State and federal regulators have approved the design for the cover that will go over part of the U.S. Department of Energy’s desert site after crews are done removing transuranic waste from it.

The U.S. Environmental Protection Agency and state Department of Environmental Quality approved the plans for the subsurface disposal area cover earlier this summer, said Erik Simpson, spokesman for cleanup contractor Fluor Idaho.

The entire footprint of the cover is 157 acres, Simpson said, including the 97-acre subsurface disposal area, the landfill where crews have been digging up decades-old Cold War-era waste that came from nuclear weapons production in Rocky Flats, Colo., for processing and disposal. The point of the cover is to prevent the remaining buried waste from leaking into the East Snake River Plain Aquifer, an important source of irrigation and drinking water for southern Idaho that lies 585 feet under the site, by preventing rain and snowmelt from trickling through the remaining debris and contaminated soil.

“When the state, EPA and DOE conducted the environmental investigation of the entire 97-acre subsurface disposal area, the cover was a common element in all the alternatives that they evaluated,” Simpson said. “That tells you just how important this cover is to put over the remaining waste in the subsurface disposal area. It’s vitally important to the completion of this project to have that cover in place, and having an engineered drainage system channels everything, to channel that water away from the subsurface disposal area.”

The plan is to knock down the Accelerated Retrieval Project and Advanced Mixed Waste Treatment Project buildings in that area that have been used for waste processing and also cover those sites. Simpson said he doesn’t have a completion date yet for waste processing at the Accelerated Retrieval Project IX building but he expects it to be in late 2020.

The evapotranspiration cover will be about 10 feet thick. It is designed to withstand a 1,000-year flood event and has a canal system to channel excess water away from it, Fluor said in a news release. Two ancient dry lake beds near the landfill will provide some of the more than 3.5 million cubic yards of material that will be needed to build it.

“What’s been decided is, because of the arid environment we live in out here, a cover consisting of native soils and rocks would be the most effective at minimizing the infiltration of water through the remaining buried waste,” Simpson said.

After six years of litigation and negotiation, Idaho reached an agreement with the DOE and EPA in 2008 to remove waste from a section of the subsurface disposal area most likely to contain transuranics, uranium and some other targeted waste. Crews have removed more than 9,000 cubic meters of targeted radioactive waste so far from a 5.69-acre area. Fluor said 88 percent of the targeted waste has been removed and a little more than two-thirds of an acre is left to exhume. Workers are eight months ahead of schedule, Fluor said.

Construction of the cover will start after the targeted waste is all exhumed and the buildings are knocked down. It is expected be in place by 2028.

Reporter Nathan Brown can be reached at 208-542-6757. Follow him on Twitter: @NateBrownNews.