As of October 1, 2024, the California State Water Resources Control Board has implemented a new Maximum Contaminant Level (MCL) for hexavalent chromium (Cr6) in drinking water, set at 0.010 mg/L, or 10 µg/L (ppb). This MCL represents the maximum allowable concentration of Cr6 in public drinking water systems, addressing health risks posed by this known carcinogen. While chromium can exist in the less toxic trivalent form, hexavalent chromium is linked to increased cancer risk and liver toxicity.
California’s regulation of Cr6 now includes both a specific Cr6 MCL of 10 ppb and a broader total chromium MCL of 50 ppb, which was established in 1977 to cover both trivalent and hexavalent forms of chromium. California remains the only state with its own total chromium MCL.
Treatment and Compliance Requirements
The most effective technologies identified to reduce Cr6 in water include ion exchange, reduction-coagulation-filtration (RCF), and reverse osmosis (RO). Under the new regulation, all community and non-transient, non-community (NTNC) water systems are required to:
- Complete Initial Sampling by April 1, 2025: Initial samples can include any taken within two years prior to this deadline.
- Submit Compliance Plans: Systems exceeding the Cr6 MCL must submit a compliance plan within 90 days of compliance dates
Monitoring and Compliance Timelines
- Systems that meet or fall below the MCL must monitor once every three years (groundwater) or once every year (surface water) and may apply for a monitoring waiver after three compliant rounds. Systems above the MCL must monitor quarterly, with the option to reduce frequency after two compliant rounds. Any treated water must be sampled monthly.
- Compliance Timeline: Smaller systems with fewer than 1,000 service connections must comply with the MCL by October 1, 2028, while larger water systems have earlier compliance timelines.
For further details, visit the California State Water Resources Control Board’s hexavalent chromium page.
Need Support?
Is your water system preparing for Cr6 monitoring and compliance? Our Water System Operations & Maintenance (O&M) Team is ready to assist with water quality monitoring, treatment solutions, compliance preparation, and regulatory guidance. Contact us for support today.
