Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

eview of these hydrographs indicates that water levels fluctuate between 1 and 19 feet
on a seasonal basis. Those wells located adjacent to Pilarcitos Creek and screened
opposite the terrace marine aquifer show the greatest fluctuations, ranging between 10
and 15 feet and comparable to those observed at Well 29F4. This suggests significant
stream recharge in the winter.
Aquifer Storage and Usable Storage
Aquifer storage can be estimated for the marine terrace aquifer beneath the
coastal plain of Half Moon Bay. The areal extent of the aquifer (1,765 acres) is assumed
to be coincident with the area of the Lower Pilarcitos Creek groundwater basin. The
thickness of the aquifer ranges between 20 and 60 feet, while the specific yield of the
aquifer is about 15 percent (0.15). Therefore the total groundwater storage of the marine
terrace aquifer ranges from 5,290 to 15,870 AF, averaging 10,600 AF. Assuming that
only one-third of the stored water can be used, then the average usable storage is 3,533
AF. The seasonal water level changes result in significant changes in usable storage;
summer declines result in summer/autumn usable storage of about 1,763 AF.
Groundwater Discharge
Groundwater discharge to the Pacific Ocean in the Lower Pilarcitos Creek
groundwater basin can be estimated using a modification of Darcy’s law which states
that groundwater flow is proportional to the T-value (K-value times aquifer thickness), the
hydraulic gradient, and the width of the aquifer. The width of the aquifer from
Frenchmans Creek to Higgins Canyon is about 14,000 feet. The average summer and
winter hydraulic gradients are 0.0079 and 0.0093 ft/ft, respectively, and the T-value
ranges from 9,400 gpd/ft near the Balboa wellfield to 16,000 gpd/ft near the proposed
Lower Pilarcitos Creek wellfield. Groundwater flow ranges between 74.26 gallons per
day per unit width (gpd/unit width) of aquifer in the summer to 148.80 gpd/unit width of
aquifer in the winter. Calculations show that groundwater outflow to the ocean ranges
between 1,175 AFY under summer conditions to 2,334 AFY under winter conditions.
Saltwater Intrusion
The geologic units comprising the Lower Pilarcitos Creek groundwater basin
extend to the west underneath Half Moon Bay. Near the shoreline, an interface exists
between the onshore fresh water and saltwater beneath the ocean. The density of
saltwater (1.025 grams per cubic centimeter [g/cm 3 ]) is greater than that of fresh water
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