Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

aquifer. The S-value ranges between 0.00071 and 0.0013, indicating that the marine
terrace aquifer is confined near the Balboa wellfield.
The informal pumping tests performed by drilling contractors are usually recorded
on the DWR Water Well Drillers reports and provide basic hydraulic information including
static water level (SWL), pumping water level (PWL), length of pumping test, and the
approximate discharge. These are reported in Appendix 2, well inventory information.
The well inventory lists 380 wells (62 percent) with hydraulic information.
Because the T-value (and, by inference, the specific capacity) varies with geologic
materials, the well inventory was sorted by aquifer type (alluvium, marine terrace, and
bedrock) and summarized to determine the minimum, maximum, and average specific
capacities. The specific capacity for these wells varies by at least three orders of
magnitude and ranges between 0.006 (bedrock) to 7.06 gpm/ft of drawdown (alluvium
and marine terrace). The alluvium and marine terrace aquifers are similar in hydraulic
properties, averaging 2.82 and 2.64 gpm/ft of drawdown, respectively; the average for
bedrock is 0.66 gpm/ft of drawdown. This suggests that the T-values of the geologic
units range between 1,000 gpd/ft for bedrock and 5,500 gpd/ft for the alluvium and
marine terrace units.
In summary, the marine terrace aquifer near the proposed Lower Pilarcitos Creek
wellfield has a T-value of about 16,000 gpd/ft, an aquifer thickness of about 32 feet, a K-
value of 500 gpd/ft 2 , and is a confined aquifer. The marine terrace aquifer near the
Balboa wellfield has a T-value of about 9,400 gpd/ft, an aquifer thickness of about 42
feet, a K-value of about 224 gpd/ft 2 , and an S-value of 0.0011 (confined aquifer). In
regions south of the proposed Lower Pilarcitos Creek wellfield, the T-value is lower
ranging between 1,500 and 7,000 gpd/ft. Regional informal pumping tests and empirical
analysis of the data suggest that the T-value may range between 1,000 and 5,500 gpd/ft
for bedrock and marine terrace aquifers, respectively. In general, the informal pumping
test data are consistent with formal aquifer and well testing.
Groundwater Elevation Maps
Geoconsultants (June 1987, August 1991, and September 1993) constructed
four groundwater elevation or potentiometric maps for the Lower Pilarcitos Creek coastal
plain between Arroyo de en Medio and Cañada Verde. The four maps include (1) a
composite map for 1978 to 1987, (2) winter 1990-91, (3) summer 1991, and (4) summer
25