Conceptual Site Model

Figure 1 provides a schematic illustrating the various ways in which biota can be exposed to radiation or radioactivity in the environment. There is transfer between abiotic environmental media; in the simplified representation provided in Figure 1, this is shown by radioactivity present in the air and in rain being transferred into the aquatic environment. The aquatic environment has different components: in the case of the freshwater environment this includes streams, rivers, lakes, and sediments; and in the marine environment includes tidal zones, coastal

Table 1 Simplified framework for assessing risks to plants and animals

Conceptual site

Define pathways of exposure

model

Identify possible exposed biota and characterize reference biota or indicator species

Consider individual biota when species are rare or endangered

Exposure

Estimate levels of radiation and radioactivity in the environment by use of measured data and/or models

Determine spatial and temporal patterns of radiation and radioactivity

Determine uptake by organism

Dosimetry

Estimate absorbed dose (by whole body or by tissue/organ)

Perform geometry corrections for size of organism

Utilize radiation weighting factors for internally deposited radionuclides

Reference dose

Define appropriate population-level effects such as mortality or reproductive capacity; and corresponding

rate

reference doses

Establish corresponding reference dose rates below which effects on populations of biota are unlikely, in many

cases a value of 10mGyd~1 is appropriate

Assessing effects

Compare estimated dose rate to reference dose rate and develop a screening index value which takes into

account:

spatial and temporal aspects of exposure

natural population variability

background radiation levels

Determine the possibility for effects in biota

Figure 1 Simplified schematic illustrating pathways of exposure for nonhuman biota.

waters, and marine sediments. Radioactivity can then be transferred into biota such as plants, phytoplankton, zooplankton, macro-invertebrates, sessile aquatic plants, fish, water-based amphibians, crustaceans, mammals, and birds which obtain dietary components from the aquatic (freshwater/marine) environment.

Radioactivity can also be transferred into the terrestrial environment which includes soils which can then be taken up into terrestrial biota such as plants, invertebrates, and vertebrates (mammals, birds, reptiles, and land-based amphibians).

It is important to note that the terrestrial and aquatic environments are not totally separate since some terrestrial biota obtain food or drinking water from the aquatic environment. For example, bears eat fish and moose and waterfowl feed on aquatic plants. Therefore, care is needed in defining the ways in which such animals may be exposed within the conceptual model.

As discussed above, there is a vast array of biota that can be considered in the development of the conceptual model. However, it should be recognized that it is not possible to develop information on all biota; therefore, the concept of a set of reference organisms that are representative of common ecosystems has been developed to focus the effort on methods and data needed to assess these reference organisms. The results of such dose assessments for predefined reference organisms facilitate a basic assessment of possible biological effects. This approach provides a strategy that allows assessment efforts to be focused and thereby reduced to a more manageable size.

The International Commission on Radiation Protection (ICRP) defines ''a Reference Animal or Plant (RAP) as a hypothetical entity, with the assumed basic characteristics of a specific type of animal or plant, as described to the generality of the taxonomic level of family, with precisely defined anatomical, physiological, and life-history properties that can be used for the purposes of relating exposure to dose, and dose to effects, for that type of living organism'' and describes the reference animals and plants in groups (family or taxonomic level). The reference organisms selected by the ICRP cover a range of global ecosystems and taxonomic families.

Therefore, the first step in the environmental risk assessment (ERA) process involves developing a conceptual model of the study area, an understanding of the sources and routes of exposure, and, based on this understanding, the selection of representative organisms for assessment purposes. It is important to understand that biota considered as reference organisms in a particular ERA need to be representative of the particular location and, therefore, the reference organisms used will vary from assessment to assessment. In selecting reference organisms for the assessment, consideration should be given to the value of the biota to the local ecosystem. For example, special consideration is generally given to rare or endangered species as these need to be protected.

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