Mount Rushmore hasn't held an Independence Day fireworks display for almost seven years, but a new study says those fireworks are still having an impact on the park.
A study released Monday by the U-S Geological Survey says the chemical perchlorate, used as an oxidizing agent in fireworks, is still being found in groundwater and surface water in the park.
The fireworks were held at Mount Rushmore starting in 1998, with the last display in 2009.
The study says they found high levels of perchlorate in 106 water and 11 soil samples taken over a five-year period and says they were highest in the northeast side of the memorial, where the fireworks were launched and where debris landed.
USGS hydrologist Galen Hoogestraat says, "The debris would fall onto the ground surface and infiltrate down into the ground water. And once the contaminant's into the groundwater, this particular contaminant, perclorate, is very mobile and persists. It can stay in the groundwater for a very long time. What we expect to see over time, the concentration should get lower as the water continues to be purged out."
Hoogestraat says the drinking water at Mount Rushmore is safe for visitors and meets all regulations.
He says the main danger of perchlorate is lifetime exposure.