The Marin Municipal Water District took another step this week in pursuit of what the agency says is its largest supply and drought resiliency project in 40 years.

The district board voted unanimously Tuesday to authorize spending $9.7 million to design a pipeline that would tap into an existing aqueduct system to get Sonoma County water to Marin reservoirs.

The pipeline project was selected in February as the district’s priority effort to boost supply. If completed, it would be the largest water supply project since Kent Lake was expanded in 1982, according to the district.

“This is where the rubber hits the road; it’s actually a pretty exciting evening,” Matt Samson, board president, said ahead of the approval Tuesday.

“This is the logical next step in our water supply road map,” he said, adding that bringing the project online, along with other planned efforts, “will allows us to then have more breathing room.”

The project was derived from the district’s water supply “road map” — a strategy developed in response to the 2020-2021 drought.

About 75% of the Marin Municipal Water District supply comes from the Mount Tamalpais watershed and hills of West Marin. Its seven reservoirs have a combined storage capacity of about 80,000 acre-feet of water. The district serves about 191,000 residents.

The rest of the water comes from the Russian River watershed. Treated water from the river is channeled through an aqueduct along Highway 101 for blending into Marin’s distribution system. Excess water flows into the ocean. Right now, there is no way to capture that water for later use.

Estimated at $167 million, the proposed project would construct a 13-mile, 36-inch pipeline and a pump station to redirect some of that water into the Nicasio Reservoir for storage. The pipeline could yield 3,800 to 4,750 acre-feet of water a year.

Paul Sellier, the district’s water resources director, said staff expect the design and environmental review to be completed by early 2027.

Construction could beginning soon after, and the pipeline could be operational by 2029, he said.

Potential future phases of this project, which would cost more money, could increase replenishment of stored water to up to 8,100 acre-feet, Sellier said.

The vote Tuesday awarded the design contract to Carollo Engineers, a firm headquartered in Walnut Creek.

District staff had reached out to Carollo and four other firms to solicit bids for the project design. Carollo was the only one to submit a proposal, staff said.

Board members said they were concerned that the process wasn’t more competitive.

Sellier said the firm is the same group that performed the pipeline alternatives analysis that led to the project selection. If staff chose, they could have awarded the contract to them without making additional solicitations.

Lucy Croy, the district’s water quality manager, said the firm also previously drafted design plans for a proposed emergency pipeline across the Richmond-San Rafael Bridge amid the 2021 drought, and has worked with the district on other projects.

Typically, design contracts equal about 8% to 10% of the estimated project cost. Carollo’s fee, is about 5.7% of the total cost, staff said.

“We are confident that the Carollo team can and will deliver on this project,” Croy said.

General Manager Ben Horenstein agreed, adding that it’s the same team that’s been part of the project since the inception.

“We really do have confidence in their ability to execute,” Horenstein said.

Some residents who attended the meeting had concerns about the district’s naming of the project. When talked about in previous meetings, it was identified as the “PETA-3” project. For this meeting it was renamed to the “Atmospheric River Capture (ARC) project.”

Those who complained said it was confusing and could be perceived as misleading. Staff and board members said they would consider a name change, and be consistent with that name moving forward.

Some showed support for the project.

“I’ve been here for five years watching this project, and I will say that it has been fully transparent,” said Larry Minikes, a board member of the Marin Conservation League. “I’m very pleased with the progress that the board is making, and the rubber is hitting the road.”

Croy said staff issued a request for proposals for the environmental review reporting, which is required to meet the California Environmental Quality Act. Staff expects to return to the board next month to recommend a consulting firm to hire to do that work.

At the same time, the district is pursuing a separate water supply project to modify the Nicasio Reservoir. That plan, estimated about $5 million, involves modifying dam spillway gates to add 3,000 acre-feet of water storage. The district aims to have that project ready for approval next year.