E.1.2 Regulatory considerations

order of hundreds of meters, the evaluation of mixing-zone sizes must

necessarily be based on calculation of near-field dilution and dispersion

processes.

There are a variety of possible estimation techniques for most real mixing-

zone problems, but any choice of a suitable technique involves some trade-offs.

The available techniques may be thought of as ranging from sophisticated

computer models, which are sometimes capable of very accurate predictions, to

simple approximations that yield order-of-magnitude estimates. The most

sophisticated models may not run on a microcomputer, and they may require a

considerable amount of effort and measured data for calibration of the model to a

single site. By contrast, the simplest of approximations may be made on the

basis of several simplifying assumptions and hand calculations.

E.1.2 Regulatory considerations

Any evaluation of potential water column effects from effluent surface

runoff discharges from CDFs should consider the effects of mixing. Section

230.3(m) of the 404(b)(1) Guidelines (USAEWES 1976) defines the mixing

zone as follows:

The term "mixing zone" means a limited volume of water serving as a

zone of initial dilution in the immediate vicinity of the discharge point

where receiving water quality may not meet quality standards or other

requirements otherwise applicable to the receiving water. The mixing

zone should be considered as a place where wastes and water mix and

not as a place where wastes are treated.

Further, Section 230.11(f) (USAEWES 1976) requires that:

The mixing zone shall be confined to the smallest practicable zone within

each specified disposal site that is consistent with the type of dispersion

determined to be appropriate by the application of these Guidelines. In a few

special cases under unique environmental conditions, where there is adequate

justification to show that widespread dispersion by natural means will result in

no significantly adverse environmental effects, the discharged material may be

intended to be spread naturally in a very thin layer over a large area rather than

be contained within the disposal site.

E.1.3 Potential applications of initial mixing

There are three potential applications of initial mixing evaluations:

a. Screening calculations under Tier II for water quality evaluations.

b. Evaluation of contaminant concentrations by comparison of discharge

concentrations with WQS after allowance for mixing under Tier III.

Appendix E Evaluation of Mixing in Surface Waters