Workers on Saturday plugged a broken pipe that leaked up to 82,000 tons of power plant coal ash into a North Carolina river, Duke Energy Corp said.
Crews filled part of the stormwater pipe with concrete and capped it, and the rest of the pipe will be filled with concrete as well, said Duke, the biggest U.S. electric power provider.
"Plugging the pipe was clearly job one, but we're continuing our efforts and working closely with all the agencies involved in this response," Charlie Gates, senior vice president for power generation operations, said in the statement.
Duke's next step is to keep monitoring water quality, he said.
The ash release was discovered on Sunday at the power plant in Eden, North Carolina, which was retired in 2012. The company said the pipe under a 27-acre (11-hectare) ash pond released enough coal ash to fill between 20 and 32 Olympic-size swimming pools.
Up to 27 million gallons of ash basin water also reached the river, Duke said.
Before the pipe was sealed, Duke installed a catch basin and pumps at its end to reduce outflow to the river. Samples by Duke and the state Department of Environment and Natural Resources show that water quality is improving, the company said.
Waterkeeper Alliance, a water advocacy group, said this week that analysis of Dan River samples showed "extremely high levels" of arsenic, chromium, iron, lead and other toxic metals typically found in coal ash.
A Duke Energy spokesman has said tests by the utility and North Carolina officials indicated no adverse impact on the water supply.