Problems With Perchlorate

For years, the Mount Rushmore National Memorial held a fireworks show on Independence Day, but U.S. Geological Survey (USGS) scientists later found the celebrations may have contributed to groundwater contamination at the site. USGS hydrologist Galen Hoogestraat spoke with WQP Associate Editor Bob Crossen about the study that revealed the perchlorate problem at the memorial.

Bob Crossen: Tell me a little bit about this study. When and why was it conducted? 

Galen Hoogestraat: This study originated from discussions between USGS and Mount Rushmore staff in 2011 regarding concerns about the water resources in the area. Initially, the biggest concern was with the pesticide carbaryl, which Mount Rushmore was using to control the spread of mountain pine beetles that were decimating certain parts of the ponderosa pine forest in the Black Hills. The two other issues addressed in the water quality sampling plan were fireworks (perchlorate) and runoff from the large parking lot structure (BTEX–benzene, toluene, ethylene and xylene). After initial sampling in the summers of 2011 and 2012—funded by Mount Rushmore—it became evident that perchlorate concentrations were of concern, but there were not detections of carbaryl or BTEX compounds.  

Crossen: What did USGS learn from the study at Mount Rushmore? 

Hoogestraat: Perchlorate concentrations were greatest in the Lafferty Gulch basin, ranging from less than 0.2 to 38 ppb in groundwater samples and 2.2 to 54 ppb in surface water samples. All groundwater and surface water samples at reference sites outside the park boundary had perchlorate concentrations less than 0.2 ppb. Perchlorate concentrations in samples collected at the groundwater wells used for public supply ranged from 2.1 to 38 ppb with a median of 17 ppb. Perchlorate concentrations in the treated water supply were similar to the concentrations from the supply wells. 

Crossen: How are fireworks related to the water contamination found at the memorial?

Hoogestraat: Perchlorate is a common component in firework displays, such as those at Mount Rushmore from 1998 to 2009. The main perchlorate contamination was clearly isolated to one drainage basin within the park boundary (Lafferty Gulch), where the trajectory of the firework displays carried the fallout and debris. Based on this information, and lack of alternative perchlorate sources, fireworks were indicated as a probable source of the perchlorate. However, this is still open to interpretation if additional evidence comes to light. 

Crossen: In what ways can contaminants at the site affect humans? What about wildlife? 

Hoogestraat: Perchlorate can interfere with function of the thyroid gland in humans at high levels when consumed regularly. The U.S. Environmental Protection Agency (EPA) does not currently have a Maximum Contaminant Level established for perchlorate; however, EPA established an Interim Lifetime Drinking Water Health Advisory of 15 ppb. Perchlorate is not considered carcinogenic. There hasn’t been much research on potential effects on wildlife or aquatic species.  

Crossen: How does this study relate to July Fourth celebrations throughout the country? 

Hoogestraat: Although not common, several other cases of perchlorate issues in groundwater and surface water following firework shows have been reported, and each presents its own unique hydrologic circumstances (our report also references cases in Oklahoma and Massachusetts). In some situations, contamination is not a concern because the water is not consumed or is not susceptible to surface infiltration of contaminants. An important message is to understand where important water resources are located and if they are susceptible for contamination from any source, such as pesticides and fertilizers, not just focusing on fireworks.

Crossen: How will Mount Rushmore conduct Independence Day celebrations in the future? 

Hoogestraat: Mount Rushmore still provides a special Independence Day celebration at the memorial, but it is unknown if fireworks will ever return. This perchlorate study was not the reason fireworks stopped at Mount Rushmore. Firework displays at the memorial pose a large risk to public safety in terms of forest fire danger and emergency evacuations of the large crowds. 

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About the author

Galen Hoogestraat is hydrologist for USGS. Hoogestraat can be reached at [email protected].