The Marin Municipal Water District is revisiting the possibility of constructing a desalination plant, this time taking a closer look at the cost to produce drinking water during a drought.
The reason for the focused cost analysis is because while a desal plant could provide a great supply, the district has limited storage capacity, Paul Sellier, water resource manager, told the board of directors at its meeting on Tuesday.
“Our reservoirs are going to be spilling,” Sellier said. “So we’ll be operating this plant in years when we don’t really need to.”
With that in mind, staff presented findings where the cost of water produced by such a plant is based on a four-year drought scenario. The analysis also assumes a 25-year service life before plant upgrades would be needed.
The cost for drinking water purchased from Sonoma Water is around $1,600 an acre-foot, which is equal to about 326,000 gallons. A district study shows that it could cost $273 million to $401 million to construct a desalination plant. Annual operations and maintenance would range between $13.4 million for a 5,601-acre-foot-per-year capacity and $30 million for a 16,802-acre-foot capacity.
Annually, it would cost $5,100 per acre-foot for the lower capacity project, versus $3,100 per-acre foot for the higher capacity plant.
However, assuming a four-year drought, the estimated cost per acre-foot range changes to $10,000 for the highest yield to $15,300 for the lower yield, according to staff.
The district serves 191,000 residents in central and southern Marin. Its seven reservoirs make up about 75% of the district’s water supply. The reservoirs can hold up to about 80,000 acre-feet of water, about a two-year supply.
The district launched a study into new water sources in 2022 after facing potential reservoir depletion from the drought. Rains in late 2021 nearly refilled the district’s reservoirs, giving the county’s largest water supplier more time to study the costs and benefits of potential new sources of supply.
The resulting study is the water supply roadmap that was approved last year. In addition to desalination, the plan explores expanded recycled water opportunities, conveyance and storage.
For comparison, staff showed that recycled water projects under consideration range from $4.3 million to $452 million to construct, with an annual expense of $18,700 to $65,200 per acre-foot during a four-year drought. Water conservation efforts could cost about $12,100 per acre-foot during drought conditions.
“Getting a real cost of water would kind of require us to have that crystal ball,” Sellier said. “We would need to know when’s the drought coming, how severe is it going to be, how much are we going to spill certain years and not others.”
The district is also considering reservoir expansion proposals that would cost upward of $290 million just to construct. Proposals to connect pipelines between Sonoma and Marin have costs of $140 million to $380 million. A drought-focused cost analysis of these projects is coming, Sellier said.
Sellier said considering water supply projects is similar to buying insurance.
“If you’re thinking about buying insurance coverage, you want to make sure you buy the best, cheapest insurance coverage, from a firm that’s going to pay,” Sellier said.
One positive that has emerged from the analysis, Sellier said, is that a $5 million plan to modify the spillway gates at the Nicasio Reservoir is expected to cost around $1,600 per acre-foot of water annually.
“So it’s really exciting that we have a real viable option,” Sellier said.
“I’m really excited to see how those larger storage projects as well as the conveyance projects are going to be stacked up,” said Matt Samson, a member of the district board.
Board member Monty Schmitt said that while he appreciates the staff’s effort, the numbers are staggering. He questioned the accuracy of the cost comparisons that assume a four-year drought. He was specifically concerned about the $12,100 per acre-foot price tag for conservation, he said.
“I would really like us to have that peer reviewed if we’re going to be making decisions around it, because it is just so wildly different than what we see in other estimations of conservation costs,” Schmitt said.
Ben Horenstein, the district’s general manager, said staff also took pause when looking at the numbers.
“This is strictly and narrowly trying to look at, in context of drought, to help the board think about where do we invest limited dollars to achieve … supplemental supply and resiliency,” he said. “We’re not in any way suggesting that conservation doesn’t have so much value and benefits beyond this narrow look in this context.”
Board member Larry Russell said while his colleagues react to the numbers as being high, “I react to them being reality.”
“Well, my friends, fasten your seatbelts, this is where we’re going,” Russell said.
