Water district plans more testing, considers treatment of ‘forever chemicals’ in Big Sioux River
- Steve Jurrens
- Apr 12
- 3 min read
By: South Dakota Searchlight Makenzie Huber - April 12, 2026 - 10:00 am:
The highest concentrations of “forever chemicals” in the Big Sioux River are in Sioux Falls and just south of Watertown, according to a report from South Dakota Mines.
The study, conducted by East Dakota Water Development District and researchers at the school, tested water for per- and polyfluoroalkyl substances (PFAS) at 13 sites from northeast South Dakota to the Iowa border between September 2024 and the end of 2025.
The chemicals have been used in industry and consumer products — from nonstick cooking pans to firefighting foam — since the 1940s and don’t break down easily in the environment or in the human body. PFAS exposure may be linked to negative developmental and reproductive effects, and an increased risk of some cancers, according to the Environmental Protection Agency.
The federal government finalized limits on some types of PFAS in drinking water in 2024, with some changes announced in 2025 that will see EPA reconsider limits on some classes of PFAS but retain others. The 2025 changes also extended compliance deadlines and offered more options for exemptions for water systems in municipalities experiencing “economic hardship” and those that “lack of alternative water sources.” The limits will be implemented in phases, starting in 2027.
The maximum contaminant limits don’t apply to surface water, East Dakota director Jay Gilbertson told South Dakota Searchlight previously, but managers of public drinking water systems located near the river should be aware of them. Under the right conditions, Big Sioux River water can infiltrate wells near the river.
The report presented on Thursday showed 15 types of PFAS compounds in the Big Sioux River, some in concentrations higher than the EPA drinking water limits.
Eleven of the sites along the river were tested five times, in September 2024, followed by April, June, August and October 2025. Two sites were added after the initial September test and were only tested in the 2025 dates.
High concentrations were found consistently at two Sioux Falls locations: at Falls Park and near the Sioux Falls Regional Airport. One compound was detected at higher levels than the drinking water limit near Watertown.
The highest amounts of PFAS tied to firefighting foams in the report were detected near the airport, with samples showing concentrations of up to 25 times the drinking water limit.
The second-highest concentration of the compounds was found at Falls Park, also over the drinking water limits, but not by as much. A still-lower concentration was detected once at a site south of Watertown, putting that site’s sample over the drinking water limits in the September 2024 test.
PFAS was previously detected in wells near the Sioux Falls Regional Airport and its co-located Air National Guard base, where the chemicals were used in firefighting foam. Sioux Falls suspended the use of 21 wells suspected of PFAS contamination after those earlier detections. Since then, all raw water samples collected by the city, which can include water from the Big Sioux River, have been tested for PFAS compounds.
A forever chemical has been detected in drinking water wells near Ellsworth Air Force Base at Box Elder, at National Guard facilities near Custer and in Rapid City, and in three private wells in South Dakota. A federal test of drinking water sources identified forever chemicals at Mount Rushmore in 2024.
“Don’t drink the water at Mount Rushmore,” Gilbertson said. “Or make sure you see it come out of a bottle that was sealed.”
Lisa Kunza, South Dakota Mines professor and lead researcher, said her team is developing a water filtration system for PFAS, which was designed for use at Ellsworth Airforce Base. Her team could test the system on the Big Sioux River, she said.
Gilbertson said the water development district plans to add PFAS sampling to its 2026 collection routine. The South Dakota Department of Agriculture and Natural Resources is conducting its own study of PFAS in South Dakota surface waters, which is expected to conclude this year.
