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From the Bennett Valley
On Wednesday evening,
4/21/04, Bennett Valley Homeowner's Association sponsored
the Groundwater Forum, held at the Bennett Valley Grange.
After attending the City of Sebastopol Groundwater Forum,
on 3/30/04, we were able to add to our list of excellent
panelists. . . Valerie Brown, County Supervisor, Pete
Parkinson and Dwayne Starnes, Director and Deputy Director,
Sonoma County PRMD, Jay Jasperse, Deputy Chief Engineer,
Sonoma County Water Agency (SCWA).
[also presenting were]
Carl Hauge, Chief Hydrogeologist, California Dept. of
Jane Nielson, Ph.D., Geologist, retired from U. S. Geological
Survey (USGS), and Steve Carle, Ph.D., Hydrologist, Lawrence
Livermore Lab. Somewhere between 200 and 300 people attended.
It was an excellent meeting with a tremendous amount
of information presented on groundwater.
Bennett Valley Geology
and Groundwater (Excerpts from write-up prepared for
the Bennett Valley Groundwater Forum on 4/21/04, by Jane
Nielson, Ph.D., Geologist, Sebastopol Water Information
The history of Bennett Valley settlement suggests that
its water-scarce character has been well known for at
least a quarter of a century…A broadscale study
of Sonoma County groundwater resources was carried out
by the State Department of Water Resources in the 1970’s
and early 1980’s, in which the Bennett Valley area
was classified as either lacking water supplies, or as
having “contiguous or detached groundwater areas
outside of boundaries of ground water basins.”
Water that infiltrates below the surface may eventually
reach the top of a water-saturated subsurface groundwater
zone, called the water table. Rocks that store and
transmit groundwater are called aquifers. The process
infiltrated water to groundwater is called recharge.
Recharge does not happen everywhere. …Groundwater
will move toward any area of lower pressure, discharging
to the surface at a spring, steam, pond, or lake. It
will also move toward water table depressions, caused
by pumping wells…The amount of fresh water available
for use depends on:
* The amount of annual
* The extent of recharge area that can collect the water
so that it doesn’t simply run off…
* The capacity (and ability) of a rock to transmit
water to wells, which depends on both the texture
of the rock or soil.
The locations of recharge areas and aquifers are
determined by the geology: if soils and rocks exposed
at or near
the surface cannot absorb and hold water, and also
transmit water, they cannot recharge underlying aquifers.
is when the amount of groundwater being extracted is
greater than the amount of recharge…” Edward
J. Casey, Chief Water Resource Attorney, Weston Benshoof
Bennett Valley is underlain
by (rocks, mainly impermeable, called) Sonoma Volcanics
and Petaluma Formation. Most of its
comes from thin… (surface) sedimentary units, including stream sediments
and slope deposits eroded off the mountainsides. Fragmental volcanic rocks
may occur at the surface, but there’s no known connection between
such exposures and deeper rocks with aquifer characteristics. If any recharge
the volcanic rocks, it’s strictly luck.
The 2003 Kleinfelder report showed that most Bennett
Valley well owners with water problems had wells drilled
into the Petaluma Formation, in
and southern part of the area.
Recent wells as much as
1,200 feet deep have been reported on the southern and
western sides of the valley. The
landowner indicated several property
owners in this same area have drilled multiple wells to depths greater
than 700 feet.
A 700-ft. dry hole was drilled for a home site on the Sonoma Mountain
Road corridor. That property owner has resorted to capturing stream
the creek (which
is then) stored in cisterns for use.
Community supply wells for the Woodside Subdivision, Hidden Acres,
and Bennett Ridge residential developments, and agricultural irrigation
to be deep and pump a lot more water than domestic wells. Regarding
agricultural wells, when the Matanzas Creek Winery well, located in
the heart of Bennett
Valley and adjacent to the north side of the Woodside Subdivision,
began irrigating vineyards in the 1980’s, Woodside community-supply
wells lost pressure (probably in summer, the major stress time for
supplies in our climate).
Subsequently the Winery agreed to irrigate at night. Not all competitive
water supply issues can be settled so easily.