A $25 million water storage project in the Ross Valley area is resuming this month after a pause due to environmental and weather restrictions, the Marin Municipal Water District said.
The project, which began last summer, will decommission the century-old Pine Mountain Tunnel used as a water storage tank, and replace it with two 2-million-gallon storage tanks for treated water. The tanks will be on the western side of Concrete Pipe Road near Bolinas and Sky Oaks roads on the outskirts of Fairfax.
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 project is increasing our resiliency, making sure our storage is seismically strong,” said Alex Anaya, director of engineering. “And it’s increasing storage for supply and for fire protection.”
The project will be completed in two phases.
The work underway is part of phase one, which is estimated at about $5.27 million, Anaya said. Maggiora & Ghilotti has been hired to prepare the site for the new storage tanks through grading work and building a 420-foot-long soil nail wall to stabilize the nearby hillside.
The project will require the district to remove 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 loaded trucks will be tarped and will take a one-way route through the watershed and exit out of Natalie Coffin Greene Park. The work will result in the temporary closure of some parking spaces at the park.
The remaining soil will be deposited at Bullfrog Quarry, with some being used to create the soil nail wall. The first phase is set for completion in early 2025.
Road and trail access will be limited near the construction site on weekdays during construction hours. Mount Tamalpais visitors are encouraged to review notices from the district at marinwater.org/visiting-mt-tam before making the trip.
The larger second phase, set to begin next summer, will build the new water tanks and cap off Pine Mountain Tunnel. Anaya said the district plans to advertise work for phase 2, estimated at about $20 million, later this year.
Construction of the tanks is expected to continue on a seasonal basis through January 2029.
Some Fairfax residents have raised concerns about the impact of the project on a nearby Carey Camp Creek, a portion of which lies within Elliott Nature Preserve, a home for endangered and threatened animals. They think leaks from the tunnel have been a primary reason for the year-round flow of the creek, a tributary to the San Anselmo Creek.
Concerned neighbors say they fear that Carey Camp Creek will dry up and the water district should prepare some sort of plan to keep it flowing.
Scott Davis said the creek is a habitat for yellow legged frog, rainbow trout and steelhead trout, and a water source relied upon by northern spotted owls and the dusky-footed wood rat.
“It’s a really valuable ecosystem for plants and animals in that area,” Davis said. “Bottom line, it would be really a shame to lose this beautiful area.”
In response, Jessie Underhill, spokesperson for the district, said, “This legacy infrastructure is more than 100 years old.”
“There is some leakage that is inherent to the age of the infrastructure,” Underhill said. “Eliminating the loss of water from those leaks is a significant benefit of these infrastructure investments.”
Sandy Guldman, president of Friends of Corte Madera Creek Watershed, said it is still unclear how much the leaky tunnel contributes to the creek’s supply, or if it’s actually groundwater that fills the channel.
Guldman said there is riprap that blocks fish migration, so if the creek runs dry there would be nowhere for them to swim. A fix would involve a lengthy study and lots of money, she said.
Guldman said that while she would like to see robust flow in the creek, “we will not support the transfer of water from Lagunitas watershed to Carey Camp Creek. It does more good in Lagunitas Creek.”
“Right now we are biding our time to see what happens when that tunnel is closed, emptied and abandoned,” Guldman said.
