Three-year, $15 million Sonoma sewer line upgrade to begin this summer

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Work is to begin this summer on the next phase of replacing an old and failing sewer main that runs through Sonoma Valley, periodically leaking wastewater during heavy winter storms.

Crews are going to be tackling three pipeline segments totaling 1.8 miles over the next three years, beginning with a section likely to be the most disruptive, given its intersection with a key transportation corridor in northwest Sonoma. The first stretch of pipeline to be upgraded starts on Sixth Street West but quickly aligns with West Napa Street/Highway 12 and then turns north toward Maxwell Farms Regional Park.

The Sonoma Valley County Sanitation District, which manages the area’s wastewater treatment system, has laid plans for the roughly $15 million project to proceed as smoothly as possible, officials said.

The impacts on traffic and daily life are a trade-off to upgrade and enlarge a roughly 60-year-old sewer main. The work is meant to stop the accidental discharge of untreated wastewater that has occurred over several decades when abundant rainfall hits the area.

“We’re going to do some pretty heavy outreach as it gets closer, just so people know,” said Mike Thompson, assistant general manager with Sonoma Water, which operates the system through the sanitation district. “It looks simple but it’s extremely complicated, and we’ll be happy when it’s done.”

Spills come primarily during heavy storms that allow rain runoff and groundwater to seep into cracks in deteriorating sewer laterals and collection lines — a situation known as inflow and infiltration — overwhelming the system with so much volume that wastewater can leak out or overflow, usually through manhole covers.

The discharge is highly diluted, agency spokesman Barry Dugan said. But it contains untreated sewage, and it floods onto the street and into storm drains, often working its way into local creeks, potentially risking wildlife and water quality, and putting the district in violation of the California Environmental Quality Act.

Heavy rains last month led to eight such overflows on two occasions, Jan. 6 and 16, involving a combined 498,843 gallons of wastewater.

Separately, a faulty valve on the Sonoma Valley system failed to close Jan. 12 and released more than 2 million gallons of wastewater into a slough before being discovered and stopped.

Nearly 170 known spills from the Sonoma Valley system totaling more than 2.5 million gallons have been reported to the San Francisco Bay Regional Water Quality Control Board since 2007, most as a result of such accidental discharges, though Sonoma Water staffers say the frequency has diminished as repairs have been made in recent years.

They also note that aging collection systems and related overflow problems are widespread in California and elsewhere, a point validated by reports and enforcement by state regulators in other areas.

Private property owners in the Sonoma Valley system, which serves more than 17,000 accounts, also are partly to blame. They’re responsible for maintenance and replacement of the sewer laterals that connect their homes and businesses to the public sewer main. The city, believing that many are likely in need of repair, given their age, has made allowances for video inspections and rebates for repair and replacement.

As far back as 2010, San Francisco Bay water quality regulators said the Sonoma Valley district was not moving fast enough to replace and enlarge capacity in the public sewer main.

A 2015 cease and desist order escalated the state’s crackdown, along with a $732,000 fine tied to a series of 52 sanitary sewer overflows between 2010 and early 2015, the majority the result of heavy rain, flooding or insufficient capacity.

The sanitation district has replaced some of the oldest, most deteriorated pipeline, beginning at the waste treatment plant on Eighth Street South outside Sonoma and edging along the southwest side of town to Studley Street and West Sixth Street. That’s where work crews will pick up next, hopefully in June, said Kent Gylfe, a principle engineer with the water agency.

The compliance order from regulators gives the sanitation district until Oct. 31, 2022, to complete the three-phase, 1.8-mile upgrade, and then until 2024 to finish roughly 1.5 miles more.

The work is set to last about eight months this year, with workers digging trenches as deep as 28 feet for new pipelines along west Napa Street and Highway 12.

Crews will pick up the project in 2020 at Ramon Street, skirt the edges of the Sonoma Oaks Mobile Home Park and Maxwell Farms Regional Park, cross Verano Avenue and end south of Old Maple Avenue. That project is to last about six months.

The 2021 segment runs from the south end of Buena Vida Drive through private easements north to Happy Lane, ending just north of Anthony Court. It should last about six months.

“It’s almost like widening 101, but this is our big trunk main. It’s the freeway of the sewer system,” Thompson said, “and we started that many years ago, starting at the treatment plans making our pipes bigger, working their way upstream to where we’re at right now.”

See for more information and a link to a project map.

See for more information and a link to a project map.

You can reach Staff Writer Mary Callahan at 707-521-5249 or On Twitter @MaryCallahanB.

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