ESCONDIDO -- The Rincon del Diablo Municipal Water District has become the third agency in the county to agree to buy desalinated ocean water from a plant proposed for Carlsbad.

Under the agreement approved unanimously by the district board on Tuesday, Rincon will buy 4,000 acre-feet of desalinated ocean water a year over 30 years from the plant, which is proposed by Connecticut-based Poseidon Resources. The district consumes about 8,000 acre-feet a year.

The district will pay no more for the water than it would pay the San Diego County Water Authority, according to district officials.

The district will not actually receive the desalinated sea water, but Poseidon will arrange with another agency to swap the desalinated water for treated water available from other sources.

Carlsbad and the Valley Center Municipal Water District also have signed agreements to purchase water from the $270 million plant, which Poseidon plans to build by 2009.

Poseidon proposes to desalinate 56,000 acre-feet of ocean water a year at the plant, proposed for the Encina Power Station at Cannon Road and Carlsbad Boulevard.

The desalination plant is undergoing environmental review by the city of Carlsbad, and the county water authority is reviewing a similar proposal.

An acre-foot is the amount of water it takes to cover one acre one foot deep, or about 326,000 gallons, which is enough to supply two average households a year.

Rincon Municipal Water District encompasses 42 square miles in an area that encircles Escondido and includes portions of that city, San Marcos, San Diego and unincorporated San Diego County. It serves about 28,000 people.