E.1.4 Evaluation of dissolved contaminant concentrations by comparison with WQS

c. Evaluation of concentrations of suspended plus dissolved constituents by

comparison with toxicity test results after allowance for mixing under

Tier III.

E.1.4 Evaluation of dissolved contaminant concentrations by

comparison with WQS

If necessary, the potential for water quality effects may be evaluated by

comparison of predicted contaminant concentrations, as determined by screens

or laboratory tests, with the WQS, considering the effects of mixing. The mixing

evaluation need only be made for the contaminant requiring the greatest dilution

to meet its WQS. The key information derived from the model is the maximum

dissolved concentration of the contaminant at the boundary of the mixing zone.

This concentration is compared to the applicable WQS. See Section 2.3.2, Chap-

ter 2, maintext, for additional discussion of applicable WQS.

E.1.5 Evaluation of concentrations of suspended plus dissolved

constituents by comparison with toxicity test results

The potential water column toxicity of the discharge material may be

determined with toxicity tests evaluated in consideration of mixing. In this case,

the dilution of the discharge expressed as a percent of the initial volume of

disposed fluid in a given volume of water column is calculated. The key

parameters derived from the evaluation are the maximum concentration of the

discharge in the water column at the boundary of the mixing zone. These

concentrations are compared to toxicity endpoints such as LC50 or EC50 as

determined by toxicity tests.

E.1.6 Physical characteristics of dredged material discharges

Discharges of effluent or runoff from CDFs can be introduced to the

receiving waters in a variety of ways including direct pipeline outfalls or open

channels. For purposes of evaluation of initial mixing, barges or hopper dredge

discharges are discrete discharges, while direct discharges of effluent, runoff, or

leachate to surface water should be considered continuous discharges.

E.1.7 Confined disposal facility (CDF) effluent discharge

Dredged material, hydraulically placed in a confined disposal area, settles

into a thickened deposit of material overlaid by a clarified supernatant. The

supernatant waters are discharged from the site as effluent during active dredg-

ing operations. The effluent may contain both dissolved contaminants and

suspended colloidal particles with associated (adsorbed or held by ion exchange)

contaminants. Supernatant waters from confined disposal sites are discharged

after a retention time of up to several days. Furthermore, actual withdrawal of

the supernatant is governed by the hydraulic characteristics of the ponded area

and the discharge weir. The effluent suspended solids concentration is typically

Appendix E Evaluation of Mixing in Surface Waters