Elevated levels of arsenic have been found in Mountain Island Lake, Charlotte’s main water supply, near Duke Energy’s Riverbend power plant.
Two ponds at Riverbend hold 3.6 million tons of coal ash, which contains arsenic. Duke has been draining water from the ponds in preparation for excavating the ash.
The cancer-causing element was detected near the plant on June 20 at 95 parts per billion, Mecklenburg County officials say. Duke acknowledges even higher levels recently, fluctuating between 100 and 150 ppb.
The state safety threshold for arsenic in surface water such as lakes is 10 ppb.
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Mecklenburg County officials say there is no threat to drinking water because arsenic was not detected in the lake, including Charlotte’s water intake, other than near the power plant.
Duke Energy says it expects the state to issue a permit allowing arsenic at up to 72.5 ppb in an area near the power plant where water drained from the ash ponds mixes with lake water.
When county tests confirmed an upward trend in arsenic levels last month, Duke said, the company stopped draining water from the ponds. Arsenic levels in the lake fell, the county says.
A treatment system to clean water from the ash ponds before it is drained into the lake will start operating in September, Duke spokeswoman Zenica Chatman said. Discharges into the lake will resume then, she said.
County officials also found elevated levels of arsenic in the lake near the power plant on April 27, but levels dropped in May.
Water tested 50 feet from the plant’s discharge point in April held arsenic at 35.5 ppb. No arsenic was detected in the lake farther from Riverbend’s discharge.
The 95 ppb level found in June is well above the limit state regulators are expected to permit. Mecklenburg County reported the elevated reading to the N.C. Department of Environmental Quality, which regulates Duke’s discharges.
Elevated arsenic levels were also detected in the lake near Riverbend in 2010 and 2011, Mecklenburg County water quality chief Rusty Rozzelle said.