Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

(1.000 g/cm 3 ), allowing the fresh water to float on top of the saltwater (Todd, 1980) and
to seep directly into the ocean. The fresh-salt water interface is a broad zone of mixing
rather than a sharp boundary. The ground surface expression of the interface usually is
defined by mean sea level along the coastline.
The shape of the interface is defined by a non-linear polynomial equation (Todd,
1980). The equation relates the width of the seepage face along the coastline, the depth
below sea level of the interface, the density contrast between saltwater and fresh water
(40), the hydraulic conductivity of the aquifer (224 to 500 gpd/ft 2 ; averaging 362 gpd/ft 2 ),
and the unit discharge of groundwater per unit length of shoreline (11,500 feet). The
fresh-salt water equilibrium requires that the water table or groundwater elevation
(potentiometric surface) be above sea level and slope toward the ocean, otherwise
seawater will advance directly inland (Todd, 1980).
Calculations show that the dynamic location of the toe of the fresh-salt water
interface changes with the amount of groundwater discharge from the aquifer. Between
an average winter discharge (2,334 AFY) and summer discharge (1,175 AFY), the toe of
the fresh-salt water interface moves inland from 34 feet in winter to 83 feet in summer
while the seepage faces decreases from 10 feet in winter to 5 feet in summer. Additional
pumpage from the marine terrace aquifer would reduce the aquifer discharge and result
in additional inland migration of the fresh-salt water interface. If 80 percent of the
summer discharge (940 AFY) is pumped from the aquifer, then the groundwater
discharge to the Pacific Ocean would be 235 AFY. The toe of the fresh-salt water
interface would move inland by 437 feet to a new dynamic position. The submarine
seepage face would decrease to one foot in width.
Land Subsidence
Land subsidence is the lowering of the natural ground surface, which can result
in damage to structures and disrupt the gradient of streams and canals (Todd, 1980). It
has been documented in areas of intensive groundwater pumping and groundwater level
decline, including portions of the San Joaquin and Santa Clara valleys. Such subsidence
occurs in groundwater basins with a significant thickness of saturated fine-grained
sediments and significant lowering of the water table. It results from loss of hydrostatic
pressure and subsequent compaction of clay layers. Coarse-grained sediments are
subject to smaller amounts of compaction (Borchers, 1998). Subsidence is not an issue
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