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Around half of California waterways tested by regulators are contaminated with pesticides considered Pfas, “forever chemicals”, a new analysis of state and federal records shows, highlighting a risk in the substances’ wide use that is only beginning to come into focus.

The pesticides are linked to a range of health problems, including cancer, and the review is the first to systematically check for the dangerous substances in streams and rivers, which include drinking water sources.

More than half of sediment samples also showed some levels of the pesticides, which are increasingly applied to California and the nation’s food crops.

The review of California department of pesticide regulation and United States geological survey data was released this week, just days before a proposal to eventually ban Pfas pesticides failed to make it through the state assembly. However, pieces of the legislation, including a moratorium on approvals of the new pesticides, passed.

The findings are “alarming but not surprising”, said Varun Subramaniam, one of the report’s co-authors with the Environmental Working Group (EWG) non-profit.

“It’s concerning that we’re finding these levels of Pfas pesticide … but they were applied at really high rates on produce, so it makes sense that they’re in the streams and sediment,” he said.

Pfas are a class of at least 16,000 compounds most frequently used to make products water-, stain- and grease-resistant. They have been linked to cancer, birth defects, decreased immunity, high cholesterol, kidney disease and a range of other serious health problems. They are dubbed “forever chemicals” because they can persist for thousands of years in the environment, and are designed to be indestructible.

The chemicals are added as an active ingredient to food crop pesticides to kill weeds or insects, or used as an inert ingredient. At least 60% of active ingredients in pesticides fit the most widely accepted definition of Pfas, a 2023 analysis of EPA data found.

California farms applied an average of 2.5m lb of Pfas per year on cropland from 2018 to 2023, a review of state records last year revealed. Recent regulatory testing found the pesticide residues on 37% of all produce. But about 90% of peaches, plums and nectarines contained Pfas, while 80% of strawberries and grapes showed them.

Public health advocates have said the pesticides almost certainly pollute nearby water sources, and the new analysis confirmed their theory. Regulators only tested streams from 10 counties, and found the highest concentrations in agricultural areas, including Monterey and San Luis Obispo counties.

Previous research showed the highest applications of Pfa pesticides in Fresno and Kern counties, but water pollution data was not collected in those regions. The results are “almost certainly an undercount” because of the limited scope, Subramaniam said.

At least 10 Pfas in pesticides were identified across the state. The pesticide bifenthrin, which is among the most toxic and linked to cancer, was found in all San Luis Obispo waterways, and more than 80% of samples in Stanislaus county, which holds Modesto and portions of the Central valley.

The health threat is difficult to determine because pesticide laws do not require companies to assess many of the health threats common to Pfas, like immunotoxicity, reproductive harm, or hormone disruption, in part because the regulators do not have proper resources, Subramaniam said. Regulators also do not consider the cumulative effect of drinking water contaminated with the Pfas, then eating produce containing its residues.

“The fact that the chemicals are permitted is largely because we’re not considering all the ways that they can harm us,” Subramaniam added.

Amid pressure from the powerful agricultural lobby, the proposal to fully ban Pfas pesticides by 2035 failed, but the moratorium on new pesticides survived. The California Farm Bureau, in its opposition to the bill, wrote that the legislation is an “overly broad and unworkable approach that departs from science-based regulation”. It argued that California farmers would be put at an economic disadvantage with little health benefit to the public.

The legislation still includes a new requirement that pesticides come with warnings to farmers that alert them that they are using Pfas with health and environmental risks. Susan Little, California legislative director for EWG, said most farmers are unaware that they are spreading Pfas on food crops.

The legislation also gives local leaders more power to limit the pesticides’ use, and defines the pesticides as Pfas, which is in line with most scientific definitions. California’s pesticide office currently uses a narrow definition of Pfas that is favored by industry in part because it excludes smaller compounds like those used in pesticides.

Advocates say the legislation is especially needed with the Trump administration rushing to approve more Pfas pesticides for use. California’s pesticide office also recently reapproved the Pfas insecticide sulfoxaflor despite that its approval has repeatedly been struck down by state and federal courts over its high toxicity to honeybees and other pollinators.

Little said advocates are “disappointed” the ban did not make it into the final bill, but added the bill, if approved by the state senate, “will continue to address and reduce the use of Pfas pesticides in California”.