Page 783 - The Toxicology of Fishes

Ecological Risk Assessment 763

Problem Formulation
Available Information:
Stressor: Zinc
Source: Abandoned mine
Exposure Characteristics: Mine leachate is entering a stream that is valued
as a trout fishery
Effects Characteristics: Zinc is known to be acutely toxic to fish, including
trout
Assessment Endpoint:
Self-sustaining trout population; survival, growth, and reproduction of trout
Analysis Plan:
Measure in-stream zinc concentrations over range in streamflow

Analysis
Exposure Characterization
In-stream zinc concentration = 1000 µg/L with excursions to 7000 µg/L
during runoff and storm events

Effects Characterization
Laboratory-derived acute toxicity of zinc to trout:
Mean Acute Value (LC ) = 689 µg/L
50

Risk Characterization
Ambient Exposure Scenario
Hazard Quotient = Exposure Estimate = 1000 µg/L = 1.5
Effect Benchmark 689 µg/L

Event Exposure Scenario
Hazard Quotient = Exposure Estimate = 7000 µg/L = 10
Effect Benchmark 689 µg/L

High likelihood of fish kill events; maintenance of trout fishery unlikely.

FIGURE 18.3 Example of risk assessment components.

and between geographic locations, spatial scales, and time scales. Extrapolations may be based on
empirical models derived from experimental or observational data, on mechanistic models that simulate
key processes, or on professional judgment when such information is lacking. The selection of the
appropriate exposure–response and extrapolation approaches to be used depends on the scope and nature
of the ecological risk assessment and should be defined prior to conducting the effects assessment (i.e.,
during the problem formulation phase).

Risk Characterization
Risk characterization is the final component of the risk assessment and has as its objective the description
of risk in terms of the assessment endpoints identified in the problem formulation. The exposure and
effects data are integrated to formulate a risk estimate, typically in the form of a probability or likelihood
that adverse effects are occurring or will occur as a result of the existing or projected exposure. Risk