Ecological Risk Assessment

Ecological risk assessment is a scientific process that evaluates the potential adverse impacts to plants, animals, and the environment from exposure to toxic contaminants. An ecological risk assessment does not consider the impacts to humans or domesticated species. The goal of the risk assessment is to evaluate actual and predicted potential effects on animal and plant populations. It also examines communities that occupy or could occupy habitats (such as lakes, wetlands, streams or estuaries) affected by contamination.

The U.S. EPA defines an ecological risk assessment as the process that evaluates the likelihood that adverse ecological effects are occurring or may occur as a result of exposure to one or more stressors (see definition below).

The Ecological Risk Assessment Process

There are three basic parts to an ecological risk assessment: Problem Formulation, Analysis Phase, and Risk Characterization.

Problem Formulation

Problem formulation is a systematic planning step that identifies the major factors to be considered in the risk assessment. It primarily involves a review of existing data about the site. The end product is a conceptual model that identifies the stressor characteristics, ecosystems potentially at risk, and ecological effects to be evaluated. In this step, assessment and measurement endpoints for the ecological risk assessment are identified.

Some Definitions:

• A stressor is any physical, chemical, or biological entity that can cause an adverse response, such as a toxic chemical.

• Assessment endpoints focus a risk assessment on particular components of the ecosystem that could be adversely affected at the site. These endpoints are expressed in terms of individual organisms, populations, ecosystems, or habitats with some common characteristic, such as ingestion or exposure.

• Measurement endpoints provide the actual measurements used to evaluate ecological risk and are selected to represent mechanisms of toxicity and exposure pathways. A measurement endpoint is a measurable biological response to a stressor that can be related to the valued characteristic chosen as the assessment endpoint. In most cases the endpoint is determined by modeling the site characteristics, such as the contaminants, soil/sediments, plants, and organisms, in the laboratory. In other cases, the endpoint can be measured directly.

In this phase, an exposure assessment and an ecological effects assessment are conducted.

Exposure Assessment

Exposure assessment quantifies the exposures of ecological receptors (animals, plants and microorganisms) to site contaminants. The key elements are quantification of contaminant release, migration and fate; determining the receptors that may be exposed; and measuring the concentrations to which the receptors are exposed.

The amount of contaminant a receptor takes in depends on such factors as:

• The properties of the contaminant.
• The way the organism takes it in (skin absorption, ingestion, etc.)
• The nature of the receptors (behavior, life patterns)

This step seeks to link concentrations of contaminants to adverse effects in receptors. Literature reviews, field studies, and toxicity testing provide this "dose-response" information—that is, how much toxicant is associated with how much of an adverse effect. Organisms vary widely in their ability to tolerate toxicants, and the research literature can provide information of the species under study. Field studies offer direct evidence of a link between contamination and ecological effects.

Finally, toxicity tests evaluate the effects of contaminated media on the survival, growth and reproduction of test organisms.

Risk Characterization

Risk characterization, the final phase of the risk assessment, compares the results of the exposure assessment with the results of the ecological effects assessment. Investigators seek to answer the following questions:

• Are ecological receptors currently exposed to site contaminants at levels capable of causing harm, or is future exposure likely?
• If adverse ecological effects are observed or predicted, what are the types and severity of these effects?
• What are the uncertainties associated with the risk assessment?

The Bottom Line

Ecological risk assessments can have their greatest influence on risk management at a site in the selection of site remedies. The ecological risk assessment should identify threshold contamination levels for adverse effects. These threshold values provide a yardstick for evaluating the effectiveness of remedial options and can be used to set cleanup goals.

For More Information

• EPA’s ecological risk assessment site

(www.epa.gov/superfund/programs/risk/ecolgc.htm)

• General overview of topic

(www.iet.msu.edu/journalists/ecotox/Default.htm)