Residuals of Glyphosate Linger in California Creeks Nearly 2 Years After Spraying Halt

Nearly a year and a half after Orange County stopped spraying glyphosate in creeks that empty into its beaches, independent testing still found traces of the chemical.

The push to halt herbicide use has rippled throughout Orange County for months, leading many to question what could be lingering after public workers stop spraying. During the California Courier’s initial investigation into the county’s spraying, both major labs, Eurofins and Weck, turned down a request to sample soil and water from both the Santa Ana River and Doheny State Beach. Both labs have had contracts with the county.

Due to the denial over concerns about contracts, samples were sent to the Health Research Institute in Iowa to be studied for a variety of different chemicals.

Concerns over the amount of herbicides being sprayed first ignited in January after Creek Team OC, a resident-motivated group, started pushing the issue. Public pressure first focused on the Orange County Board of Supervisors and on Supervisor Katrina Foley, since Creek Team OC began its work in her district and tried reaching out to her office multiple times. 

By February, Foley addressed the public pressure in a statement, noting that she wanted to assure constituents that Roundup or glyphosate had not been “applied to the Trabuco or San Juan Creek flood control since January 2025” and wouldn’t be applied in the channels again. 

“The County is only permitted to apply State Water Board and EPA-approved herbicides to manage vegetation in flood control channels. We will continue balancing environmental needs while protecting lives and property from the threat of flooding,” her post stated. 

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The Results

In May, samples were collected from two downstream locations at opposite ends of the county. Both showed residual glyphosate and its primary breakdown product, AMPA. Glyphosate turns into AMPA as it breaks down, and that resulting product sticks around longer in soil and sediment than glyphosate itself.

At Doheny Beach, downstream of the San Juan and Trabuco system, water tested at a “trace” level of glyphosate. Soil from the same area contained 2.17 ng/g of glyphosate and 2.73 ng/g of AMPA. 

The same Doheny soil sample also showed a trace of TCP (3,5,6-trichloro-2-pyridinol), a breakdown product that can come from the insecticide chlorpyrifos and, in some cases, from other related chemicals like triclopyr. 

At the Santa Ana River outlet near the jetties, water showed no detectable glyphosate but 0.04 parts per billion of AMPA. Soil from that site had no detectable glyphosate and 2.59 ng/g of AMPA. 

The numbers are below California’s drinking-water limit of 700 parts per billion for glyphosate. 

While officials could potentially point to new findings arguing that the levels are too low to matter, those limits were built mostly on studies the manufacturers themselves submitted. Independent long-term research on repeated low-level exposure is thinner and hasn’t all landed in the same place.

The residuals raise additional questions, and Orange County has not publicly released independent testing of residual glyphosate or AMPA levels in these channels since January 2025. 

The Concerns

Glyphosate usually breaks down in weeks to a few months. AMPA, the broken-down product, lasts a lot longer. Field studies have found that AMPA can show up in soil and sediment many months later—sometimes more than a year after the last spraying. 

What those lingering amounts could mean for health is still contested. The International Agency for Research on Cancer, part of the World Health Organization, classified glyphosate as “probably carcinogenic to humans,” pointing mainly to links with non-Hodgkin lymphoma. 

However, regulatory agencies like the Environmental Protection Agency have disagreed, stating the chemical is not likely to cause cancer at typical exposure levels. 

California’s drinking-water standards for glyphosate track the federal standards, basing the limits primarily on non-cancer endpoints. At the same time, the state previously listed glyphosate under Proposition 65 in 2017, marking it as a chemical known to the state to cause cancer following the cancer research agency’s classification. 

The dual posture leaves a gap: The chemical is treated as a carcinogen for warning purposes, but the standard allowed for drinking water remains relatively high. 

Some independent studies have also raised questions about glyphosate’s effects on hormones, the gut microbiome, and development. But those findings are not settled and have not been adopted into the current drinking-water limits.

As for AMPA, it has been studied far less than glyphosate. 

TCP also has its own set of questions. According to data, it tends to last longer in the environment and move more easily through soil and into water than some of the parent chemicals it comes from. 

Lab studies on TCP have shown liver and kidney effects in mice after repeated oral exposure, with other liver cell studies showing signs of oxidative stress, DNA damage, and cell death.

Currently, the EPA and International Agency for Research on Cancer do not classify TCP as a known or probable human carcinogen. However, there are no long term human studies. Studies on the chemical show it can be toxic to aquatic life, including fish and invertebrates.

The Background

For years, Orange County Public Works has sprayed herbicides across the county’s flood control channels, stating it is necessary to keep vegetation from blocking water flow. Public records reviewed by the Creek Team OC put the annual total at more than 100,000 gallons for some years, with some accounts close to 150,000 gallons. 

The chemicals used throughout the years have included glyphosate—the active ingredient in Roundup—triclopyr, imazapyr, and other herbicides, along with additives meant to help them stick and spread. Those channels then drain toward the ocean, including into the same system that empties at Doheny State Beach. 

Notably, the same San Juan and Trabuco creeks are also the focus of a long-running effort to restore habitat for the endangered Southern California steelhead trout. 

Federal and state agencies have poured over $20 million into the protection of the endangered steelhead trout for over two decades, seeking to build fish passage and recovery projects in the watershed.

In addition to the glyphosate pause in 2025, Foley announced a broader countywide pause on herbicide use in flood control channels.

With concerns around herbicide use reigniting in July at a recent Ladera Ranch town hall focused on a cluster of rare childhood cancers, Foley said she has long opposed the use of herbicides in public spaces and waterways.

Board records from 2021 and 2022, however, show the Board of Supervisors—including Foley, Supervisor Don Wagner, and Supervisor Doug Chaffee—approved amendments that continued to fund the county’s main herbicide application contracts during those years.

Board documents show the primary herbicide application contracts were authorized at nearly $3 million between 2018 and 2023. Actual spending under those contracts reached about $1.47 million by late August 2023, according to records. 

The California Courier reached out to the Orange County Board of Supervisors, Orange County Public Works, and Foley’s office for comment, but none responded.