Bennett Valley Water Story

by Chris Gralapp

BVCA Board

Everyone in Bennett Valley depends on a well for water—and we are all faced with 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 waterwise, the Bennett Valley Grange in partnership with the Bennett Valley Community Association presented a very well-attended Water Symposium on October 25 at the Grange Hall.

Featured were two Sonoma County water experts: Marcus Trotta(Principal Hydrogeologist for Sonoma Water) and Robert Pennington (Senior Geologist, 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. An emphasis was put on the recent California state mandate requiring all counties to comply with the Groundwater Sustainability Management Act (SGMA).

Marcus Trotta described what water conditions we have in BV, using a set of updated hydrogeology maps and historic studies.BV is nestled in 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, which 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 yield 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 ~200 feet in the 1940’s, and in the ‘90’s they averaged ~400 feet in depth.

Another interesting fact: some of the deepest wells contain water that is dated to 34,000 years old—ancient indeed—and the water comes out of the ground hot.

Marcus showed us 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 BV. 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—good for wildlife, and good for our groundwater.

Permitting / Metering

Robert Pennington spoke about the permitting side of the water equation. In Sonoma County ~250 – 350 new wells are permitted each year. In BV, permits for 4 – 12 new wells are issued. Based on the geology, 4 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--water yield scarce—Bennett Valley is in this class

Class 4—Franciscan formation--water yield very scarce—Coastal areas

Well yield tests are required for new commercial and agricultural wells. Water conservation and monitoringrequirements may apply to new residential and other low-water usages. Well permits are denied in cases that present impacts to groundwater.

Metering of 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, right now, no new well permits are being issued until April 4, 2023.In the interim Permit Sonoma is rewriting the well ordinance (Ch 25B) 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 spread sheet—the county just cannot say with any specificitywhat the water table is like across the valley. To that end, plans are underway for a ‘citizen scientist’ 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.


-Trotta Powerpoint presentation: Bennett Valley Groundwater Conditions

-Pennington Powerpoint presentation: Sonoma County Well Ordinance and Permitting