Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

3,000 homes exist within the basin. Most of these are single-family homes. The average
water usage of a single family home within the service area as of 2001 is 244
gallons/day (244 gpd or 0.273 AFY; see CCWD, March 2002, Table 6). Accordingly,
water imported for residential purposes in the basin amounted to 820 AF in 2001 (3,000
x 0.273).
These amounts do not account for the various commercial, multiple-unit
residential, and public service water users. Review of water sales data for the entire
service area (Table 5 in CCWD, March 2002) indicate that urban water sales (not
including floriculture and irrigation sales) are mostly residential (67 percent in both 1987
and 2001). The remaining water sales are for various recreational, commercial, school,
and multiple-unit residential customers. Applying this ratio to the groundwater basin
indicates that total water import for residential, commercial, and public service uses
amounted to 1,224 AF in 2001 (820 AF for residential and 404 AF for commercial,
multiple unit, and public service uses).
Of this water, some is used indoors, where it is either consumed by evaporation
or discharged as wastewater. The City of Half Moon Bay, including the groundwater
basin area, is predominately sewered and the collected wastewater is discharged to the
ocean.
The remaining portion of the imported water is used outdoors for landscape
irrigation. This irrigation water is consumed by evapotranspiration or percolates
downward to recharge groundwater. The amount of water used outdoors in Half Moon
Bay is estimated at 40 percent. This proportion is relatively low, reflecting the locally low
water use rates (CCWD, 2000) and the cool climate. Accordingly, the estimated amount
of water used outdoors is 490 AF (1,224 x 0.4). Of this amount, 15 percent or 73 AF is
assumed to percolate to groundwater.
In sum, estimated deep percolation of water from agriculture is 45 AF and from
landscape irrigation is estimated at 73 AF. The total deep percolation, 118 AFY under
current conditions, represents a small but significant inflow.
Subsurface Inflow. Groundwater flows into the groundwater basin along its
eastern boundary with the bedrock uplands and through the alluvium that underlies the
upper Pilarcitos Creek valley. The amount of inflow from bedrock is small and not
quantified for this study.
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