Sampling Lafferty Gulch surface water that was analyzed for perchlorate and metals on July 10, 2012. (Brian Engle/USGS)

Fireworks at Mount Rushmore were probably a bad idea to begin with. In South Dakota’s woodsy Black Hills, the thousands of onlookers who flocked to the display had to use the same entrance, which is also the exit.

What if the pyrotechnics sparked a forest fire? And what if there were some other emergency? Those are two questions that officials at the national memorial site grappled with during the 11 years starting in 1998 that the event was held. But there was at least one other threat, something officials at the national memorial didn’t consider until a U.S. Geological Survey investigation recently uncovered it in the drinking water.

The fireworks were discontinued in 2011 but they left high levels of a chemical called perchlorate in water used by 3 million people who visit the memorial yearly. The Environmental Protection Agency doesn’t regulate the chemical under the Clean Water Act, but Galen Hoogestraat, the lead author of the investigation, said the agency does provide guidelines that say that any presence over 15 parts per billion is a possible health risk.

According to the report issued two weeks ago and announced by USGS on Monday, percholate in the groundwater is many times higher than that. “We’re finding concentrations of over 50 parts per billion,” said Hoogestraat, a USGS hydrologist in South Dakota. 

Perchlorate can interfere with the function of the human thyroid gland when consumed at high levels. It’s used as an oxidizing agent in fireworks, and is commonly found in rocket fuel, explosives and fertilizer containing nitrogen, the USGS said in a statement. The statement also hastened to say that Mount Rushmore’s drinking water meets current health regulations.

But those regulations don’t account for perchlorate. At any rate, people who visit the area for a few days don’t drink enough water for the chemical to have any effect. However, the thousands of people who live in the Black Hills, including workers at the memorial, might understandably have a concern, Hoogestraat said.

Officials at the memorial, which partnered with USGS to produce the study, said they recognize that. “The park will continue to strive to provide safe drinking water that meets and exceeds current standards,” said Cheryl Schreier, the Mount Rushmore National Memorial superintendent. She called the USGS “an excellent partner” that used “sound science throughout the process of sampling, analyzing, reviewing and finalizing” its report.

For employees, Hoogestraat said, the memorial has “point source devices” — water filters clamped on faucets — to remove chemicals such as arsenic, and can be adjusted to remove perchlorate.

Hoogestraat and his co-author, Barbara Rowe, traveled to the site to investigate whether high concentrations of a pesticide used to fight pine beetle infestation and oil from parking lots were contaminating the water. Later, he recalled, they threw perchlorate into the mix of items to test, kind of as an afterthought.

As it turned out, the entire investigation centered around that chemical. At first, they wondered where it came from. Because no military facilities were nearby, and because perchlorate isn’t found in dynamite used to blast away rock in the Black Hills, they knew it had to come from fireworks. That presented another problem, because the fireworks hadn’t been used for five years.

“When we started putting the pieces together and digging further, we found that perchlorate didn’t degrade,” Hoogerstraat said. “Once it’s in the water, it stays there.”

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