The monthly flea control you squeeze between your dog’s shoulder blades is built to spread. It collects in the oil glands of the skin and seeps back into the coat for weeks, which is how it reaches the flea. It also comes off on your hands, into the dog’s bed and down the bathtub drain. On July 31, state regulators said they have five years of sampling that shows where it goes next.
The California Department of Pesticide Regulation released its final risk memo on fipronil, the active ingredient in many over-the-counter spot-on flea and tick products. The department posted the memo along with its written answers to seven comment letters filed on the December draft. Both documents are on the department’s website here and here.
The finding, in one line: the insecticide reaches wastewater treatment plants, the plants can’t remove much of it, and what comes out unscathed at the other end is routinely above the level federal regulators consider safe for insects and crustaceans at the bottom of the food chain.
Between June 2019 and May 2024, the department’s wastewater monitoring program sampled 33 treatment plants in 10 California counties — 241 samples of what went in, and 241 of the effluent, the treated water that comes back out.
Fipronil turned up in 100% of the samples going in, and it was still detectable in 93% of the effluent. In 81.3% of those treated samples, the concentration was above 11 nanograms per liter — parts per trillion — which is the lowest benchmark the U.S. Environmental Protection Agency uses for chronic harm to freshwater invertebrates. The median was 21.7 ng/L. The highest single reading was 411.
Better wastewater treatment doesn’t solve the issue. Published removal rates run from statistically indistinguishable up to 36%, the memo says, and one plant using advanced tertiary treatment managed to clear only 24%.
Ukiah runs an advanced treatment plan
Under discharge order R1-2024-0016, Ukiah sends tertiary-treated effluent to the Russian River during the wet-weather season, Oct. 1 through May 14, and disinfected secondary effluent to percolation ponds beside the river the rest of the year. Willits discharges enhanced secondary effluent to Outlet Creek, a tributary of the Eel, on the same seasonal calendar, and its permit bars that discharge from exceeding 10% of the creek’s flow.
Whether either plant is among those the state sampled isn’t public. The memo doesn’t name the 10 counties, and the regions it does list — Southern California, the Bay Area, the Central Coast and the Central Valley — don’t include the North Coast. Either way, the study measured what leaves treatment plants, not what’s in Mendocino County’s creeks and rivers.
Fipronil washes off in the bath, on your hands after you apply it and in the wash when the dog’s bedding goes through the machine. Timing is most of the story. Research the department cites found that 21% of the dose came off a dog bathed two days after treatment, and 4% came off a dog bathed at 28 days. The department calculated that 3.5% is enough to push a receiving stream past the benchmark for acute harm.
Meanwhile, sales have climbed. Pet products account for about 56.7% of all fipronil sold in California, and annual sales of the pet formulations rose from an average of 16,600 pounds a year between 2013 and 2019 to 41,600 between 2020 and 2022. Fourteen California water bodies are already listed as impaired for fipronil under the federal Clean Water Act, including the Petaluma River and Nathanson Creek in Sonoma County,
Nothing changes on a shelf yet. The memo carries no label change and no use restriction. The department said it will next produce a mitigation scoping document and an assessment of alternatives, both expected to go out for public comment in the third quarter of this year.
The comment letters ran in both directions. One argued the department’s model is stacked with conservative assumptions and overstates the risk; the department answered that its case rests mainly on the monitoring data, not the model.
Others urged the state to cancel the products outright, or reclassify them so only veterinarians and licensed applicators could apply them. One commenter warned that one in four pet-owning households in California can’t afford veterinary care for pest control, and that cheap over-the-counter products are what those households can use.
For anyone who wants to act ahead of the state, the Bay Area Clean Water Agencies, a joint powers authority consortium of the five largest wastewater treatment plants in the Bay Area, has issued some guidance. Ask a veterinarian whether a chewable or injectable works for your animal, skip sprays and foggers, wash pet bedding, vacuum often and use a flea comb. Its list of topical ingredients to avoid includes fipronil — and imidacloprid, another systemic insecticide, which one commenter singled out for the department as the likely substitute if fipronil goes away.
This story originally appeared in The Mendocino Voice.
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