North Carolina officials admitted on Sunday that estimated levels of arsenic in the Dan River taken two days after a massive coal ash spill at a Duke Energy plant last week were wrong and that the water was in fact highly toxic and unsafe for public exposure.

The state’s Department of Environment and Natural Resources previously said water tests showed levels of arsenic to be at safe levels. However, the Department now admits those levels were four times higher than safety standards.

Officials claim the false readings were a mistake.

While officials said Saturday that the leak is officially plugged, environmental groups warn that massive damage has already been done.  More than 82,000 tons of toxic ash is now contaminating the Dan River in North Carolina and Virginia, which, according to eyewitness accounts, has filled the river with an unnatural gray sludge.