Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

aquifer is defined primarily by a distinct geophysical signature while interpretation of the
bottom of the aquifer has been based on the geologic logs. Ground surface elevations
are provided by Nelson (1998). The Phase One test borings indicate that the aquifer
thickness ranges between 27 and 33 feet. The aquifer is underlain by the consolidated
Purisima Formation and overlain by about 30 feet of silts and clays. The base of the
aquifer ranges between 23 and 31 feet below msl while the top of the aquifer ranges
between 10 feet above and 4 feet below msl. The aquifer was absent from TB4 (Nelson
1997), which is located approximately 600 feet northeast of TB3 where the underlying
Purisima Formation was encountered at an elevation of about 24 feet below msl.
Phase Two. All test borings except TB4 were selected for follow-on test well
construction. The purpose of Phase Two was to determine the long-term yields for the
test wells near the Phase One borings. The test wells were installed, constructed, and
developed by Pitcher Drilling Company using a Failing 1500 direct mud rotary drilling rig.
The five test wells were drilled and reamed (), constructed, and developed between
November and December 1997 using a 14-inch diameter tricone and drag drill bits. The
test wells were drilled to depths between 82 and 85 feet, providing 10 to 15 feet of
additional hole beneath the aquifer for a sump or tail pipe.
Eight-inch diameter Schedule 40 PVC (0.25-inch wall) casing and horizontal
perforations were installed in each boring. The perforations consist of 43 rows of 0.060-
inch (60 slot) per foot and three perforations per row (), resulting in a relatively small
open area. The estimated open area is 18.576 square inches (in 2 ) or 6 percent open
area. In contrast, the open area of similar diameter and slot wirewrap well screen is
123.840 in 2 or 40 percent open area.
AMC Lonestar medium aquarium sand (6 X 16) was installed as a filter pack
envelop surrounding the screen. The depth to the top of the filter pack ranges between
25 and 37 feet. A concrete grout surface seal was installed in the annular space
between the 14-inch diameter hole and the 8-inch diameter PVC casing from ground
surface to the top of the filter pack. The length of the perforations range between 40 and
45 feet. These are between 30 percent to 105 percent longer than the aquifer thickness.
This placement of perforations opposite other geologic formations (especially the
Purisima Formation with potentially poor water quality) may significantly impact the
measured water quality parameters from the test wells.
Well development consisted of thinning the drilling fluid and partially removing
the fluids prior to well screen installation and then completely removing the fluids after
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