U.S. drinking water is permitted to contain up to 10 parts per billion of arsenic, according to the National Institute of Environmental Health Sciences. Some states have stricter standards, but Montana adheres to the federal level.

Tests by the Montana Department of Environmental Quality in 2015 showed high arsenic levels in rivers that drain from Yellowstone National Park into Montana, including 367 ppb in the Firehole, 197 ppb in the Gibbon and 300 ppb in the Yellowstone River.

In 2015 the Montana Department of Environmental Quality advised the Gardiner Water and Sewer district that sludge should be removed from its treatment ponds and new liners installed. But undertaking that task made no sense if the arsenic problem was not addressed.

After twice advising Yellowstone officials of the problem by letter with no response, the complaint said the park’s staff finally acknowledged its role for the issue during a meeting and agreed to help fund the sludge removal from the wastewater ponds, a project estimated to cost about $2 million.