High Arsenic Concentrations Lurk In Underground Lakes At Wind Cave National Park

Underground lakes at Wind Cave National Park are beautiful, but also high in arsenic, so no sampling! NPS photo.

Coming upon an underground lake at Wind Cave National Park can be stunning, a pool of water surrounded by rock. But don't think about sampling the water, as it likely is high in arsenic.

A four-year study by the U.S. Geological Survey and National Park Service detected the high arsenic levels, which are associated with the surrounding geology of the Minnelusa Formation. As part of the study, the scientists collected 100 water samples from 60 sites in and around the park from sources such as stream sinks, cave drips, cave water bodies, springs, and wells.

The arsenic concentrations are thought to have originated from shale layers in the formation, according to a release from the park.

“Our drinking water has always been in compliance, including for arsenic. This report talks a lot about water in the cave, but our drinking water doesn’t come from the cave,” stressed Wind Cave Superintendent Vidal Davila. “We regularly work with the State of South Dakota Department of Environment and Natural Resources to ensure visitors and employees continue to have safe drinking water at Wind Cave National Park.”

The park has been using a groundwater well for its water supply for more than 50 years. The U.S. Environmental Protection Agency lowered the maximum contaminant level of arsenic in 2006 from 50 parts per billion (ppb) to 10 ppb. Since 2003, the well water has been tested nine times for arsenic with only one reading over 10 ppb, according to park officials. That reading of 11 ppb occurred in 2004, before the new regulations went into effect, and upon retesting the level was back down to 8 ppb.

Groundwater is an important resource for the park and, in particular, for Wind Cave. Groundwater drips from the ceiling of the cave at numerous locations and exists in ponds, lakes, and streams on the cave floor. For more information, or to read the entire report, visit this site.