4.3 Water Resources
movements. Measured contaminant concentrations indicate little potential for biological effects from
contaminant release during sediment resuspension. Dissolved sulfide concentrations were below detection
at all sites and total sulfide concentrations were low in borrow site sediments. Given the predominantly
sandynature ofthe borrow site sediments, and the mixing and diluting capacities of the coastal environment,
any increases in nutrients are anticipated to be short-term and not of sufficient concentrations to result in
plankton blooms that could cause red tides. The mixing and diluting capacity of waters at the borrow sites
are considered sufficient to avoid changes in pH as a result of dredging activities. No violation of the
California Ocean Plan objective would occur from dredging any of the borrow sites.
The potential for turbidity impacts associated with dredging at the borrow sites is also addressed in detail
in Appendix D. The California Ocean Plan specifies that discharges shall not result in significant reductions
in natural light at any point outside the initial dilution zone. Water clarity may be determined by numerous
methods including measurement of light transmissivity, irradiance, or secchi disk. Turbidity also may be
determined by collection of water samples and measurement of total suspended solids, chlorophyl
concentrations, and/or national turbidity units (NTU).
The duration and extent of turbidity relates to the concentration and settling rate of the resuspended
material, water depth, turbulence of the water, and current speed. Silts/clays remain in suspension longer
than sands. Turbulence contributes to sediment resuspension. High current speeds will transport turbidity
plumes greater distances than low currents. At the subject borrow sites, suspended sand-sized particles
would tend to settle through the 40-to 80-foot depths of the water column and reach the bottom within
minutes, while finer silt/clay particles could take several hours.
In the open ocean environment of the borrow sites, the extent of the turbidity plume will relate to particle
concentration, size, and settling rate, water depth, current speed and direction, and turbulence. The average
downcurrent turbidity plume distances that may be generated from dredging each borrow site were
estimated based upon the median grain size diameters of the sediment, and three different current speeds,
assuming the utilization of a hopper dredge. (The plume from a cutter dredge would be approximately one
half of those calculated for the hopper dredge.) As shown in Table 4.3-1, the estimated turbidity plume
would range from less than 100 feet to over 500 feet at average current speeds. Visible turbidity plumes
could extend several thousand feet at several of the sites under maximum current conditions, although the
concentration of particulates within the plumes would be highest near the dredge and would decrease with
distance away from the dredge.
Regional Beach Sand Project EIR/EA
Page 4.3-5
99-69\SANDAG EIREA 4.1 to 4.13.wpd 7/17/00
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