Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

The overall capacity of the five well pumps is 441 gpm (0.64 MGD). It is expected
that the average annual flow from the wellfield will be a slightly less than half of the
maximum quantity (194 gpm [102 MGY]); but may be reduced to 75 MGY (0.21 MGD;
143 gpm) to maintain a 3:1 blend ratio since a review of the current wet-season flows
produced by the Nunes WTP are about one MGD (700 gpm). Cost projections of
economic feasibility are based upon that quantity of water (75 MGY).
The estimated well construction cost is $71,000, based upon a recent bid
received by Todd Engineers for a well of similar depth capacity, design, and size at
Stinson Beach, Marin County. The cost of the pump units for 460-volt, 3-phase motors
varies from $3,700 to $4,200. Electrical service to the wells would be obtained from
nearby PG&E 3-phase power poles and was estimated at about $29,000 for each well.
An additional 40 percent ($42,000) is included for well construction and design services,
wellhead piping and appurtenances, and pump design.
Transmission Pipeline. The proposed transmission pipeline between the wells
and Nunes WTP is shown in Figure 22. All buried pipe would be ductile iron Pressure
Class 350 pipe, suitable for 350 pounds per square inch (psi) working pressure. Fourinch
diameter pipe would be used to connect individual wells to the main transmission
pipelines to be located in the paved streets west of Highway 1. Those south of Pilarcitos
Creek would connect into a 6-inch diameter pipeline and bore and jack pipeline crossing
beneath Pilarcitos Creek north of Pilarcitos Avenue, while wells north of Pilarcitos Creek
would be routed southeasterly on St. James Avenue. A bore and jack pipeline would be
provided north of Main Street. This pipeline would proceed up Foster Drive past the high
school to the access road, where it would proceed up the hill to the Carter Hill Water
Storage Tanks to the Nunes WTP.
The cost of this pipeline is based in part upon recent water distribution pipeline
costs in the area (Teter, January 2003). The Engineer’s Estimate of probable
construction cost of the transmission pipeline is $693,000, which is adjusted to $832,000
in the Final Estimate to provide for a 20 percent contractor overhead, profit, and
construction contingency.
Water Treatment. The proposed water treatment for the well water is to aerate
the water to oxidize the iron, strip carbon dioxide, and increase pH to about 7.8
(Summerfield, 1999). The aerated water will be (1) treated with potassium
permanganate to oxidize and precipitate the remaining iron and manganese, (2) blended
with surface water for removal of the solids by coagulation, flocculation, sedimentation
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