5.4 Tier III Runoff Water Quality and Toxicity Evaluations

5.4 Tier III Runoff Water Quality and Toxicity

Evaluations

If Tier III is entered from Tier II because there was not sufficient information

to make a decision about WQS, the evaluation of runoff water quality should

proceed as described in Sections 5.4.1 through 5.4.5. If Tier III is entered from

Tier II because of the absence of applicable WQS or because of concern about

interactive effects, the evaluation of runoff toxicity should proceed as described in

Sections 5.4.6 through 5.4.8.

5.4.1 Tier III - Runoff Simulation Approaches

Two laboratory tests are available in Tier III for prediction of runoff quality, the

Simplified Laboratory Runoff Procedure (SLRP) and the Rainfall Simulator/

Lysimeter System (RSLS). The SLRP is a simple and cost-effective batch

extraction test for runoff quality prediction. The RSLS is a more costly,

time-consuming, and logistically demanding test in that it requires use of a

mechanical rainfall simulator and a large volume sediment sample exposed in a soil

bed (lysimeter) to a simulated rainfall runoff event. The SLRP is a more

conservative test procedure than the RSLS with respect to the predicted contaminant

release to the dissolved phase because the procedure exposes all particles in the test

sample to the extraction, while the RSLS only exposes the surface of the sediment

sample to the runoff simulation. Since the RSLS makes use of a simulator and

large-scale movable soil bed, it provides a more accurate simulation of runoff quality

by accounting for field conditions such as rainfall intensity, CDF slope, surface

exposure to runoff, and dredged material profile conditions to include crust

formation and cracking. Based on these considerations, the recommended

approach for Tier III runoff evaluations is to conduct the SLRP procedure

initially. If more accurate data are considered necessary prior to a decision, the

RSLS procedure can then be conducted.

5.4.2 Tier III - Simplified Laboratory Runoff Procedure (SLRP)

The SLRP is a predictive laboratory test consisting of an oxidation and

suspension simulation of the runoff generated within the CDF (Figure 5-3). The

occurrence of precipitation events on freshly placed dredged material will

normally produce water quality similar to the effluent during disposal and

dewatering operations. However, differences in carrier water (receiving water vs.

precipitation) and other exposure characteristics prevent the effluent data from

being used to predict precipitation runoff at this time. The SLRP also evaluates

potential oxidation and increased solubility of metals resulting from long-term

drying of dredged material.

Depending on the basis of applicable WQS, the prediction of the quality of

runoff from CDFs accounts for the dissolved concentration of contaminants and

may also consider that fraction associated with the released total suspended solids.

Although total contaminants in runoff are generally not required for water quality

comparisons, these data can be determined by analysis of unfiltered SLRP

5-9

Chapter 5

Surface Runoff After Disposal Operations