Marin desalination prospects fade in favor of imports

The Marin Municipal Water District is moving away from plans to acquire temporary desalination plants and instead is exploring purchasing more water from Sonoma County during the winter months.

“We are determining that this is really not a feasible approach for the current drought where winter water does seem promising,” Paul Sellier, the district operations director, told the board on Oct. 19.

The district, which serves 191,000 residents in central and southern Marin, faces the potential of depleting local reservoir supplies as soon as next summer if this winter is as dry as the last. The recent storms have put the district in a better starting position, but district staff said reservoir levels are still well below average.

The district’s main backup plan is to potentially construct a pipeline across the Richmond-San Rafael Bridge to bring in water purchased from the Sacramento Valley by next summer. The project could cost up to $90 million.

However, the East Bay Municipal Utility District, which would partner with the Marin district to bring the water over the bridge, said it can only pump in about 60% of the 13.5 million gallons per day that the pipeline could carry because of water pressure issues.

Marin water officials said they would need about 10,000 to 15,000 acre-feet of water per year to ensure residents have enough water for vital indoor uses such as cooking, sanitation and hygiene. Outdoor uses such as lawn watering would be banned.

The pipeline would only be able to bring in about 9,000 acre-feet of water with the East Bay district’s limitations, requiring Marin to find additional sources to make up the difference.

The Marin Municipal Water District has been looking into purchasing or renting between two to three portable desalination plants as another water supply option. All three plants would be able to provide about 5.4 million gallons of treated San Francisco Bay water per day, or about 7,000 acre-feet of water per year.

The three facilities would cost about $35 million to rent and $6 million per year to operate, according to district staff.

However, the Marin Municipal Water District staff has been in discussions with the Sonoma Water agency to purchase more Russian River water during winter and spring when flows are naturally higher. Sonoma Water already provides about 25% of the district’s water supply.

This option would allow Marin to secure the water without having to make significant investments and undergo a lengthy permitting process among state and federal agencies, according to Sellier.

Sonoma Water engineer Donald Seymour said the Russian River water that would be provided to Marin would otherwise flow out to the ocean.

“We don’t have another reservoir to put it in, but MMWD can use it to offset the use of their own reservoirs and make sure they fill and have adequate water supplies for the summer,” Seymour said.

Sellier said Marin could secure about 2,000 to 6,000 acre-feet of additional Russian River water through the North Marin aqueduct between October and May.

“The facilities already exist to deliver it,” Sellier told the board.

Seymour said the district would be paying the same rate as it already does under its agreement. The agency charges Marin Municipal Water District about $1,200 per acre-foot of water.

Unlike desalination, where there is a seemingly endless supply of water to treat, Marin would be relying on adequate flows in the Russian River. In August, the state ordered 1,500 water rights holders to stop diverting water from the Russian River in response to low reservoir levels.

But Seymour said there are usually natural excess flows in the river during the winter even during dry years. And the agency would still work to ensure that state-mandated minimum flows for wildlife are met.

“These are flows that are above and beyond during the winter that would just end up flowing out into the ocean,” Seymour said. “We’re not talking about huge amounts of water either.”

Seymour said the agreement would be similar to what the North Marin Water District did last winter when it bought 1,100 acre-feet of Russian River water to put into its Stafford Lake reservoir.

One issue that would need to be worked on before winter water purchases begin is Sonoma Water’s agreement to reduce water diversions, including those to Marin, by 20% until Dec. 10. The reduction was part of an agreement with the state that allowed the agency to release less reservoir water into the Russian River for wildlife in order to preserve more water for people.

Seymour said the agency is working with the State Water Resources Control Board to make an exemption for high winter flows.