The Bennett Valley water story
By Chris Gralapp, Bennett Valley Community Association Board
Everyone in Bennett Valley depends on a well for water — and we all face the possibility that one day we will turn the tap and nothing will come out. It happens! The issues around groundwater sustainability are top of mind for us. With new development and the seemingly ever-present and increasing drought situation, we have a right to be worried.
To help address some of our concerns, and to better understand what’s under our feet water-wise, the Bennett Valley Grange, in partnership with the Bennett Valley Community Association (BVCA), presented a very well-attended Water Symposium on Oct. 25 at the Grange Hall.
Featured were two Sonoma County water experts: Marcus Trotta, principal hydrogeologist for Sonoma Water, and Robert Pennington, senior geologist with Permit Sonoma, who together painted a picture of the water situation in Sonoma County with a focus on the unique water issues in the 25 square-mile zone identified in the Bennett Valley Area Plan. Emphasis was on the recent California state mandate requiring all counties to comply with the Sustainable Groundwater Management Act (SGMA; pronounced sigma).
Trotta described the water conditions in Bennett Valley using a set of updated hydrogeology maps and historic studies. Bennett Valley is nestled between two major aquifers, the Santa Rosa Basin and the Sonoma Basin, but derives water from neither — we are sort of on our own, so to speak. Much of the floor of the valley is composed of the Petaluma formation — the familiar, dense “blue clay” — interspersed with thin sand layers. This is a low-yielding substrate that can be very thick, and water here is deemed scarce. The surrounding uplands are identified as Sonoma volcanic, which is deeply and unevenly fractured rock that yields somewhat more water flow, but in a spotty fashion.
An interesting fact: Bennett Valley’s wells are deeper on average than wells in other parts of the county. Well depth data comes from drilling logs filed by well drilling companies. Sonoma County wells averaged about 200 feet in the 1940s, and in the ’90s they averaged about 400 feet in depth.
Another interesting fact: Some of the deepest wells contain water that is 34,000 years old — ancient — and the water comes out of the ground hot.
Trotta showed data collected over several decades demonstrating fluctuations in the water table based on volunteer and agency soundings of a cross-section of wells in Sonoma County, highlighting two in Bennett Valley. Overall declines in the local water tables are noted, but in some areas water levels seem to be holding steady (so far) during the past five years.
Creeks are important recharge areas, so conserving and protecting both forks of Matanzas Creek serves many purposes: It’s good for wildlife, and good for our groundwater.
Permitting and metering
Robert Pennington spoke about the permitting side of the water equation. In Sonoma County, 250 to 350 new wells are permitted each year. In Bennett Valley, permits for four to 12 new wells are issued.
Based on the geology, four tiers classify the groundwater situation in Sonoma County: – Class 1: Major basins with high, dependable yield (Santa Rosa, Sonoma Basins);
– Class 2: Recharge areas, mostly good yield, primarily in West County;
– Class 3: Petaluma formation, with scarce water yield (Bennett Valley is in this class);
– Class 4: Franciscan formation, with very scarce water yield, primarily in coastal areas.
Well yield tests are required for new commercial and agricultural wells, Pennington said. Water conservation and monitoring requirements may apply to new residential and other low-water usages. Well permits are denied in cases that present impacts to groundwater.
Metering wells has been in the news lately. The good news for Bennett Valley proper is that no metering or fees are anticipated for us. They are proposed for agricultural and commercial users in the two adjacent major basins (residential wells are not expected to be metered anywhere).
However, no new well permits are being issued until April 4, 2023. In the interim, Permit Sonoma is rewriting the well ordinance to comply with the state mandate. The exception to this is in cases of emergency: If a well should fail in this time frame, emergency permits can be issued.
Monitoring: Let’s get involved
Bennett Valley well levels represent a “data gap” in the county’s spreadsheet: The county just cannot say with any specificity what the water table is like across the valley. To that end, plans are underway for a “citizen science” project, co-sponsored by the Grange and the BVCA, to train Bennett Valley residents to voluntarily self-monitor their wells and supply real-world water table data to the county. Stay tuned for details in the coming months. This new project will provide valuable information to help us plan for our collective water future.
If you missed the presentations, you can see them here: https://app.box.com/s/ 3zfbsqps15yflh66sb21vi9hskd1l3z0.