5.1.2 Influence of CDF Design and Operation on Runoff

5.1.2 Influence of CDF Design and Operation on Runoff

The method of filling can affect the erosiveness of the dredged material, the

rate of runoff, and resulting suspended solids generation. Hydraulic disposal

tends to provide a smoother surface while mechanical disposal from a conveyor or

truck tends to provide a rougher surface unless altered by grading equipment.

Most runoff studies to date have addressed the hydraulic disposal option. No data

have been gathered to determine if there is any significant difference in runoff

characteristics as a result of mechanical disposal. It is assumed that although

suspended solids generation may be different between disposal options, the effects

on soluble contaminants would not be significantly affected and the current testing

approach is suitable for both.

5.1.3 Regulatory Considerations

If there is a reason to believe that surface water runoff might contain

contaminants, evaluations using this chapter will be required. As defined, surface

water runoff is considered as a discharge of dredged material to waters of the

United States and is subject to the same nationwide permit as effluent discharge,

requiring Section 401 Water Quality Certification. The water quality certification

issues for surface runoff should be addressed at the same time that certification is

obtained for the effluent discharge.

In addition to typical contaminants, WQS may exist for nutrients and physical

parameters such as turbidity or TSS. Chlorides should be considered a potential

COC whenever there is the potential for runoff from saline dredged material to

enter a fresh water system.

5.1.4 Mixing Zones

As for effluent, the evaluation of runoff discharges should consider the effects

of mixing and dispersion. Mixing zones are normally defined by the State

regulatory agency as part of the CWA Section 401 Water Quality Certification

requirements. When runoff enters receiving waters, it is dispersed by natural

physical processes so that the concentration decreases spatially and temporally

beyond the point of entry. This phenomenon is important in determining the

potential for effects, because effects depend on both the concentration to which

organisms are exposed and the length of time for which they are exposed. Effects

are generally less at lower exposure concentrations or shorter exposure times, and

for each COC, there are exposure time-concentration combinations below which

effects do not occur. The Federal regulations implementing Section 404(b)(1) of

the Clean Water Act (40 CFR 230) recognize this and explicitly provide for

consideration of mixing in evaluating dredged material releases.

Mixing calculations describe the spatial and temporal boundaries within

which the discharge will reach the WQS. If these boundaries are within the

established mixing zone limits, there should not be an effect. If these boundaries

exceed the established mixing zone limits, there may be an effect.

5-3

Chapter 5

Surface Runoff After Disposal Operations