Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Lower Pilarcitos Creek Test Well Analysis
In the autumn of 1997, CCWD conducted a two-phase investigation of the Lower
Pilarcitos Creek groundwater basin to determine the thickness, permeability, and
lateral extent of aquifers beneath and adjacent to Pilarcitos Creek and west of the
Highway 1 bridge crossing. Phase One included the drilling of six test borings
while the follow-on study, Phase Two, completed the project with the installation
and aquifer testing of five test wells. This section summarizes the investigation and
provides an analysis of available pumping test data.
Drilling and Construction
Figure 15 shows the location of the test borings/wells. Three test borings were
installed north of Pilarcitos Creek and three south of Pilarcitos Creek. Ground surface
elevations range from 60 feet near Kelly Avenue and Highway 1 to 30 feet on
Chesterfield Avenue near Pilarcitos Creek. Table 4 summarizes the construction details
for the test borings/wells.
Phase One. The test borings were installed by Pitcher Drilling Company from
East Palo Alto, California using a Failing 1500 direct mud rotary drilling rig. The six test
borings were drilled between September 2 and 9, 1997 using a combination of 5-inch
diameter tricone and fish-tail drill bits. Discrete formation samples were collected
generally every ten feet using a split-spoon sampler providing a 1-inch diameter by 18-
inches long core sample. These cores were used to describe the subsurface materials
and were supplemented with examination of returned formation cuttings mixed with
bentonite drilling fluid. The borings were drilled to depths ranging between about 64 and
75 feet below ground surface.
A suite of geophysical logs were conducted by Welenco (September 11, 1997)
on four of the test borings (TB1, TB2, TB3, and TB5) on September 8, 1997. The suite of
geophysical logs included spontaneous potential (SP), short (16-inch) and long (64-inch)
normal resisitivity, and single point resisitivity. Two (TB1 and TB2) of the four test
borings were drilled, left as open holes, and logged 142 and 120 hours, respectively,
after mud circulation had stopped. This non-standard logging procedure can impact the
interpretation of the geophysical logs. Nelson (1997) provides the geologic and
geophysical logs.
Based on the geologic logs and geophysical logs of the test borings, the area is
underlain by a thin and uniform aquifer (see Figure 16). The top of the sand and gravel
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