The USGS study, in cooperation with the National Park Service, revealed that arsenic concentrations in Wind Cave groundwater exceeds 10 micrograms per liter, the Maximum Contaminant Level for drinking-water supplies set by the U.S. Environmental Protection Agency. The high arsenic concentrations are likely naturally occurring and originated from shale layers in overlying rocks.
"The fact that the source of the contamination is from the rock layers through which the ground waters naturally flow means that it is not possible to remediate the arsenic in the water," explained USGS Director Marcia McNutt. "This study points out the fact that part of what makes the park a geologic wonder also creates challenges for the water supply."
The study also found that the water may be flowing among various connected cave water bodies within the park. Groundwater in Wind Cave drips from cave ceilings and exists in underground streams, pools, and lakes. As part of the study, a harmless dye was injected into one of the Wind Cave lakes. The dye was subsequently detected in two other cave water bodies within the park, traveling fast at a minimum velocity of nine meters per day.
"The rate of flow is very fast for groundwater," said Dr. Andrew Long, USGS hydrologist and lead author of the report. "This indicates that groundwater in Wind Cave is connected to a unique aquifer consisting of a network of underground pipes."
According to a further groundwater chemistry analysis, the water in Wind Cave is a mixture of local precipitation that has infiltrated the ground and groundwater inflow from multiple aquifers.
For more information, visit the USGS South Dakota Water Science Center website.