3.3 Water Resources
and/or dredging activities. Increases in turbidity can affect light levels in the water which can reduce
photosynthesis and plant growth. Additional effects of turbidity may result in impaired feeding and
respiration of fish and invertebrates.
Suspended silt particles in the water column will increase turbidity; however, larger sand particles (greater
than 63 microns [m]) will settle out rapidly and do not cause a significant increase in turbidity. Sampling
of the water near the receiver beaches indicates that nearshore water visibility typically ranges between 5
and 20 feet; however, visibility is significantly reduced in the surf zone due to sediment disturbance from
wave action and rip currents. Sediment testing has shown that the average percentage of fines in the
Oceanside Littoral Cell native sediments is approximately three percent above mean sea level (MSL) and
12 percent below MSL. The higher percentage of fines below MSL is attributed to the fact that finer
grained materials reside at equilibrium below the shorebase. Generally, intertidal waters of the receiver
beaches are characteristically turbid due to the high energy activity in the nearshore environment.
Water clarity for light (transmissivity) tends to increase with distance from shore. Transmissivity levels
typically range from 40 to 90 percent at depths of the borrow sites in the study area. In the June 1999
survey of proposed borrow sites SO-5 and SO-7, surface water transmissivity ranged from 83.9 percent
to 84.3 percent and bottom water transmissivity ranged from 64 percent to 69.6 percent.
Turbidity concentrations may be substantially elevated in coastal lagoons due to shallow depths, river
discharges, storm runoff, and/or algal blooms. Suspended particle concentrations of 100 mg/l were
recorded just inside Batiquitos Lagoon at the same time concentrations of 20 mg/l were recorded in the
adjacent nearshore during a non-storm period.
Sediment Characteristics
Trace metal and organic contaminants discharged into coastal waters can settle to the bottom. Finer
sediments (silts and clays) generally have higher contaminant concentrations thancoarser sediments (sands).
Contaminants may be remobilized through strong currents, storms, or mechanical disturbance such as
dredging. Thus, grain size characteristics and sediment contaminant concentrations at the borrow sites are
important to the evaluation of the potential for contaminant release and turbidity during dredging.
Most borrow sites are located miles from wastewater outfall point source discharges and several thousand
feet from non-point source river discharges, which represent the major sources of contaminate input to the
Page 3.3-4
Regional Beach Sand Project EIR/EA
99-69\SANDAG EIREA 3.4.wpd 7/17/00
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