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Trends in Ecological Risk Assessment

For more information, contact Joe Zupan at 512.329.5544.  

While the science of human health risk assessment has been steadily advancing over the past 15 years or so, assessing risks to ecological receptors is typically more complex and time-consuming, and the state of the art is not so advanced as for human receptors. The process for quantifying risk to either human or ecological receptors is more or less the same:

1. Identify chemicals (or ecological stressors) of concern;

2. Evaluate concentration and persistence of chemicals in source media;

3. Evaluate contaminant migration pathways;

4. Identify receptors and quantify exposure;

5. Evaluate toxicity of chemicals/stressors to receptors;

6. Quantify and characterize risk.

In the case of human health risk assessment, this process is somewhat simplified since only one "receptor species" is involved, and since there is more toxicity data for human health effects (even if gathered from the use of animal models) than for other species.

As an example, toxicity data for the effects of cadmium in soil on a specific perennial grass or an earthworm may be nonexistent, and must be extrapolated from data based on other species. In addition to this lack of data, the broader context of ecological receptors includes a consideration of so called "communities," "feeding guilds," and selection of "representative species." This complexity is also sometimes described as the "food web," a term that implies an interlocking pattern of "food chains."

Much state and federal guidance attempts to manage this complexity by using a "tiered" approach to the ERA, depending on the complexity of the ecology at the site. An example is Texas' three-tiered approach as specified in Guidance for Conducting Ecological Risk Assessments at Corrective Action Sites in Texas. This guidance specifies three tiers:

1. Exclusion Criteria Checklist - "exclusion criteria" refer to conditions at an affected property that preclude the need for a formal ERA because there are incomplete or insignificant ecological exposure pathways.

2. Screening-Level Ecological Risk Assessment - the purpose of a SLRA is to eliminate chemicals of concern (COCs) that do not pose an ecological risk and to calculate protective concentration levels (PCLs) for those COCs that do pose an unacceptable risk to selected ecological receptors.

3. Site-Specific Ecological Risk Assessment - the purpose of this most-intensive tier of assessment is to incorporate additional information obtained through site-specific studies designed to provide a more empirical evaluation of ecological risk at an affected property. This level of assessment is typically applied when a Tier 2, or screening-level ERA, suggests that site conditions pose an unacceptable risk. A third-tier assessment can sometimes be useful to refine risk calculations using actual site measurements versus default parameters.

One area into which the state of the art of ERA is advancing is the use of distributional data and probabilistic techniques (e.g., "Monte Carlo" analysis). Probabilistic risk assessment (PRA) has been used to a limited extent for human health risk assessment for several years, and the various complexities and uncertainties inherent in ERA may make the use of probabilistic techniques even more applicable in this arena.

Once an ERA has been performed and an affected property has been characterized, it can often be somewhat difficult to establish how the site should best be remediated. Many traditional remediation techniques ("dig and haul," "cap and cover," etc.) can sometimes pose more risk to ecological receptors than the original contamination.

One of the ecological risk management options available under remedy selection guidance in state and federal ERA programs includes the use of an ecological services analysis (ESA). The ESA considers the present and predicted ecological "services" of an affected property, as well as the beneficial and/or detrimental effects on services associated with potential response actions. At certain sites, an ESA may reveal that the best approach is to perform compensatory ecological restoration to address ecological concerns by providing or restoring alternative services when a response action at an affected property may cause additional damage to environmental receptors. For example, it may be possible to purchase adjacent or nearby property and maintain it in a natural state to promote the health of ecological receptors that are adversely impacted at an affected property.

The bottom line: a complete and thoughtful consideration of ecological risks may ensure that an affected property is adequately characterized and remediated in the most cost-effective manner, even if the remediation includes the use of a more novel approach such as compensatory ecological restoration. Factoring in this consideration of the ecology early in the process will promote better planning for characterizing and rehabilitating affected properties.