The USGS reports that past fireworks displays are the probable cause of elevated concentrations of perchlorate in groundwater and surface water within Mount Rushmore National Memorial. It is a compound used as an oxidizing agent in fireworks, and is a common component in rocket fuels, explosives and certain nitrogen fertilizers. At high levels in drinking water, it can interfere with the function of the human thyroid gland. However, the drinking water at Mount Rushmore meets current regulations and is safe for public consumption. Aerial fireworks displays took place at the memorial around the 1998-2009 Independence Days. The USGS and National Park Service studied perchlorate and metals associated with fireworks in 106 water and 11 soil samples taken from Mount Rushmore during 2011-2015.
Perchlorate concentrations were greatest in samples collected from the northeast side of the memorial, and the scientists found perchlorate in soil where the fireworks were launched and where the debris landed. The lack of alternative perchlorate sources in the area, such as a military site or agricultural land with applied fertilizers, and the presence of firework debris suggest that past fireworks are the perchlorate source, said Galen Hoogestraat, a USGS scientist and the lead author of the report. Their results can help park managers protect water quality at this iconic national landmark. The main findings include: A maximum perchlorate concentration of 54 micrograms per liter (ug per L) measured in a stream sample, and 38 ug per L measured from a monitoring well. In contrast, all groundwater and stream samples collected from sites outside the memorial boundary had perchlorate concentrations less than 0.2 ugL. Concentrations of metals in water samples were similar at sites within and outside the memorial, indicating little evidence of metal contamination due to human-made factors. Dynamite that was used to blast the memorial in the 1930s is not a likely source.
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