MMWD water storage project near Fairfax advances to next stage
The Marin Municipal Water District is poised to begin the next phase of its water storage project in the Ross Valley.
The project is to construct a pair of 2-million-gallon storage tanks for treated water in the Mount Tamalpais watershed on the outskirts of Fairfax. Eventually, the century-old Pine Mountain Tunnel used as a water storage tank will be decommissioned.
The new tanks will be placed on the western side of Concrete Pipe Road near Bolinas Road and Sky Oaks Road. Construction is set to begin in June.
“The launch of phase 2 marks a major milestone for this generational infrastructure project,” said Matt Samson, president of the district board. “This capital investment not only strengthens Marin Water’s overall system reliability, it also expands system capacity for our Ross Valley customers and provides additional water storage for firefighting — both now and for generations to come.”
Typically such work would be prohibited until August because of environmental constraints. The major factor is the northern spotted owl nesting season from Feb. 1 to July 31. Work cannot happen during those months if the raptors are present.
The district’s bird surveying consultant, Point Blue Conservation, surveys all nest sites in the watershed, said Adriane Mertens, spokesperson for the district.
“When they are able to confirm that a nest is non-active or young have fledged the nest, the noise disturbance restriction can be lifted,” she said. “This year, after multiple rounds of surveys, the biologists confirmed no nesting activities at or near the project site, so construction is now able to start sooner than originally planned.”
The 9,000-foot-long Pine Mountain Tunnel was built in 1919 to transport raw water from the Alpine Lake reservoir to Cascade Canyon for drinking water. After new water treatment standards were implemented in the late 1960s, the tunnel was disconnected from Alpine Lake and repurposed in 1971 to store up to 3 million gallons of treated water.
A risk assessment performed by the district in 2019 found that the tunnel had one of the highest risks of failure from disasters such as earthquakes, which could threaten water supply and fire response for Ross Valley communities. The tunnel was also found to have cracks, resulting in water loss.
The first phase of construction began in the summer of 2023 and cost an estimated $5.27 million. The work prepared the site for the new storage tanks by grading and building a 420-foot-long soil nail wall to stabilize the nearby hillside.
That work required the removal of 45,000 cubic yards of soil, or enough to fill more than 2,800 dump trucks. About 5,000 cubic yards of topsoil will be hauled to a landfill to prevent the potential spread of a pathogen that causes sudden oak death disease.
The larger second phase will cost about $23.18 million. Alex Anaya, director of engineering at the district, presented the details to the board at a planning committee meeting in February.
Construction involves manufacturing and installing new seismically-fit, pre-stressed concrete tanks standing 45 feet tall with a diameter of 90 feet. Work will span four years, he said.
The water stored at Pine Mountain Tunnel serves 44,000 district customers in the Ross Valley. That’s about 23% of the district’s service area.
In March, the board authorized a construction contract with Azul Works Inc., a San Francisco company, for $19.14 million, plus a $1.92 million contingency.
Because of the complexity of the project, Anaya said staff needed the support of a construction management firm as well. The board authorized a $1.12 million contract with Park Engineering Inc. of Orinda, to provide the service. That amount includes a $300,000 contingency.
District board member Ranjiv Khush thanked staff for recommending they hire a construction management firm to support the project.
“We’ve been talking about options for managing the ambitious level of capital improvement that we have lined up,” Khush said. “This seems like a good step in that direction.”
During construction, road and trail access near the site will be limited. Prospective watershed visitors are encouraged to check for notices and restrictions in advance at marinwater.org/visiting-mt-tam.
Anaya said once the tanks are installed, the district will shift focus to decommissioning the tunnel.
More information on the project is at marinwater.org/PMTtanks.