B.3.3 Effluent Elutriate Toxicity Evaluation

B.3.3 Effluent Elutriate Toxicity Evaluation

The end result of this evaluation is the 96-hr LC50 or 96-hr EC50 expressed as

a percentage of the suspended dredged material concentration (or 100 percent

elutriate). This result is then compared with the concentration of the suspended

dredged material at the boundary of the allowable mixing zone.

B.4 Column Settling Tests for Effluent TSS/

Turbidity

If turbidity or SS are identified as COCs, or if water quality standards (WQS)

are specifically defined in terms of whole water (total) concentrations of COCs,

settling tests are necessary to provide data for design or evaluation of disposal

areas for retention of suspended solids and to compare to WQS (Figure B-2).

These tests are designed to define the settling behavior of a particular sediment and

to provide information concerning the volumes occupied by newly placed layers of

dredged material. If WQS exist for turbidity, a sediment-specific correlation of

suspended solids and turbidity must be developed (Thackston and Palermo 2000).

Sedimentation of freshwater slurries (mixtures of sediment and water) of

concentration less than 100 g/L can generally be characterized as flocculent

settling. As slurry concentrations are increased, the sedimentation process may be

characterized as a zone settling process, in which a clearly defined interface is

formed between the clarified supernatant water and the more concentrated settled

material. Zone settling also occurs when the sediment/water salinity is

approximately 3 parts per thousand (ppt) or greater. Flocculent settling also

describes the behavior of residual suspended solids in the clarified supernatant

water above the sediment/water interface for slurries exhibiting an interface. The

procedures described below define the sedimentation of suspended solids under

flocculent settling conditions or above the settled material/water interface under

zone setting conditions. The settling test procedures consist of withdrawing

samples from the settling column at various depths and times and measuring the

concentrations of suspended solids. Additional data should be collected from the

column settling test for purposes of CDF design for initial storage and minimum

surface area for a given inflow rate. These procedures are provided in Engineer

Manual 1110-2-5027 (USACE 1987).

B.4.1 Column settling test apparatus

An 8-in.-diam settling column such as shown in Figure B-3 is used. The test

column depth should approximate the effective settling depth of the proposed

disposal area. A practical limit on the depth of the test is 6 ft. The column should

be at least 8 in. in diameter with interchangeable sections and with sample ports at

1/2-ft or closer intervals.

Appendix B Column Settling Test and Effluent Elutriate Procedures