Mountain View finds chromium-6 above state public-health goal but well below federal limit; treatment study estimated at $48 million

Mountain View — City staff reported to council Tuesday that routine testing of groundwater wells detected levels of hexavalent chromium (chromium‑6) above California’s public-health goal but far below the state maximum contaminant level used for regulatory compliance.

Utility Services Manager Mike Vasquez and Assistant Public Works Director Lisa Au told the council the report covers calendar years 2022–2024 and fulfills a required triennial update to the state on any exceedances of public‑health goals (PHGs) set by the California Office of Environmental Health Hazard Assessment.

What the tests showed: Vasquez said routine monitoring of the city’s four groundwater wells detected chromium‑6 concentrations in the range of about 0.36 to 1.6 parts per billion. Those results exceed the PHG of 0.02 parts per billion established by OEHHA — a conservative, non‑enforceable health goal meant to guide protective action. However the measured values were well below the state’s regulatory maximum contaminant level (MCL) of 10 parts per billion.

“The city’s water meets all regulatory water-quality standards,” Vasquez said. He added that the PHG exceedance triggers a reporting requirement, not an enforcement action, because the MCL — the enforceable standard — was not exceeded.

Staff analysis and options: The U.S. Environmental Protection Agency has identified anion‑exchange treatment as an effective technology for removing chromium‑6 from groundwater. Vasquez said the city’s preliminary, high‑level estimate for installing treatment at all four wells would be roughly $48 million in capital costs and roughly $9.6 million in annual operations and maintenance.

Vasquez cautioned that the studies used to create these estimates were aimed at meeting the MCL; meeting the much lower PHG would carry additional uncertainty and possibly higher cost.

Why the wells exist: Council members asked how and when the wells are used. Vasquez and Public Works staff said the four groundwater wells are exercised weekly for a few hours to keep them ready. They account for a small fraction of the city’s total supply under normal operations (staff estimated about 2 percent), but are a locally controlled resource and provide important resiliency in the event of a disrupted imported supply or a major earthquake.

Next steps: Staff told the council there is no recommended follow-up action required because the MCL is not exceeded. The city will report the PHG exceedance in the required triennial report and will continue annual monitoring and include the results in the city’s annual water quality (consumer confidence) report.

A selection of speakers: Mike Vasquez, utility services manager; Lisa Au, assistant public works director; Council members and the public asked questions about the role of the wells in emergencies and about the meaning of PHG versus MCL.

Ending: The council accepted the 2025 public-health goals report and asked staff to continue routine monitoring and to publish the full results in the consumer confidence report this summer.