Texas (Cont.)

Tier II, risk assessment. Simple PLCs and analytical models are developed in a

Tier II assessment. If PCLs are exceeded or risk cannot be ruled out at Tier II, the

investigation proceeds to Tier III. Detailed, site-specific PCLs are derived in a

Tier III assessment. Through this tiered approach, a cost-efficient remediation

option that is protective of human health can be selected.

Document:

"Guidance for conducting ecological risk assessments under the

Texas Risk Reduction Program." (TNRCC 1996b).

Contact:

TNRCC, Office of Waste Management, Austin, TX.

This guidance incorporates the tiered structure of the Texas Risk Reduction

Program and is consistent with the USEPA's Framework for Ecological Risk

Assessment. It also includes some modifications to make this document specific

for Texas, such as the addition of state-developed criteria (Texas Surface Water

Quality Standards) and the consideration of livestock and crops as potential

ecological receptors.

As in the human health assessment process, the ecological risk assessment uses

a three-tiered approach.

1. The purpose of Tier I is to characterize the site and identify potential

exposure pathways.

2. Tier II is a screening-level ecological risk assessment, in which the

contaminants at a site that are likely to pose an ecological risk are

identified. There are three levels in a Tier II assessment:

a. The first level compares established ecological benchmarks to the

site data.

b. The second level uses toxicity reference values derived from

literature. This level involves problem formulation, an ecological

effects evaluation, exposure estimates, and risk characterization.

c. The third level reduces the Hazard Quotient by justifying the use of

less conservative toxicity values than in the second level.

3. A Tier III assessment is a quantitative ecological risk assessment in which

site-specific cleanup levels are developed.

A Tier III evaluation uses the effects and exposure information from the

second level of a Tier II evaluation to develop a problem formulation. After the

problem formulation, the study design is developed and verified in the field. Once

a sampling plan is established, the samples can be collected and analyzed. The

final step is a risk characterization, which can be used to select remediation

alternative(s). Risk characterization involves risk estimation, risk description, and

uncertainty analysis. The risk description of cleanup levels includes the threshold

for effects on assessment end points as a range of values between the NOAEL (no

observed adverse effects level) and the LOAEL (lowest observed adverse effects

level) for a particular contaminant. Decisions regarding remediation, no further

A27

Appendix A Summary of Federal, State, and Regional Guidance