Regional Scale Ecological Risk Assessment and the Relative Risk Model
Regional Scale Ecological Risk Assessment Defined
Ecological risk assessment calculates the probability of an impact to a specified set of assessment endpoints over a defined period of time. In the risk assessment of chemicals, exposure and effects are estimated and the probability of the intersection of those functions calculated. Impacts typically considered are mortality, chronic physiological impacts and reproductive effects. Most often these risk assessments deal with single chemicals in such classic cases as pesticides, herbicides, organic solvents, metals, polychlorinated biphenyls, and dioxins. Most often the risk assessments dealt with only one or a few biological endpoints.
During the 1990s there was an effort to expand ecological risk assessment to more accurately reflect the reality of the structure, function and scale of ecological structures. Hunsaker, O’Neil, Suter and colleagues (Hunsaker et al 1990, Suter 1990, O’Neil et al 1997) formulated the idea of performing regional risk assessments at a landscape scale. There have been attempts to perform risk assessment based upon the classical USEPA paradigm, but each has had limitations (Cook et al 1999, Cormier et al 2000) imposed by a risk assessment framework originally designed for single chemicals and receptors. A principal difficulty is the incorporation of the spatial structure of the environment and the inherent presence of multiple stressors.
We (Landis and Wiegers 1997, Wiegers et al 1998) adopted a definition that naturally incorporates multiple stressors, historical events, spatial structure and multiple endpoints. Our working definition of a regional scale risk assessment is:
Regional scale ecological risk assessment. A risk assessment deals at a spatial scale that contains multiple habitats with multiple sources of multiple stressors affecting multiple endpoints and that the characteristics of the landscape affects the risk estimate. Although there may only be one stressor of concern, at a regional scale the other stressors acting upon the assessment endpoints are to be considered.
The Goals of an Regional Scale Risk Assessment
The goals of regional scale risk assessment are those of any ecological assessment. As stated by the U. S. EPA, “Ecological risk assessment is a process used to systematically evaluate and organize data, information, assumptions, and uncertainties to help understand and predict the relationship between stressors and ecological effects in a way that is useful for environmental decision making” (1998 U.S. EPA Guidelines for Ecological Risk Assessment; EPA/630/R-95/002F; Published on May 14, 1998, Federal Register 63(93):26846-26924). Similarly, regional risk assessments are most effective when they target the decision-making needs and goals of environmental managers. This is the cornerstone from which the regional assessment develops. Important, and perhaps difficult or even conflicting goals and decisions must be identified early in the process. Without identifying, discussing, and resolving these issues, the assessment results will not appear to be useful to managers, and in fact may not be usable for the decisions at hand.
The stated goal of regional risk assessment requires that the process and output must be communicated clearly to the decision makers and environmental managers. Risk, uncertainty and sensitivity must be appropriately communicated. The myriad decisions and assumptions made in the risk assessment must also be documented.
Framework of the Relative Risk Method
The framework for the relative risk model for regional risk assessment was outlined by Landis and Wiegers (1997). EcoRA methods traditionally evaluate the interaction of three environmental components: stressors released into the environment, receptors living in and using that environment, and the receptor response to the stressors (Figure 1a). Measurements or estimates of exposure and effect quantify the degree of interaction between these components. At a single contaminated site, especially where only one stressor is involved, the connection of the exposure and effect measurements to the assessment endpoints can be relatively simple. However, in a regional-multiple stressor assessment, the number of possible interactions increases dramatically. Stressors arise from diverse sources, receptors are often associated with a variety of habitats, and one impact may lead to additional impacts. A complex background of sets of natural stressors and effects further clouds the picture.
Expanding an assessment to cover a region requires consideration of larger scale regional components: sources that release multiple stressors, habitats where the multiple receptors live, and the multiple impacts to the assessment endpoints (Figure 1b). The three regional components are analogous to the three traditional components, but the emphasis is on location and groups of stressors, receptors and effects.
Traditional risk assessment estimates the level of exposure and effect to calculate risk. However, exposure and effect cannot be directly measured unless a specific stressor and a specific receptor are identified. At a regional level, stressors and receptors can be represented as groups: a source as a group of stressors, a habitat as a group of receptors, and an ecological impact as a group of receptor responses. These combinations involve the use of a variety of distinctly different measurements. For example, the measurement of a polychlorinated organic compound will results in units, mg/L, distinctly different from the occurrence of an invasive species, number of organisms/m2. Yet both can be present within the area of study. Impacts can be similarly varied, mortality may have to combined with an decrease with the occurrence of non-native species. It is very intractable to attempt to combine measurements taken with distinctly different units.
However, it is possible to combine these measurement based on the establishment of ranks. In this manner a concentration of a chemical that may cause a high degree of mortality can be combined with an invasion of a new species that will alter a small amount of habitat. The criteria for setting ranks is discussed later, but the crucial feature is that this approach allows the evaluation of multiple stressors being derived from multiple sources impacting a variety of species in a variety of habitats in variety of locations.
Relative regional assessment identifies the sources and habitats in different locations of the site, rank their importance in each location, and combine this information to predict relative levels of risk. The number of possible risk combinations resulting from this approach depends on the number of categories identified for each regional component. For example, if two source types (e.g., point discharge and fish waste) and two habitat types (e.g., the benthic environment and the water column) are identified, then four possible combinations of these components can lead to an impact. If in addition we are concerned about two different impacts (e.g., a decline in the sport fish population and a decline in sediment quality), eight possible combinations exist.
Each identified combination establishes a possible pathway to a risk in the environment. If a particular combination of components interact or affect each other, then they can be thought of as overlapping. When a source generates stressors that affect habitats important to the assessment endpoints, the ecological risk is high. A minimal interaction between components results in a low risk. If one component does not interact with one of the other two components, no risk exists. For example a discharge piped into a deep water body is not likely to impact salmon eggs, which are found in streams and intertidal areas. In such a case, the source component (an effluent discharge) does not interact with the habitat (streams and intertidal areas), and no impact would be expected (i.e., harm to the salmon eggs). This is analogous to the overlap between the stressor, receptor and hazard in conventional risk assessment. Impact 1 may also be due to the overlap of several sources of stressors with several habitats, all altering the risk. Integrating these combinations demonstrates that impact 1 is actually the result of several combinations of sources and habitats. To fully describe the risk of a single impact occurring, each possible route to the impact needs investigation.
Integration of these routes is not always a simple matter and is again facilitated by the use of ranks. Often, measurements of various exposure and effect levels cannot be added together to determine the overall impact to the assessment endpoint. For example, a decline in wild salmon populations can result from a combination of eggs in the spawning grounds being exposed to chemicals and increased predation when the juveniles migrate out of the Port. However, chemical exposure to the eggs may also influence growth of the juvenile fish. Smaller fish are less able to avoid predation; therefore, mortality from predation may increase beyond what would be expected if the effect to the eggs was not considered.
Our regional approach is a system of numerical ranks and weighting factors to address the difficulties encountered when attempting to combine different kinds of risks . Ranks and weighting factors are unit-less measures that operate under different limitations than measurements with units (e.g., mg/L, individual/cm2). In a complex system with a wide range of dissimilar stressors and effects, few measurements exist that are additive. For example, there is little meaning in adding toxicant concentrations to counts of the number of introduced predators in order to determine the total risk in a system. However, knowing that a particular region has both the highest concentrations of a contaminant and the most introduced predators is useful in a decision making process.
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