Initial test finds ‘forever chemical’ in Mount Rushmore drinking water at level exceeding new limit

The busts of U.S. presidents George Washington, Thomas Jefferson, Theodore Roosevelt and Abraham Lincoln tower over the Black Hills at Mount Rushmore National Memorial on July 2, 2020, near Keystone. (Scott Olson/Getty Images)

A sample of Mount Rushmore National Memorial’s drinking water had levels of a “forever chemical” exceeding new limits established by the federal government.

Perfluorooctane sulfonic acid, or PFOS, is a member of the synthetic chemical group known collectively as perfluorinated alkylated substances, or PFAS. The chemicals have been used in industry and consumer products since the 1940s and don’t break down easily in the environment or in the human body. Research indicates PFAS exposure may be linked to negative developmental and reproductive effects, and an increased risk of some cancers.

A 2023 test of Mount Rushmore’s drinking water showed a PFOS concentration of 9.8 parts per trillion, more than two times higher than the Environmental Protection Agency’s new limit of 4 parts per trillion. The EPA finalized limits for several types of PFAS last month.

The result doesn’t necessarily mean Mount Rushmore is out of compliance with the EPA’s new rule, which will be implemented in phases. Current testing is preliminary. Tests won’t count toward the new limits until 2027, and the EPA will use annual running averages to determine compliance. The EPA won’t begin issuing violations until 2029.

The current sampling is part of a multi-year, nationwide testing effort by the EPA. The South Dakota Association of Rural Water Systems is conducting a majority of the testing in the state.

In publicly available results published so far, Mount Rushmore is the only site in South Dakota to test above the new EPA limits for forever chemicals. Additional Mount Rushmore sampling results will be published in the coming weeks and months. Elsewhere in South Dakota, some of the chemicals have been detected at levels below the new EPA limits. 

PFAS from firefighting foam was previously known to have contaminated groundwater at military installations in or near locations including Rapid City and Sioux Falls, leading to mitigation efforts in those areas.

In an emailed statement, Mount Rushmore National Memorial spokesman Earl Perez-Foust said the National Park Service is monitoring the results and considering any mitigation that may be necessary.

“This could include treatment or considering a new water source,” Perez-Foust said. “Public health and safety is always our top priority.”

Reverse osmosis, granular activated carbon, nanofiltration and other methods have been identified as methods of removing PFAS from drinking water, according to the EPA.

The exact source of the contamination at Mount Rushmore is unknown, said Galen Hoogestraat, a hydrologist for the U.S. Geological Survey’s Dakota Water Science Center. 

“In general, PFAS sources can come from anywhere humans are interacting with the environment: food wrappers, water-resistant clothing, common products and waste,” Hoogestraat said.

For over a decade, Hoogestraat has studied perchlorate contamination in Mount Rushmore’s groundwater and local streams from former fireworks displays at the memorial. He said the amount of perchlorate in the water has “dropped substantially” in the last decade.

Hoogestraat said the memorial provides water to over 2 million visitors every year from a “very small postage stamp of an area in the Black Hills,” because the memorial is limited to using water from within the park boundaries.

That source is a fractured rock system that collects rain and groundwater, which makes it susceptible to contamination.

“There’s very little soil on top of the rocks, so there’s very little filtration of anything that comes from the surface — good or bad,” Hoogestraat said.

That can create volatile test results, since concentrations of contaminants can vary based on the seasons and weather conditions. 

“There needs to be more sampling done to assess the variability around this,” Hoogestraat said, “and wrap our arms around the trends of this: Is this a long-term, persistent thing, or will this be variable over time?”

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