DETROIT LAKES, Minn. - The Fourth of July fireworks display near the Pavillion on the shore of Detroit Lake draws throngs of spectators who line the beach for a mile while others crane their necks from their perches on an armada of pontoon boats.

The annual aerial display, which has grown over the decades, is reflected on the water, adding a shimmering mirror image to the pyrotechnics. Smaller versions, private displays from the hundreds of people who live around the lake, add more celebratory color and noise.

"It's gorgeous," said Brad Wimmer, a member of the Lake Detroiters Association and a homeowner on Little Detroit Lake. "It's absolutely gorgeous."

But, he added, referring to the debris cast off from the aerial explosives, "It all goes into the water."

So does a chemical called perchlorate, salts of potassium and ammonium that serve as the primary oxidant in fireworks. Perchlorates are an emerging concern, and the Environmental Protection Agency classified them as a contaminant in 2011.

Perchlorates pose a health concern because they can interfere with the production of thyroid hormones.

A study published in 2007 found that perchlorate concentrations rose sharply after fireworks displays, with levels spiking from 24 to 1,028 times their average baseline levels 14 hours later.

Repeated fireworks displays over the same site, allowing pollutants to accumulate, can result in contamination. Fireworks displays were routinely part of Independence Day celebrations at Mount Rushmore in the Black Hills of South Dakota, for instance, from 1998 to 2009.

A study by the U.S. Geological Survey and National Park Service found perchlorate and metals associated with fireworks in 106 water samples and 11 soil samples taken between 2011 and 2015. Officials attributed the contamination to pyrotechnics, citing firework debris.

The fireworks shows were stopped at Mount Rushmore because of wildfire worries, but the perchlorate and metals remained.

So far, at least, perchlorate tests in Minnesota and North Dakota have not detected the compound, or the tests have found it in concentrations below the level of concern, officials said.

Minnesota drinking water systems tested in 2001 and 2002, both large municipal and a sampling of smaller community water systems, did not have any confirmed detections of perchlorate, said Cathy O'Dell of the Minnesota Pollution Control Agency.

The MPCA does not monitor for perchlorate in the air and neither the Minnesota Department of Health nor the EPA have developed "inhalation health benchmarks" for the chemical, she said.

"It is likely that some of the perchlorate released during a fireworks display could be deposited in nearby water bodies, which might be used for drinking water," O'Dell said. "However, as fireworks are only set off a few times a year, they represent an intermittent contaminant source, at best."

Karl Rockeman, director of water quality for the North Dakota Department of Health, said 13 drinking water systems were tested for perchlorate around 2000 to 2001.

"We have done some sampling for perchlorates," he said. But perchlorate was not detected in any of the water systems, Rockeman said, and to date "is not a concern" in North Dakota.

"It certainly hasn't been an issue for Fargo," said Troy Hall, the city's water utility director.

In Moorhead, testing around 2001-2002 found no perchlorate four times, and detected concentrations below the level of concern once, said Kristofer Knutson, the city's water division manager.

"Perchlorates are an issue throughout other areas of the United States, but it's never been an issue here," he said.

In Canada, perchlorates were found in Lake Huron and Lake Erie, among other sites, and were detected at several sites, including Hamilton Harbor, where Canada Day fireworks displays are ignited.

Perchlorate was detected four days after the Canada Day show, but was not detected at the same site after a week had passed, showing that concentrations can be quickly diluted in large bodies of water.

In California, concerns about ocean pollution caused by major fireworks displays resulted in a threatened lawsuit in 2006, prompting SeaWorld San Diego to cancel its fireworks show that year.

Fireworks displays have also caused air pollution concerns. A study published in 2005 found that pollutants like dust, dirt and soot present in the air increased by 42 percent, on average, across the U.S. on July 4. Monitoring in Ogden, Utah, has shown the worst air quality happens on July 4.

Wimmer said possible contamination of water is something the Lake Detroiters Association likely will discuss at a future meeting.

"Is there a problem?" he said. "I've never thought about it."