PFAS in Clark Fork River fish prompts consumption advisory
Laura Lundquist
(Missoula Current) Recent sampling around Montana has found “forever chemicals” in some fish tissue, so the state is advising against eating too much fish from some locations, including the Clark Fork River.
After state biologists caught and sampled fish for polyfluoroalkyl substances, or PFAS, in various locations in 2023 and 2024, the results prompted the Interagency Fish Consumption Advisory Group to issue new fish consumption advisories for certain waterbodies. The Interagency Group includes representatives of the Montana Department of Public Health & Human Services, Department of Environmental Quality and Fish, Wildlife & Parks.
Fish in the Clark Fork River are already known to be affected by mercury from mine waste, PCB’s, and wood-processing chemicals including furans and dioxins. Now, perfluorooctanesulfonic acid, or PFOS, has been added to the list of pollutants, affecting fish in the river below the confluence with the Blackfoot River. However, the addition of PFOS didn’t change the fish consumption advisories for the Clark Fork, which were issued in 2021 after the discovery of significant levels of wood-processing chemicals.
In the Clark Fork River between the Blackfoot and Bitterroot rivers, women and children should avoid eating more than one 14-18” rainbow trout per month, and men can eat one 10-14” trout or two 14-18” trout. However, downstream of the confluence with the Bitterroot River, no one should eat any species of fish.
Technicians also detected PFOS in various fish species in stretches of the Yellowstone, East Gallatin and Missouri rivers and Prickly Pear Creek, and in Fort Peck Reservoir, Lake Helena, and Nelson Reservoir.
PFAS are called “forever chemicals” because their synthetic bonds break down extremely slowly in the environment. PFOS, a specific group of PFAS compounds, were manufactured between the 1940s and the 2000s as a repellent coating that is resistant to water, heat, stains or grease. PFOS were used for firefighting foams, stain-resistant fibers in carpets and clothes, food packaging and industrial plating.
Due to their wide use and persistence, they’ve unfortunately made their way into the nation’s waters, affecting an estimated 20 million Americans. Following scientific studies that found PFAS contamination may be more widespread than previously understood, DEQ and FWP partnered on a targeted study that began in 2023. The 14 sample locations were selected based on proximity to confirmed or potential sources of PFAS and distributed across important regional fisheries.
That same year, the Environmental Protection Agency proposed to regulate PFAS in drinking water, because new technology can filter out the chemicals. In April 2024, the EPA concluded there is no safe level of PFAS exposure, which has been shown to promote the development of certain cancers, interfere with the reproductive system and fetal development, cause developmental effects or delays in children, reduce the ability of the immune system to fight infections, and disrupt the body’s natural hormonal system.
However in September 2025, the Trump administration announced it would no longer enforce rules to monitor and clean PFAS pollution in drinking water.
Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.