Water from one of the Mariposa Public Utility District’s four wells has tested above California’s response level for the PFAS chemical PFHxS in every notice the district has sent since March 2025, and its plan puts construction of a treatment system in 2029 and 2030.
The latest notice, distributed Oct. 14, reported that Well 06 measured 37 nanograms per liter in the third quarter of 2026, compared with a response level of 20.
Customers in distribution zone 1, the town area, and zone 2, the Smith Road area, received it.
PFAS, short for per- and polyfluoroalkyl substances, are manmade chemicals that resist water, grease and stains and break down very slowly, which is why they are often called “forever chemicals.” PFHxS is one specific PFAS.
All notices above the level
The district’s first notice, distributed March 7, 2025, said Well 06 measured 34 nanograms per liter.
It was one of 25 PFAS compounds tested, and PFHxS was the one above the response level, the notice said. The district was testing under a 2024 state order.
MPUD’s General Manager Susan Wages said the district first got the result back in a letter dated December 2024, saying it had found the chemical measured at 34 nanograms per liter.
MPUD was then allowed to do a resampling and received results on Jan. 28, 2025 measuring at 38 nanograms per liter.
Six more notices have followed, with results between 32 and 40 nanograms per liter.
Under the state order MPUD’s notices cite, a system whose source exceeds a response level must take the source out of use, provide treatment or blending, or notify the public within 30 days, according to the order’s text.
Wages said MPUD is unsure whether the 30 days began Jan. 28 or whether the order applied to the district then.
What customers were told
People who are pregnant or breastfeeding should not drink the water or use it to make infant formula, the notices say.
PFHxS has been shown to interfere with thyroid hormone levels, which are needed for normal growth and development before birth and in infants and children.
Anyone with other health concerns may want to consult a doctor.
The notices say boiling, freezing, filtering or letting the water stand does not reduce PFHxS.
Wages said the chemical can be filtered out some ways, but residents should make sure any filters used are rated to remove it.
‘Recommended, really not enforceable’
California set a notification level of 3 nanograms per liter and a response level of 20 for PFHxS in 2022.
The state’s own order describes them as nonregulatory, health-based advisory levels and they are “recommended, really not enforceable,” said Robert Brownwood, assistant deputy director for the Division of Drinking Water at the State Water Resources Control Board.
What is required comes from the state’s monitoring orders.
Brownwood said systems have 30 days to notify the public and the board.
Wages said the well is in service “all the time” and has not been put off service.
Blended, but not tested
MPUD blends Well 06 with water from the surface water treatment plant and from Well 05, which Wages said have shown no PFAS. The district tests Well 06 quarterly and recently tested its surface water sources, finding none, she said.
It does not test the blended water because it isn’t required to. “The levels are not going to go up because it’s blended. They’re going to go down,” she said.
Brownwood compared blending to baking. If a recipe calls for a teaspoon of salt, you mix it in and don’t check each cookie.
“If they were to do it (blending) by pipeline, that’s a different story,” he said.
In a hypothetical, a well at 40 mixed with two cleaner wells would end up under 20, and testing the blend might not be required.
How MPUD blends matters.
Irrigation and the source
Asked whether irrigating with the water could spread PFAS, Wages said she didn’t know, noting that MPUD supplies drinking water, not irrigation water.
Brownwood said “yes, it would spread,” but compared the amount to a cookie with a little salt and ice cream on top, meaning it becomes heavily diluted.
“You can even drink it,” he said. However, he said children should avoid it as they are in their developmental stages and the water might affect them.
Wages said no source has been identified. The state is doing studies, she said, but there is no definitive answer for MPUD’s well and any investigation would be up to the state.
A delayed study and a 2031 deadline
The State Water Board arranged for Stantec Consulting Services to study treatment at no cost to MPUD or its customers.
The options are ion exchange, granular activated carbon, a new well and consolidation with a nearby water system, according to MPUD’s notices.
The study has slipped. MPUD’s June, September and December 2025 notices said it was due Feb. 27, 2026. A July 2026 notice listed a draft Sept. 4 and a final study Dec. 4, with a close-out Feb. 5, 2027.
Stantec will present its findings at a public meeting Tuesday, Dec. 1, at 6:30 p.m. at Mariposa County Fire Headquarters, 4802 Highway 140, Wages said, and she hopes the district’s board will vote that night on moving forward.
Wages said the timeline calls for engineering and design in 2027, a pilot study, design completed in 2028 and construction around 2029 into 2030, with treatment in place by 2031, when state technical assistance funding ends.
The district hasn’t applied for construction money. Wages said Stantec will help with applications for state grant money, available because Mariposa is a disadvantaged community and the cost will depend on which treatment the board picks.
“We’re working on this as quickly as we can,” she said.
State and federal rules
California says it intends to set an enforceable limit for PFHxS.
At the federal level, the EPA set a limit of 10 nanograms per liter in 2024 and proposed rescinding it in May.
Residents with questions can call MPUD at (209) 966-2515. Notices are posted at mariposapud.org/pfas-public-notifications.
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