San Geronimo pipeline upgrade project approved

Seismically vulnerable segments of a pipeline that carries untreated water across the San Geronimo Creek are slated for replacement.

The board governing the Marin Municipal Water District unanimously approved the project on Dec. 16. The vote included the adoption of an environmental assessment that imposes steps to minimize potential disturbances to the creek and surrounding habitat.

The pipe is part of the district’s “North Marin Line,” which was constructed in 1957 to convey raw water from Kent Lake and Nicasio Reservoir to the San Geronimo treatment plant. Pipe crossings are supported by concrete trestles installed in the creek. The pipe is leaky and at the end of its useful life.

“It’s such a critical line in our system that we want to make sure that it’s operable, right?” said Elysha Irish, the district’s engineering manager. “Nicasio and Kent Lakes are some of our largest lakes in our system. Also, Soulajule (Reservoir) would be fed through that line should we choose to operate it.”

Irish said eventually the whole pipeline would need to be replaced as part of a larger project.

“These parts of the pipeline, they’re not underground, they’re not easily accessible, and just given the nature and complexity of these particular locations, staff wanted to repair these ahead of that larger project,” she said.

Ahead of the vote, board president Matt Samson thanked district staff for making progress on a project that has been delayed since 2016.

“This project was identified as a vulnerability for the district,” Samson said. “And we appreciate you continuing to bring it back up and reminding us of how important it is, and taking steps here to make sure our infrastructure is modernized so we can still meet the demands in the next 50 years on one of the most critical pipelines.”

Initially, the district planned to replace the pipes at the two points where they cross the creek. The pipe is the only transmission line from the reservoirs to the San Geronimo plant, the largest of the district’s three treatment plants.

The project was deferred for several years, and when it was evaluated more recently, district officials determined it would be best to relocate the connection from Kent Lake away from private roads and residences.

“The homes and trees and fences are very, very close to that existing pipeline, and it would be very difficult from a constructability standpoint to put a new bridge at that location,” Irish said.

The new plan involves installing about 1,700 feet of pipe under Sir Francis Drake Boulevard at Lower Peters Dam Road. The site is east of the Leo T. Cronin Fish Viewing Area in Samuel P. Taylor State Park. The pipe under Shafter Bridge would become part of the North Marin Line. A valve would also be relocated.

The changed scope prompted the district to perform a supplemental environmental assessment. The report reached a similar conclusion to one approved in 2016 that said all potential impacts could be mitigated to less than significant.

The 263-page report, officially called a supplemental initial study/mitigated negative declaration, was circulated for public review and comment in the fall.

The modified project also includes cutting and removing some of that pipe and the concrete pilings in the creek that support it. That would reduce the risk of potential debris buildup that could restrict flow and damage the pipe, officials said.

Plans to replace the pipe crossing the creek from Nicasio Reservoir remain unchanged from the original proposal. Once that old pipe is removed, a new 36-inch pipe would be installed onto a steel truss on piers. The pipe would be elevated about 2 to 3 feet above the ground before crossing the creek.

The creek would be dried out in the work area to provide safe access during the construction.

In response to public comments, district staff amended the design of the pipe so that it blends in better with the natural environment at points where it meets the ground.

Construction is expected to begin in August after the northern spotted owl nesting season. The estimated project cost is $3 million to $5 million.