Raleigh, N.C. — A deal reached between Raleigh and the U.S. Army Corps of Engineers marks the first time the Corps has agreed to drain water from the sediment layer of a lake.

City officials wanted to tap the bottommost layer of Falls Lake as an emergency reservoir in case its water supply runs out during the spring or summer.

Levels in Falls Lake have dropped 8½ feet below normal, and officials believe the Raleigh's normal allocation of water from Falls Lake will extend until mid May.

The lowest level of Falls Lake is usually set aside to collect sediment that flows into the lake from upstream, but the soft bottom could contain up to 8 billion gallons of water. That would be enough to supply Raleigh for two to three months.

The Army Corps of Engineers agreed to help Raleigh tap into the sediment layer in stages and then store that water for up to $1 million a month. Whether the state or city will bear that cost has not yet been decided.

Water from the sediment layer will probably require more chemical treatment to make it safe for drinking.

Tapping into Falls Lake's sediment is only a temporary solution, said Tom Fransen, with the state Division of Water Resources.

Developing a plan for the project will take some time, Fransen said, because the Army Corps of Engineers has not previously drained water from a lake's sediment layer. Raleigh's attempt to do so could set a precedent for other drought-affected cities in the country.

Officials must also take into account concerns about water quality, fisheries and other environmental issues.