5.4.5 Tier III - Runoff Water Quality Decision

5.4.5 Tier III - Runoff Water Quality Decision

After consideration of the runoff Tier III water quality information based on

SLRP or RSLS results, one of the following conclusions is reached (Figure 5-2):

1. Information is sufficient to reach a decision regarding water quality

aspects of runoff. This is this case when WQS exist for all COC and are

met for all COC after consideration of mixing. No further runoff

evaluation is necessary.

2. Information is not sufficient to reach a decision. This may be the case

when:

a. SLRP results indicate WQS are exceeded for one or more COC after

consideration of mixing, and additional information using the RSLS

test is desired; or

b. There are no applicable WQS for some COC; or

c. There is concern about interactive effects.

Conducting the RSLS, evaluation of toxicity of runoff, or management

actions as an alternative to further evaluation should be considered. A decision to

implement management actions for runoff, such as placement of surface covers or

treatment, may require more detailed information for design of such actions. If

management actions are selected, no further runoff evaluation is necessary.

In determining the potential level of concern regarding interactive effects, the

number and classes of COCs that may be exceeded and the relative degree of

exceedences should be considered. Interactive effects may be purely additive,

synergistic (the resulting effect is greater than the sum of the effects from

individual COCs), or antagonistic (the resulting effect is less than the sum of the

effects from individual COCs). WQS were developed for single contaminants.

Where several are present and are close to WQS, especially if they are the same

class of contaminants (metals, chlorinated organics, metal-organic complexes,

nonpolar organics, etc.), interactive effects may be of concern.

5.4.6 Tier III - Runoff Toxicity Evaluation

Runoff should be evaluated for toxicity in Tier III if there are COC for which

there are no WQS or if there is concern regarding potential interaction of multiple

contaminants. Bioassays provide information on the toxicity of contaminants not

included in the water quality standards, and indicate possible interactive effects of

multiple contaminants. The Tier III runoff toxicity evaluation is based on use of

simulated runoff samples from the SLRP or RSLS as a medium to conduct water

column toxicity tests. Tier III toxicity testing assesses the potential toxicity of

runoff to appropriate sensitive water column organisms. As with water quality

evaluations of runoff, the results of the runoff toxicity tests should be interpreted

considering the effects of mixing (Appendix E).

5-12

Chapter 5

Surface Runoff After Disposal Operations