Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

exists, the average annual inflows and outflows balance at about 2,200 acre-feet per
year (AFY).
Rainfall recharge and subsurface inflow from the upper Pilarcitos Creek valley
are significant. The water balance equation indicates that stream recharge is an
important inflow; however data are lacking. Of the outflows, subsurface outflow to the
ocean is predominant, accounting for 77 percent of outflows, while groundwater
pumping, export, and consumption account for the remainder. Although many wells have
been drilled in the groundwater basin, many also have been abandoned. The major
existing groundwater use involves pumping and export of groundwater for golf course
irrigation to the south of the basin.
The perennial yield (safe yield) for the Lower Pilarcitos Creek groundwater basin
is defined by the amount of groundwater that can be pumped while minimizing saltwater
intrusion from the ocean. A total pumping of 1,300 AFY can be sustained on a long-term
perennial yield basis without inducing significant adverse impacts (i.e., saltwater
intrusion into the basin). This perennial yield value amounts to 60 percent of the
estimated 2,200 AFY of total inflow to the groundwater basin, reflecting the unlikelihood
that all of the inflow to the groundwater basin can be safely captured without incurring
adverse impacts.
Groundwater quality is a concern in the Lower Pilarcitos Creek groundwater
basin, especially for iron and manganese. Excessive iron and manganese likely reflects
a combination of pumping from the Purisima Formation, mixing with Marine Terrace
aquifer water, and inadequate well development. These problems can be minimized with
thorough well development and design of production wells with screens placed solely in
the marine terrace aquifer. Based on available information, water from a proposed
Pilarcitos Creek wellfield would be treated for iron and manganese and blended with
Crystal Springs water at an approximate ratio of 3:1 to provide acceptable water for
floriculture irrigation.
For evaluation of economic feasibility, plans for wellhead, conveyance, and
treatment facilities are provided for the proposed wellfield. The yield of the five wells is
variable throughout the year (reflecting changing groundwater levels), with higher yields
during the winter-spring season (November-March) and low yields during the summerautumn
dry season. Key criteria for operating the wellfield are to manage pumping rates
to avoid dewatering of the aquifer and exposure of well screens. Application of these
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