The Board of Selectmen has chosen an engineering firm to design a new ground-water treatment plant on Lary Lane. The plant is intended to relieve stress of the Exeter River, the town’s primary water source. It will be designed by Weston & Sampson at a cost of $566,700. The money will come out of the $6.35 million approved for the plant at least year’s Town Meeting. Construction is expected to begin in 2014.