Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

(Geoconsultants, June 1987 and August 1991) estimated recharge as 15 percent of
runoff. Application of the assumed recharge rates of 5 and 15 percent to the average
annual Pilarcitos Creek flow at Half Moon Bay (11,543 AFY) yields a range from 587 to
1,730 AFY. For this study, an assumed average annual recharge rate of 1.0 cfs per day
on a year-round basis results in recharge of 723 AFY.
The recharge/runoff ratios also can be applied to evaluation of wet periods and
drought. For example, application of a 15 percent recharge rate to total streamflow in the
drought year of 1990 (1,725 AF, see Table 3.1) would result in estimated recharge of
only 259 AFY. Assuming a less efficient recharge rate of 5 percent in a very wet year like
1998 (36,783 AFY) results in an estimated recharge of 1,839 AFY.
Leakage from Pipelines. The CCWD’s most recent Water Supply Evaluation
(Table 4 in CCWD, March 2002) includes a discussion of leakage from pipelines, which
represent an inadvertent inflow of imported water to the groundwater basin. According to
CCWD, unmetered water includes authorized uses such as pipeline flushing and
firefighting. It also includes unauthorized uses, such as meter inaccuracy and pipeline
leaks. Annual unmetered water has ranged between 13.4 and 95.4 million gallons (MG)
over the study period and has averaged 50.4 MG per year. The CCWD’s review of
unmetered water in 2002 indicated that 18 MG of unmetered water were the result of
authorized uses and meter inaccuracy, and the remainder was the result of leaks.
For this study, it is assumed that an average of 20 MG per year of unmetered
water are the result of authorized uses and meter inaccuracy, and that the remainder,
about 30 MG per year (or 95 AFY) is the results of leaks. Of this service area-wide
amount, 50 AFY is assumed to occur within the groundwater basin, representing a small
inflow.
Outflows
Outflows from the Lower Pilarcitos Creek groundwater basin include subsurface
outflow to the ocean, groundwater pumping and export, and groundwater pumping and
consumption. Phreatophyte water consumption is very small, but significant to
supporting the riparian corridor along lower Pilarcitos Creek.
Subsurface Outflow. As described previously in the Hydrogeologic Setting
section, the marine terrace aquifer is a relatively thin (30 to 50 feet thick) aquifer that
slopes from east to west, extending under the Pacific Ocean. Similarly, groundwater
levels decline from east to west, indicating groundwater flow toward the ocean. As
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