2.3.3 Consideration of Mixing/Attenuation/Dispersion Zones

2.3.3 Consideration of Mixing/Attenuation/Dispersion Zones

The evaluation of effluent or surface runoff discharges should consider the

effects of mixing and dispersion in receiving waters. Mixing zones are normally

defined by the State regulatory agency as part of the CWA Section 401 Water

Quality Certification requirements. When effluent or 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), Clean Water Act (40 CFR 230), and

Section 103, Marine Protection, Research and Sanctuaries Act (40 CFR 227)

recognize this and explicitly provide for consideration of mixing in evaluating

dredged material discharges.

Mixing calculations will describe the spatial and temporal boundaries within

which the discharge may reach the applicable water quality or toxicity standards.

If these boundaries are within the established mixing zone limits, there should be

no risk. If these boundaries exceed the established mixing zone limits, the

discharge may not meet the mixing zone aspects of water quality certification

requirements. Some regulatory entities make no provisions for such events, in

which case the discharge should be managed or controlled to not exceed water

quality certification requirements. Other regulatory entities have provisions for

variances, waivers, or other case-by-case approaches for dealing with releases that

exceed established mixing zone limits.

In a similar manner, attenuation of leachate in foundation soils should be

considered in evaluation of the leachate pathway, and dispersion of volatile

emissions should be considered in evaluation of the volatile pathway.

Detailed procedures for calculation of mixing zones for effluent and runoff are

found in Appendix E. Guidance on considering attenuation in evaluating leachate

and dispersion in evaluating volatile emissions is presented in the chapters on those

pathways.

2.3.4 Control Material

Use of control materials is an integral part of evaluations for toxicity or uptake

(bioaccumulation) testing. The purposes of control materials in biological tests are

to confirm the biological acceptability of the test conditions and help verify the

health of the test plants or animals. The response to the control material is not to

be compared to the response to the dredged material to determine the effect of the

dredged material. The reference material (Section 2.4) is used for this purpose.

The essential characteristics of control materials are that they be essentially free of

COC and fully compatible with the needs of the test plants or animals such that

they have no discernable influence on the response being measured in the test.

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Chapter 2 Structure and Approach of the UTM