Risk Characterization Phase

In this final phase of assessment, the likelihood and significance of adverse effects due to exposure to stressors are evaluated. Good risk characterization uses this evaluation to help build answers for decision-relevant questions. The phase includes two major steps: risk estimation and risk description. The final product of this phase is the risk assessment report, prepared for managers to support science-based decision making based on defensible assessment conclusions.

Risk estimation, the first step, integrates the exposure profiles and the stressor-response profiles developed in the analysis phase while also addressing uncertainties that arose throughout the assessment. The integration approaches can include comparing single values of effect and exposure; comparing statistical distributions of exposure and effect values; or conducting simulation modeling. In watershed assessments, the spatial distribution of exposure and effects is important to consider - GIS overlays of the two types of information can be a useful tool. In assessments where timing of events is critical (e.g., as in the acid precipitation example described previously, or in the assessment of episodic events), graphs that show the timing and distribution of excursions over an effects threshold may be a better way to integrate the information.

Also during this step, uncertainties originating in all three assessment phases and in external data should be summarized in an uncertainty analysis. Sources of uncertainty may include measurement data (inappropriate, imprecise or too few measurements), conditions of observation (such as extrapolating from laboratory tests to field predictions), or limitations of models (e.g., oversimplifying complex ecological processes). When exposure and effects data are limited or are not easily expressed in quantitative terms, qualitative evaluation techniques may be used to rank risks using best professional judgment and categories such as low, medium and high.

Risk description concludes the characterization phase with the preparation of an ecological risk summary and the interpretation of ecological significance. Summarizing risk involves making a bottom-line estimate of risk, usually in the form of a quantitative statement (e.g., there is an 80% chance of 50% forest mortality in the watershed due to air pollution). It is crucial to include a discussion of the weight of evidence supporting this conclusion, which may cover the quality of the data, corroborating information, and evidence of causality. Agreement among multiple lines of evidence increases the confidence in the conclusions, however any differences in findings need to be discussed. Useful additional analyses that could improve the assessment's certainty may also be identified.

Interpreting ecological significance translates possible risk estimates into a discussion of their consequences for the watershed. This step may address the nature and magnitude of effects, spatial and temporal patterns of effects, and the potential for ecosystem recovery. The significance of predicted effects may vary considerably in their consequences for different types of ecological systems. For example, the effect of a herbicide may be quite different in a stream that derives most of its organic carbon energy from plants as compared to a stream that utilizes predominantly detrital-based organic carbon. The loss of a small wetland area may be highly significant if it represents the only habitat available in an area for migratory waterfowl, but negligible if it occurs among thousands of other pothole-type wetlands.

After risk characterization, assessors should have a better understanding of the risks at hand and a scientifically defensible report that provides:

• A description of risk assessor/risk manager planning results
• A review of the conceptual model and the assessment endpoints
• A discussion of the major data sources and analytical procedures used
• A review of the stressor-response and exposure profiles
• A description of risks to the assessment endpoints, including risk estimates and adversity evaluations
• A summary of major areas of uncertainty and the approaches used to address them
• Documentation of science policy judgments or default assumptions used to bridge information gaps, and the basis for these assumptions.

Ecological Risk Assessment Flow Chart