Two environmental groups filed a lawsuit Monday to overturn the conditional approval given to a long-discussed Carlsbad plant that would turn seawater into "drought-proof" drinking water.

The lawsuit was filed in San Diego on behalf of the nonprofit Surfrider Foundation and the Planning and Conservation League

It alleges that the California Coastal Commission acted illegally when it granted a permit to the proposed $300 million plant despite acknowledging that commissioners needed more information about how the plant would minimize harm to marine life and offset greenhouse gases.

Poseidon Resources Inc. officials say they are still working with the commission on those demands for additional information. Poseidon, the city of Carlsbad and several local water agencies that have signed deals to take the plant's water were also named in the suit.

Surfrider representatives say the desalination plant would hurt marine life and Agua Hedionda Lagoon, where the plant would be located, and that Poseidon hasn't proved otherwise.

"The substantive issue of whether they can even mitigate (the harm) is still out there," said Marco Gonzalez, the lawyer representing the environmental groups.

Poseidon officials deny that. Poseidon Vice President Peter MacLaggan said that over the course of eight years of study, the company has proved the plant will not harm the environment.

He said the company is simply working out details with the Coastal Commission and that the suit was without merit.

"They're challenging eight years of environmental research and study by pre-eminent scientists in this field from Scripps that has been reviewed by the various permitting agencies, who all came to the same conclusion - move forward," MacLaggan said.

At the commission's November hearing, Poseidon said it would reduce the greenhouse gases that the desalting plant would emit each year in part by spending money to replace old, smog-producing equipment at other facilities with newer, more efficient equipment.

Poseidon said it planned to offset the harm it would do to marine life gets sucked out of the ocean and run through desalting filters by creating 37 acres of wetland habitat in a joint San Dieguito River Valley program.

Coastal commissioners granted Poseidon a conditional permit. But, they said, Poseidon's plans lacked enough detail to determine if they would work.

Coastal Commission Executive Director Peter Douglas said Monday that Poseidon still has to have those "detailed" plans - which are expected to be reviewed within the next six months - approved by commissioners before Poseidon can build.

"We can't issue the permit until those conditions are met," Douglas said.

MacLaggan said Poseidon is sure that the company will get its plans approved.

He said the company hopes to clear the remaining regulatory hurdles, start building by the end of this year and start churning out water by 2010.

"The bottom line here is, the region's dependent on this new source of drought-proof supply from the Pacific ocean," he said. "What we see here in this lawsuit is a desperate, last-ditch effort to slow down a project that inevitably will be needed to shore up the region's water supply."

Meanwhile, one official from a local water agency that has signed a deal with Poseidon blasted the environmentalists' opposition.

"It's very frustrating to be a water agency doing the very best to supply … the needs of your community," said Gary Arant, general manager of the Valley Center Municipal Water District. "It seems at every turn, there's somebody standing in your way.

"My question to these groups would be, 'If not desal, then what?' " he said. "They'll say 'conservation.' But we think we're all doing a pretty good job in terms of conservation. So it's very frustrating."

The lawsuit was filed just before the 60-day deadline allowed to challenge the Coastal Commission's Nov. 15 conditional permit.

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