The challenge in estimating exposure to toxicants is to properly model ecosystem function. Exposure modeling must consider a hierarchical scale, system dynamics, and use them to determine what the limits of predictability may be. A hierarchical approach allows modelers to characterize exposure components and their linkages among different scales of ecological organization and complexity. Therefore, exposure needs to be analyzed at multiple scales and appropriate levels of ecological organization in both space and time. Although estimating exposure in terrestrial and aquatic ecosystems is linked, there are special considerations that need to be taken for each.
Aquatic systems are considered primary integrators within a watershed because they potentially receive, through surface or subsurface drainage/discharge, toxicants from outfalls and other contaminant sources within the watershed. If these toxicants reach biologically significant levels, they would be expected to affect the numbers, types, and health of stream organisms. Often, biological sampling is conducted in a stream system to measure the cumulative ecological effects of contaminant sources received by the aquatic system (pond, stream system, lake, etc.). Information from biological sampling is used to assess the environmental quality based on contaminant exposure and is often termed bioassessment. For aquatic assessments, evaluation areas are partitioned spatially, based on watershed boundaries and potential contaminant sources and classified in terms of the type of surface water body (streams, lakes, etc.) and their associated wetlands, including surface water, sediment, and related biota. These systems receive potential contamination discharged to surface water or migrating through groundwater from source contaminant areas, National Pollutant Discharge Elimination System outfalls, and operational facilities to points of potential receptor exposure. Ecological receptors feeding within stream-based food chains are exposed to the cumulative effects of contaminants that are released to the stream system, and their health can be considered an integrative indicator of the severity of contamination within the watershed. Because some watersheds/stream systems are large in size, the study area may need to be subdivided into subunits to facilitate the assessment process and identify areas of possible contamination with higher precision.
Exposure assessment models have primarily focused on the mobility of contaminants in the environment using vertebrates as the assessment endpoint (e.g., exposure to each contaminant in mgkg-1d_1 or mgl-1d_1). Invertebrate models are also used especially when soil screening levels (SSLs) are of interest; however, these studies usually concentrate on transfer factors associated with bioaccumulation models. As mentioned previously, the affects assessment may be expanded beyond comparison to ESVs by using existing data to conduct trophic exposure modeling (hereafter trophic modeling). Trophic modeling can be used to refine the list of toxicants to those constituents that may pose an adverse impact (significant risk) to specific ecological receptors. The trophic modeling uses toxicant exposure models for ecological receptors to calculate an exposure dose (ED) for each constituent that poses a potential risk through ingestion of contaminated media. EDs are compared with toxicity reference values (TRVs) to identify constituents with an evaluation-level hazard quotient (HQ, i.e., ED/TRV)
greater than 1. Results of the HQ assessment and other weight-of-evidence criterion can be used to further refine the list of constituents of potential concern. The use of exposure assessments must be appropriate to the spatial scale across which the toxicants of interest are dispersed. The most influential factor for contaminant accumulation in wildlife in any ecosystem is how much time the individual spends exposed to the contaminant and how it utilizes the ecosystem. In areas with broad-scale contamination, this must be done at the landscape level and can be achieved by the implementation of spatially explicit models that are calibrated using data from long-term biomonitoring of large areas. Specifically, exposure assessment considers the following:
1. Chemical distribution which defines the extent of measured chemical contamination to each exposure area and the approximate acreage of each exposure group. The chemical exposures that may be experienced by ecological receptors are affected by the degree oftheir spatial and temporal associations with the contaminated media.
2. Receptor distribution which involves the variety of factors that may affect the extent and significance of potential exposures. Receptor exposures are affected by the degree of spatial and temporal association with the contamination. A receptors' mobility may significantly affect their potential exposures to contaminants. Many species may only inhabit the study area during the seasonal periods (e.g., breeding season, nonmigratory periods). Nonmigratory species may remain in the vicinity throughout the year. These species, particularly those with longer life spans, have the greatest potential duration of exposure. For both terrestrial and aquatic systems, some species may live their entire life cycle within the systems and others may utilize the system for forage areas, water intake, reproduction, or utilize the area for early life stages only.
3. Quantification of exposure and effects assessment defines the degree to which contaminant distributions and receptor distributions overlap and indicates which receptors are likely to have the greatest potential exposures to contaminants. This can be conducted by comparing media concentrations to ESVs or further quantify exposures by calculating an intake for each chemical in each medium (sediment, surface water, prey). The effects assessment defines and evaluates the potential ecological response to the contaminant by use of TRVs or ESVs that are the basis of the comparison. To relate these numeric comparisons to the actual receptors, biological data can be used to determine if effects are occurring in the system.
As fish and wildlife occupy different habitats within an ecosystem, they may be exposed to toxicants through three pathways: oral, dermal, and inhalation. Oral exposure occurs through the consumption of contaminants through food, water, or soil/sediment. Dermal exposure takes place when contaminants are absorbed directly through the skin. Inhalation exposure occurs when volatile compounds or fine particles are respirated to the lungs. Therefore, the total exposure experienced by an individual is the sum of exposure from all three pathways or
Etotal Eoral ^ Edermal ^ Einhalation 
where Etotal is the total exposure from all pathways; Eoral is the oral exposure; £dermal is the dermal exposure; and Einhalation is the exposure through inhalation.
In aquatic systems exposure via inhalation and dermal pathways are usually considered as one factor. This is because total uptake for free-swimming aquatic receptors is assumed to be represented by simple partitioning from surface water alone. Aquatic receptors are assumed to be in equilibrium with contaminants in the water column (this assumption in many cases is erroneous and warrants further research). Contaminant partitioning between surface water and aquatic organisms is defined by a contaminant-specific bioconcentration factor. In general, the primary mechanism of contaminant uptake for many fully aquatic species is via direct uptake across permeable membranes such as gill and gill structures (which can be addressed under dermal exposure in eqn ). This can occur as a passive transfer or an active biological process (osmoregulation). Prey consumption, incidental ingestion of sediment and pore water/groundwater during prey consumption, and incidental ingestion of surface water during prey consumption are usually treated as secondary uptake mechanisms since they are modeled via bioconcentration factors rather than exposure models. This potential exposure parameter should be considered spatially dynamic since contaminant concentrations change based on their distance from a source.
Dermal exposure is assumed to be negligible for birds and mammals on many hazardous waste sites relative to other routes in most cases. Feathers and fur of birds and mammals further reduce the likelihood of significant dermal exposure by limiting the contact of skin with contaminated media. However, when an exposure scenario for a receptor species is likely to result in significant dermal exposure such as through brood patches on birds, direct contact by burrowing mammals, or swimming by amphibians, this exposure pathway should be estimated using models for terrestrial wildlife listed in the 'Further reading' section. Moreover, if contaminants that have a high affinity for dermal uptake are present (e.g., organic solvents and pesticides), dermal pathways should be considered even if contact is minimal compared to the aforementioned taxa. Inhalation of contaminants is treated as negligible at many waste sites since quite often these sites are either capped or vegetated. This minimizes exposure of contaminated surface soils to winds which results in aerial suspension of contaminated dust particulates. Also, the contaminants most likely to present a risk through inhalation exposure, such as most volatile organic compounds (VOCs), will quickly volatilize from soil and surface water to air, where they are diluted and dispersed. As a result, significant exposure to VOCs through inhalation is unlikely. In circumstances where inhalation exposure of endpoint species is believed to be occurring or is expected to occur, models for vapor or particulate inhalation may be employed.
Based on these factors, most exposure models in fish and wildlife concentrate on exposure through ingestion. The general formulas used to estimate contaminant exposure to terrestrial and aquatic wildlife via ingestion uses the ingestion rate multiplied by the concentration of the contaminant in all possible food items in relation to the body weight of the animal. Because many waste sites (contaminated areas) do not provide suitable habitats, exposure estimates are modified to be sensitive to the home-range size (total area used by an animal) or core area (areas used most often within an animal's home range) of the species as well as the habitats that are used or the probability of the species occurring in the area. These parameters are incorporated in the following equation:
where Ejis the exposure to contaminant through ingestion (j) (mg k_1g d_1 or mg d_1); P is the probability of the receptor species inhabiting a waste site or the proportion of the waste site used; A is the area (ha) of waste site; HR is the area (ha) that defines the receptor species home range or core area; m is the total number of ingested media (e.g., food, water, or soil); IR,- is the ingestion rate for media (i) (kg d_1 or l d_1); Cj is the concentration of contaminant j) in medium (i) (mgkg~ or mgl~ ); and BW is the whole body weight of endpoint species (kg).
The area is considered in two dimensions even for aquatic species since the area-to-home-range ratio is used to determine the fraction ofthe waste site in relation to the total area used by the animal for foraging. This could of course be modified to a volumetric parameter for aquatic species where the third dimension is necessary to determine that ratio.
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