NORTH COUNTY ---- A private company studying the idea of building a Carlsbad plant that would turn millions of gallons of seawater a day into drinking water announced Tuesday that it had added Valley Center as a partner.

Officials said the deal ---- along with tentative deals with two other local water agencies ---- could bring the "critical" project at Carlsbad's Encina Power Plant closer to reality.

But the deal casts new doubts on long-running, stalled talks between the company, Connecticut-based Poseidon Inc., and the regional San Diego County Water Authority, which also wants to build the plant.

It also raises questions about political infighting among the county's autonomous water agencies, which sometimes clash over water issues more than they work together. The Water Authority supplies 90 percent of all the water county residents use annually.

On Tuesday, Poseidon officials announced that the Valley Center Municipal Water District had signed a deal to buy about 6.7 million gallons of water a day from the proposed plant if it is built.

However, Valley Center would never actually get any water from the desalination plant in the convoluted deal because there are no pipelines running to the inland North County community from Carlsbad.

Instead, a second, still-unidentified water agency would take the desalinated seawater that Valley Center buys rather than the water it would normally buy from the regional Water Authority.

Valley Center would then get 6.7 million gallons of the Water Authority's water, through the authority's pipelines. Gary Arant, Valley Center's general manager, said the water would be the same price as what it buys from the Water Authority, but would be a more "reliable" supply.

Poseidon officials Tuesday said the addition of Valley Center to the city of Carlsbad, which has signed on to buy roughly 19.6 million gallons a day, pushes the plant closer to reality.

The company, which started studying the idea in 2001, and which has a 60-year leasehold on the Encina plant site, has said it would be too expensive to build a plant unless it had commitments to buy "somewhere north" of 25 million gallons a day. However, company officials have said in the past they would prefer to build the plant to a 50 million gallons-per-day size.

Arant said Tuesday that there were still several conditions that would have to be met before the deal ever became reality.

But Arant said it was a "good deal" for Valley Center. The local, desalted water would account for 15 percent of the water that the agency supplies its ratepayers, and be more reliable than the imported water it now gets from the Colorado River and Northern California.

However, the deal also raised questions about whether the Water Authority was losing the support of its member agencies ---- which had until now largely ignored Poseidon's overtures, to let the Water Authority hammer out its own deal.

Poseidon Senior Vice President Peter MacLaggan said Tuesday that the company had also reached tentative deals with Escondido's Rincon del Diablo Municipal Water District and Encinitas' Olivenhain Municipal Water District that still must be finalized.

MacLaggan said Poseidon ---- having been unable to work out a deal with the Water Authority for a large regional plant ---- had decided to negotiate with all sides, and it didn't care if it did the deal with the Water Authority or its member agencies.

Over the last few years that Poseidon and the Water Authority have conducted often tempestuous, on-again, off-again negotiations, many experts have said that the Water Authority was the only agency large enough to reach a deal to build the plant.

Through its collective membership, the Water Authority represents the kind of water demand that could support the plant. And the agency owns the pipelines that would be needed to deliver the plant's water around the county to the agencies that want it.

On Tuesday, Water Authority officials downplayed the notion that Valley Center's deal could hurt its negotiating power with Poseidon.

Ken Weinberg, the Water Authority's director of water resources, said Tuesday that Poseidon still needed the Water Authority.

"The fact that Poseidon is still at the table with the Water Authority indicates that a regional project has the most certainty," Weinberg said.

Poseidon's agreements with Carlsbad and Valley Center do contain very interesting ---- and perhaps, risky ---- conditions for the company.

Poseidon has agreed to sell Carlsbad and Valley Center its desalted seawater at the same price the Water Authority charges for its imported water ---- now $565 per acre-foot ---- even though the desalted water will cost more, $861 per acre-foot, to produce.

MacLaggan said the company is offering the incentive because it wants to get the plant built ---- and because Poseidon believes imported water will eventually cost more than the desalted seawater, allowing the company to get its money back.

Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com.