4.4.7 LAT-E - Computer-Assisted Effluent Toxicity Evaluation

the results of the water column toxicity tests should be interpreted considering the

effects of mixing (Appendix E).

The evaluation uses the effluent elutriate to determine the potential toxicity of

effluent from the proposed operation. Results should be interpreted with

consideration of mixing. The toxicity test medium is effluent elutriate prepared to

simulate the whole effluent (i.e., not filtered). Detailed guidance for preparing the

effluent elutriate for use in toxicity tests is provided in Appendix B.

Conventional water column toxicity test procedures are used to evaluate

effluent toxicity in the water column. The toxicity tests involve exposing test

organisms to a dilution series containing both dissolved and suspended

components of the simulated effluent prepared with the elutriate procedure as

described above. The test organisms are added to the exposure chambers and

exposed for a prescribed period (usually 96 hours though some tests, e.g., bivalve

larvae, may be run for shorter periods). The surviving organisms are examined at

specified intervals and/or at the end of the test, and the concentration at which the

test material produces an effect, if it does so, is determined. The results of the

water column toxicity test are expressed in terms of the LC50 or EC50 expressed

as a percentage of the original (i.e., 100 percent) effluent elutriate concentration.

This result is then compared with the concentration of the effluent at the boundary of

the allowable mixing zone to determine the acceptability of the effluent discharge.

The detailed procedures for conducting the water column toxicity tests with

the effluent elutriate described above are provided in the ITM (EPA/USACE

1998).

4.4.7 LAT-E - Computer-Assisted Effluent Toxicity Evaluation

The LAT-E application (Brandon, Schroeder, and Lee 1997) of the

ADDAMS suite of computer programs (Schroeder and Palermo 2000) provides a

computer program to assist in the analysis of effluent toxicity. The LAT-E

application, along with documentation, can be downloaded from the USACE

DOTS website at www.wes.army.mil/el/dots. If desired, manual data analyses

procedures for evaluation of effluent toxicity are available in the ITM

(EPA/USACE 1998).

4.4.8 Tier III - Effluent Toxicity Decision

After consideration of the Tier III effluent toxicity information, one of the

following conclusions is reached (Figure 4-2):

1. Information is sufficient to reach a decision regarding effluent toxicity. In

this case either:

a. The effluent toxicity poses no risk after consideration of mixing, and

no further effluent evaluations are necessary.

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Chapter 4

Effluent During Disposal Operations