MCL. The Whirlpool WHER25 performance data sheet, certified by NSF against NSF/ANSI 58 for chromium (hexavalent), prints a challenge of 0.3 mg/L plus or minus 10 percent and a maximum permissible product water concentration of 0.1 mg/L. That is 100 ppb, ten times the California MCL, and the sheet’s worst single effluent was 0.017 mg/L from a 0.310 mg/L influent.

A unit can hold that claim honestly and still leave more than 10 ppb on an extreme feed. Your own source number decides it. The NSF/ANSI standards guide explains what each standard covers.

What should you do first?

1. Find hexavalent chromium in your utility’s latest Consumer Confidence Report, where detections must be reported, and write down the average and the range.
2. Read that number against 10 ug/L, and read total chromium against 50 ug/L separately. They are different limits on different forms.
3. Check how many service connections your system serves, which fixes its compliance date at 2026, 2027 or 2028.
4. If you are on a private well, skip steps 1 to 3 and order a test, as the well water test panel guide sets out.
5. Only if your own number is above 10 ug/L, shortlist devices from California’s searchable List of Registered Devices, filterable by treated contaminant, which lists devices certified by an accredited independent organization such as NSF International, IAPMO R&T, UL, the Water Quality Association or CSA Group.

Total chromium and hexavalent chromium stay two limits, and the EPA drinking water limits table carries the federal one that applies everywhere else.

Frequently asked questions

Sources

1. California State Water Resources Control Board, Hexavalent Chromium (Chromium-6) (government or regulatory source)
2. California State Water Resources Control Board, Hexavalent Chromium MCL (SWRCB-DDW-21-003) rulemaking page (government or regulatory source)
3. California State Water Resources Control Board, Frequently Asked Questions: Maximum Contaminant Level (MCL) for Hexavalent Chromium (updated November 2024) (government or regulatory source)
4. US EPA, Chromium in Drinking Water (government or regulatory source)
5. NSF, NSF/ANSI 58: Reverse Osmosis Drinking Water Treatment Systems (standards or certification body)
6. Whirlpool Water Solutions, WHER25 Reverse Osmosis Filtration System Performance Data Sheet (manufacturer or vendor document)
7. California State Water Resources Control Board, Water Treatment Device Registration (government or regulatory source)