Chromium 6 Informational Page

Hexavalent Chromium (Cr6)

Chromium is found naturally in rocks, plants, humans and animals. The most common forms of chromium that occur in natural waters are trivalent chromium (chromium 3) and hexavalent chromium (chromium 6). Hexavalent chromium, in high doses, can be identified as a carcinogenic. However, Patterson’s water has always had a small trace amount of chromium 6 that is naturally occurring and not caused by industrial contaminants.

The City of Patterson's water supply currently averages between 14.2 and 20.3 parts per billion – far BELOW the levels set by the State and current Federal limits. The following information on chromium levels can be helpful to keep in mind: 

  • 100 parts per billion (ppb) is the federal maximum contaminant level (MCL) set by the EPA for total chromium. Total chromium includes both trivalent chromium (chromium 3) and hexavalent chromium (chromium 6). 
  • 50 ppb is the total chromium MCL for the state of California set by the State Water Resources Control Board, Division of Drinking Water (DDW), and is still in effect. Patterson is in compliance with this standard. 

2014 California Regulation for Hexavalent Chromium 

In 2014, the State of California created a new maximum contaminant level, or MCL, for hexavalent chromium. The new MCL was adopted at 10 micro-grams per liter, or 10 parts per billion (ppb). DDW was careful to point out that the new 10 ppb MCL did not create an urgent drinking water crisis. Any potential health risks regarding Cr6 in drinking water would require decades of drinking large volumes of drinking water with elevated levels of Cr6. Therefore, DDW determined that communities with groundwater wells exceeding the new MCL could continue to use those wells while they implemented proposed system improvements to address Cr6 under compliance plans approved by DDW. 

State Rescinds 2014 MCL for Hexavalent Chromium

In May 2017, the Superior Court of Sacramento County issued a judgment invalidating the new California MCL of 10 ppb for Cr6. The court determined the State Water Resources Control Board (Water Board) failed to properly follow the requirements of the Safe Drinking Water Act for establishing the new MCL. The court ordered the Water Board to delete the new hexavalent chromium MCL from the California Code of Regulations. This change became effective on September 11, 2017. Therefore, as of September 11, 2017 the 10 ppb MCL for Cr6 is no longer in effect. The regulation of hexavalent chromium in drinking water reverts back to the previous State of California MCL of 50 ppb for Total Chromium, to which the city is in compliance. 

Recent Cr6 Exceedance

Following the court’s decision, the State Water Resources Control Board undertook a new rulemaking process, including revised health risk assessments and cost feasibility studies. In 2022, the Board proposed a new Cr6 MCL, and after a public comment period and further revisions, the revised Cr6 MCL regulation was adopted and went into effect in 2024. The reestablished MCL for Cr6 remains at 10 ppb.

As a result of this regulatory update, the City resumed Cr6 monitoring in accordance with state requirements. Based on the most recent sampling data, Well No. 07 has exceeded the new Cr6 MCL. Although this compliance order currently applies only to Well No. 07, it is anticipated that the remainder of the City’s potable wells will also exceed the Cr6 MCL when they are sampled again in August 2025. Therefore, the Division has recommended that the City’s compliance plan proactively address the likelihood of future exceedances at additional wells, even if they have not yet officially triggered compliance action.

Cr6 MCL Exceedance Letter May 2025

Initial Notification PPT 20250603

Q4-Tier 2 Public Notification
Q4-Tier 2 Public Notification (Spanish)

Chromium 6- Approved Compliance Plan 20260109

What We Are Currently Doing

The City submitted a Compliance Plan outlining how it would address Chromium-6. A draft was submitted to Division of Drinking Water (DDW) on May 22, 2025 and after several revisions, a plan was approved on January 9, 2026, with a final compliance deadline of September 2030. We will continue to issue quarterly public notifications following each sampling event. While the notifications are drafted by the State, the city updates the sampling dates, sampling results and any new activity related to mitigation. The public will be notified quarterly until mitigation is complete. Primary and secondary notifications will occur through the following methods:

  • Mail or direct delivery (utility billings)
  • Publication in a local newspaper. 
  • Posting in public places (library and front of city hall)

Feasibility Study- One of the steps for compliance was an update to the city's Feasibility Study. The study is in final draft form; however, several elements must be finalized before the City can issue an official recommendation. However, the city has narrowed the treatment options to three primary alternatives:

  • Centralized Treatment of Lower and Upper Aquifer Wells using Reverse Osmosis (RO)
  • Surface Water Treatment Plant utilizing California Aqueduct water with Interim Centralized or Wellhead Weak-Base Anion Exchange (WBA) Treatment
  • Surface Water Treatment Plant utilizing Local Irrigation District water with Interim Centralized or Wellhead WBA Treatment

Water Rate Impact- Using the same consultant that prepared the 2022 Water Rate Study, each alternative was evaluated to assess its potential financial impact. A total of seven scenarios were analyzed to determine how each option would affect residents. Based on these evaluations, the projected costs to residents could be substantial over the planning period (FYE 2027–2036). The lower range reflects interim or centralized wellhead treatment options, while the higher range represents alternatives involving advanced treatment methods, such as Upper Aquifer Reverse Osmosis or imported surface water supplies. In most cases, costs are expected to rise in the later years as projects move from design and planning to full implementation and operation. These costs would be added on top of current water bills. The City plans to reduce this financial burden by applying for grants and exploring regional projects that could help grow the customer base.

Funding Efforts- A general application for the Drinking Water State Revolving Fund (DWSRF) was submitted to the Water Board on March 19, 2025. To advance in the process, a complete application package must be submitted; however, this requires an engineer’s report, which in turn depends on identifying the selected treatment solution for Chromium-6. The project has been included on the SFY 2025-26 DWSRF Comprehensive List.

Public FAQs CY23-24

To visit the state's website regarding chromium 6, click here.