5.4.7 LAT-R Computer-Assisted Runoff Toxicity Evaluation

The evaluation determines the potential toxicity of the SLRP or RSLS

simulation of runoff from the proposed operation, considering the times and

concentrations under which water-column organisms are potentially exposed to

runoff in the field. The toxicity test medium is SLRP or RSLS samples prepared to

simulate the whole-water runoff (i.e., not filterd). Detailed guidance for preparing

the runoff for use in toxicity tests is provided in Appendix C.

Procedures to evaluate runoff toxicity in the water column are conventional

water column toxicity tests. The toxicity tests involve exposing test organisms to a

dilution series containing both dissolved and suspended components of the

simulated runoff prepared as described in Appendix C. The test organisms are

added to the exposure chambers and exposed for a prescribed period (usually 96 h

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 simulated runoff 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) runoff test medium concentration. This result is then compared with

the concentration of the suspended dredged material at the boundary of the

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

The detailed procedures for conducting the water column toxicity tests with

the runoff described above are those provided for elutriate in the ITM (EPA/CE

1998).

5.4.7 LAT-R Computer-Assisted Runoff Toxicity Evaluation

The LAT-R 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 runoff toxicity. The LAT-R application, along

with documentation, can be downloaded from the USACE DOTS website at

www.wes.army.mil/el/dots. Manual data analyses procedures for evaluation of

water column toxicity are available in the ITM (EPA/CE 1998). These are

applicable to water column toxicity tests for runoff and can be used, if desired.

5.4.8 Tier III - Runoff Toxicity Decision

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

following conclusions is reached (Figure 5-2):

1. Information is sufficient to reach a decision regarding toxicity aspects of

runoff. This is the case when runoff toxicity poses no risk after

consideration of mixing, and no further runoff evaluation is necessary.

2. Information is not sufficient to reach a decision regarding toxicity aspects

of runoff. This is the case when simulated runoff indicates toxicity after

consideration of mixing. Further evaluation of toxicity aspects of runoff

under Tier IV, or management actions as an alternative to further

5-13

Chapter 5

Surface Runoff After Disposal Operations