Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Marine Terrace Hydraulic Properties
The marine terrace deposits form the main aquifer of the Lower Pilarcitos Creek
groundwater basin. Available pumping test data from wells in the basin provide
information on hydraulic properties of the marine terrace aquifer, including
conductivity, transmissivity, and storativity. Groundwater flow is generally toward
the shoreline, where an interface exists between the onshore fresh water and
offshore salt water. Substantial discharge of groundwater occurs to the ocean.
Groundwater storage is affected significantly by seasonal fluctuations in
groundwater levels.
Pumping Tests
A handful of formal pumping tests conducted with the supervision of a
hydrogeologist and many informal tests conducted without supervision have been
performed and recorded in the Half Moon Bay and Lower Pilarcitos Creek areas. The
formal pumping tests are analyzed and discussed by ESA (December 6, 1991, February
27, 1992, and March 5, 1992) and by Luhdorff & Scalmanini (2000). Nelson (1998) did
not analyze the five 12-hour pumping tests collected during the Lower Pilarcitos Creek
test well installation and groundwater investigation but noted the data were available in
the CCWD files. Todd Engineers reviewed and analyzed the pumping test data for the
current investigation (see previous section).
Table 5 summarizes eight formal single well pumping tests conducted by CCWD.
These tests were conducted in the vicinity of the proposed Pilarcitos Creek wellfield
(cross section A-A’, Figure 16). The T-value ranges between 10,000 and 20,000 gpd/ft
for the test wells and the Holiday Inn well while the wells in the southern portion of the
cross section (Cunha and Corporation Yard) show a range between 1,500 and 7,000
gpd/ft. Nevertheless, these are similar values considering that the transmissivity can
vary by 12 orders of magnitude (Freeze and Cherry, 1979) and suggest that the aquifer
is slightly less permeable south of the proposed wellfield. The K-value averages about
160 gpd/ft 2 south of the proposed wellfield and 537 gpd/ft 2 near the proposed wellfield.
Aquifer testing was conducted at the Balboa wellfield by Luhdorff & Scalmanini
(2000). The aquifer test consisted of pumping one well at 66 gpm for eight hours and
measuring the drawdown at three observation wells. Table 7 summarizes these data.
The T-values estimated for the three observation wells range between 8,000 and 10,000
gpd/ft; equivalent to an average K-value of 354 gpd/ft 2 . Because observation wells were
used for this aquifer test, calculations can be conducted to determine the S-value of the
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