Cleanup work in Oak Ridge could shift from radiological contamination to mercury contamination, and a new $120 million water treatment plant at the Y-12 National Security Complex will help reduce mercury as workers tear down four contaminated buildings that were used to make nuclear weapons in the 1950s and 1960s, officials announced Friday.

“This water treatment plant is a major step in addressing one of the biggest problems we have from the Cold War era—mercury once used to make nuclear weapons getting into our waterways,” said U.S. Sen. Lamar Alexander, a Tennessee Republican. He said mercury contamination can cause brain and nervous system damage in people who eat contaminated fish.

Alexander was at Y-12 on Friday along with other federal and state officials to help announce the new water treatment plant, which will be at the head of East Fork Poplar Creek on the south side of Y-12′s main production area. The plant would be connected to a Y-12 storm water system, and it could begin operating in 2019. It would be able to treat 1,500 gallons of mercury-contaminated water per minute.

That is expected to be helpful as federal officials and contractors tear down the four buildings, which used mercury to separate lithium for nuclear weapons. Demolition work, which would include soil removal, could start in 2021 on the Beta 4 building at the west side of the West End Mercury Area and then move east to Alpha 5 and Alpha 4. Another contaminated building, Alpha 2, will also be torn down.

Alexander said the mercury was used in Oak Ridge a half-century ago as the United States built up its nuclear arms. There were once about 200,000 gallons of the toxic metal here, and about 18,000 gallons—enough to fill two gasoline tankers—have been lost to the environment or otherwise unaccounted for. Some of it ended up in the water, the senator said.

But most of the mercury is safely stored in casks, Alexander said. He said a review by Oak Ridge National Laboratory in 2009 said those casks shouldn’t leak.

Y-12 was built to enrich uranium for atomic bombs during World War II. The lithium separation operations started in 1955 and ended in 1963.

Mark Whitney, environmental management manager in the U.S. Department of Energy’s Oak Ridge Office, said mercury contamination at Y-12 is the greatest environmental risk on the Oak Ridge Reservation. He said remediation work began in the 1980s, and federal officials used about $250 million from the American Recovery and Reinvestment Act of 2009 on the work and to help design the new water treatment plant.

Susan Cange, deputy environmental management manager in the Oak Ridge Office, said the amount of mercury that flows into East Fork Poplar Creek varies, but the waterway generally has a concentration of about 400 parts per trillion. State water quality criteria call for a concentration of 51 parts per trillion or less, she said.

East Fork Poplar Creek starts at a spring at Y-12 and flows through Oak Ridge before joining West Fork Poplar Creek at the former K-25 site, now known as East Tennessee Technology Park. Twenty miles of the stream in Anderson and Roane counties have been included on a state list of impaired waterways due to mercury and polychlorinated biphenyls, among other pollutants, and a fishing advisory is in effect. The Tennessee Department of Environment and Conservation proposed including it again on the 2012 list.

Alexander said radioactive cleanup work at ETTP has involved hundreds of millions of dollars, and as more progress is made at the plant, including on demolition projects at the K-25 and K-27 buildings, a larger part of the roughly $400 million budgeted for environmental cleanup work in Oak Ridge each year should go toward mercury remediation.

“We have children who want to pick crawdads out of Poplar Creek and Tennesseans of all ages who want to eat the fish, but they’re warned off by signs because of dangerous mercury levels,” Alexander said. “We need to make cleanup of existing mercury contamination at the facilities at Oak Ridge a top priority in the future.”

In April, Alexander asked energy secretary nominee Ernest Moniz whether the cleanup of mercury contamination in Oak Ridge would be a priority under his leadership, and he asked Moniz to support the planned water treatment facility. Alexander said then that it will cost billions of dollars to clean up the contamination.

Other officials at Friday’s announcement included TDEC Commissioner Robert J. Martineau Jr., DOE Senior Advisor for Environmental Management David Huizenga, and Stan Meiburg, deputy regional administrator for the U.S. Environmental Protection Agency.