Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

ten gallons per minute per foot [gpm/ft] of drawdown) indicate prolific aquifers; small
specific capacities indicate poor aquifers or inefficient wells. The specific capacity is
directly proportional to the aquifer permeability. The proportionality constant is 1,500 for
unconfined aquifers and 2,000 for confined aquifers (Driscoll, 1986; AWWA, 2003).
The specific capacity for the test wells ranges from 4.64 (TW3) to 8.62 (TW5)
gpm/ft of drawdown after 720 minutes of pumping (Table 5). This means that for every
foot of drawdown the test wells can be pumped between 4.64 and 8.62 gpm. The aquifer
permeability should range between 9,000 and 17,000 gallons per day per foot (gpd/ft).
Transmissivity. The transmissivity (T-value) or field permeability represents the
ease at which groundwater flows through the aquifer. High T-values (greater than 10,000
gpd/ft) indicate prolific aquifers; small T-values indicate poor aquifers. Transmissivity is
estimated using time drawdown data collected from the pumping well and any nearby
observation wells. Table 5 summarizes the transmissivity data calculated for both
drawdown and recovery data. Average T-values for the test wells range from 9,960
(TW6) to 20,150 (TW5) gpd/ft. This is consistent with the specific capacity data.
Hydraulic Conductivity. The hydraulic conductivity (K-value) or intrinsic
permeability of the aquifer is a fundamental property of the aquifer. The K-value is
directly proportional to the transmissivity and inversely proportional to the aquifer
thickness. Like T-values, high K-values indicate prolific aquifers, while small K-values
indicate poor aquifers. Table 5 summarizes the K-values. The K-values range from 302
to 713 gallons per day per square foot (gpd/ft 2 ). These K-values represent moderate to
high values.
Storativity. The storativity (S-value) or storage coefficient is a measure of the
confinement of the aquifer and is unit-less. High S-values (greater than 0.005) indicate
water table conditions or unconfined aquifers, while low S-values (less than 0.005)
indicate confined or artesian aquifers. In unconfined aquifers, the S-value is the specific
yield of the aquifer.
Storativity can only be measured in observation wells. Insufficient data were
collected during the test boring/well program to estimate the S-value. Based on literature
review and the hydrogeologic framework, the S-value for the aquifer in the vicinity of the
test wells is about 0.0005.
Estimated Well Yields. In summary, the average T-value of the aquifer is
16,000 gpd/ft while the average available drawdown is 20.13 feet. Here, the available
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