A Marin Superior Court judge issued a tentative ruling Monday invalidating the Marin Municipal Water District's approval of a $105 million plan to desalinate bay water and pipe it to Marin homes.

Judge Lynn Duryee ruled the water district failed to comply with the California Environmental Quality Act "in several respects" when it approved an environmental impact report for the desalination plan. In particular, the district didn't properly describe the environmental setting near the project nor fully assess its impact on marine species, the judge noted.

The judge went on to write that the desalination plan is "unnecessary because water conservation costs nothing."

A court hearing on the issue will begin at 9 a.m. Tuesday, Aug. 16 in Courtroom L at the Hall of Justice.

"We are pleased with the court's ruling and expect that it will be affirmed (Tuesday)," said Steve Volker, attorney for the North Coast Rivers Alliance and other parties that challenged the desalination approval.

MMWD General Manager Paul Helliker was in the process of reviewing the ruling, but said he was confident the EIR meets state requirements.

"We will be prepared to provide evidence to show that we are meeting the requirements in the law," he said, adding that despite Duryee's assertions, water conservation costs the district several million dollars that pays for education and rebates, among other aspects.

If Duryee affirms her ruling, the water

district could appeal or fix the aspects of the EIR deemed to be flawed.

In September 2009, the North Coast Rivers Alliance and several other plaintiffs filed a lawsuit to halt progress on desalination. The suit alleged the proposed desalination plant would induce population growth of up to an additional 85,000 people in Marin, discharge up to 30 million gallons of harmful brine into San Rafael Bay, increase energy consumption by the water district by up to four times and expose the public to potential contamination.

It also alleged the environmental impact report on the project was flawed and asked the court to halt any work toward desalination, which the water district has described as a "drought proof" source of water and could save the county in case of severe drought.

The water district's Board of Directors last year decided to halt further work on the desalination project until it could get a better handle on declining water demand. The project remains on hold.

Last November, voters approved Measure S — placed on the ballot by the district board — which requires a vote of the public before the district could proceed with construction of a desalination plant, but it still allows the water district to plan for the technology.

It was prompted by Measure T, an initiative placed on the ballot by desalination critics. It was similar to Measure S in that it also would have required a vote of the public to proceed with construction of desalination, but it differed in that it would prohibit the district from approving any contracts related to planning or building a plant.

Measure S received more votes in the election, superseding Measure T.

A 5-million-gallon-per-day desalination plant, expandable to 15 million gallons per day, was the option selected by the board in August 2009. The desalination plan also includes a water conservation component. The desalination plant would take San Rafael Bay water and subject it to various forms of treatment to produce drinkable water through reverse osmosis technology.