Follow-up tests last week found no arsenic in Mountain Island Lake, according to a report from the state Department of Environmental Quality.
Tests last month had found arsenic at nearly 10 times federal limits, near where Duke Energy was draining water from coal ash ponds at the Riverbend plant in Mount Holly.
Duke halted the draining, and Mecklenburg County officials said arsenic levels returned to normal. Duke said then it won't resume draining until it completes a water treatment system for the coal ash-laden water.
A DEQ spokeswoman says tests at four locations on July 7 confirmed county tests that no arsenic is in the lake.
Samples were collected from some of the same locations Mecklenburg County uses, the spokeswoman said. (See markers MI9, MI3D, RB1, and RB-UP on the map).
Duke had been draining the coal ash ponds since January, under a federal permit issued in 2011, and Mecklenburg County has been testing the discharges regularly.
Mountain Island Lake is the public water supply for Charlotte and Mecklenburg County as well as Gastonia and Mount Holly. County environmental officials have said the water supply was never contaminated.
Duke is closing Riverbend's coal ash ponds as it cleans up the old plant site. Ash is being shipped to a landfill in eastern north Carolina.