Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Well Construction Program
The test wells provided valuable information on the feasibility of a Lower
Pilarcitos wellfield. However, estimated well efficiencies of the Lower Pilarcitos Creek
test wells range between 45 and 85 percent. For a newly installed well, typical well
efficiencies are greater than 80 percent.
In addition, the test wells were characterized by turbid discharge. This indicates
that the wells were not fully developed to remove drilling fluids introduced during drilling
and fine-grained sands adjacent to the screen, and to remedy the damage to the
borehole. The wells were not rigorously developed because of their construction with
PVC casing materials, which are a much weaker material than steel casing and provide
a much lower open area than wirewrap screen. Lower well screen open areas limit the
effectiveness of well development procedures and may restrict efficient flow of
groundwater into the well. As a result, the test wells do not adequately reflect the
potential capability of wells in that location, the local water quality, or the aquifer
response to pumping.
Should CCWD decide to proceed with evaluation of a Lower Pilarcitos wellfield,
the recommended next step is installation of a pilot production well immediately adjacent
to TW2. This pilot production well would provide the following:
• Better definition of the composition of aquifer materials,
• Water quality information with depth and geologic formation, allowing isolation
of the water quality of the alluvial aquifer,
• Assessment of the aquifer response during formal pumping tests using an
observation well, and
• Optimization of well efficiency.
The pilot well (and any subsequent production wells) should be installed
approximately 10 feet from the existing test wells. This distance will allow measurement
of water levels during pumping tests not only in the pumping well but also in the existing
test well, which would serve as an observation well. These measurements would provide
a better understanding of aquifer parameters (transmissivity and, especially, storativity).
This better hydraulic understanding is necessary to predict well interference and well
spacing in the well field.
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