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Past fireworks displays are the likely cause of contaminated water at the Mount Rushmore National Memorial, where an estimated 3 million visitors a year may have sampled the tainted water, according a report released by the U.S. geological Survey.

Though Mount Rushmore’s drinking water remains safe for infrequent public consumption, scientists have found it to have high concentrations of perchlorate, a compound common in fireworks and fertilizers that can be toxic to humans at certain levels.

Project leader Galen Hoogestraat, a USGS hydrologist based in Rapid City, said perchlorate can cause malfunctions in the thyroid gland, but only if ingested regularly over a long period of time.

“If a person lived on that property and drank that water constantly, that’s where it becomes a risk,” Hoogestraat said. “There’s not an acute risk. If you go there and fill up a water and drink it you’ll be fine. There’s not a danger in drinking small amounts.”

Scientists with the USGS found .20 to 38 parts per billion of perchlorate in groundwater samples at Mount Rushmore, said Mount Rushmore public information officer Maureen McGee-Ballinger. Though the Environmental Protection Agency does not currently regulate perchlorate, it recommends that levels not exceed 15 parts per billion.

Independence Day fireworks displays containing perchlorate were common at Mount Rushmore between 1998 and 2009, according to a statement released by the USGS on Monday. 

In partnership with the National Park Service, the USGS studied perchlorate in 106 water and 11 soil samples taken from Mount Rushmore between 2011 and 2015. The highest concentrations were found in the West Fork Lafferty Basin and in soil where fireworks are known to have been launched, Hoogestraat said.

“The lack of alternative perchlorate sources in the area, such as a military site or agricultural land with applied fertilizer, and the presence of firework debris suggest that past fireworks are the perchlorate source,” Hoogestraat said.

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According to the USGS press release, roughly 3 million visitors and park personnel drink water at Mount Rushmore every year. Increased monitoring and treatment efforts will be put in place at Mount Rushmore as a result of the USGS report.

“For people living in the park long term we’ve added some reverse osmosis units as a safety option,” McGee-Ballinger said.

“The park will continue to strive to provide safe drinking water that meets and exceeds current standards,” said Mount Rushmore National Memorial superintendent Cheryl Schreier.

McGee-Ballinger said there are no plans to allow future fireworks displays at Mount Rushmore.

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Education/County Reporter

Education/county reporter for the Rapid City Journal.