Environmental Transport and Pathways of Exposure

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Humans, believed to be far more sensitive to radiation than plants and lower animals, were historically the focus of plutonium exposure scenarios and regulations have been designed to minimize human risk as a result of plutonium exposures (Table 3). More recently, however, the distribution of plutonium in other biological systems has been considered (Table 4).

Atmospheric sources of Pu (i.e., from fallout plutonium) can lead to human and animal exposure in the form of direct radiation, inhalation of respirable plutonium-containing dust particles and aerosols, and deposition on the skin (Figure 1). In the lung, the size distribution of plutonium-containing particles is directly related to the radiological dose received, the retention rates, and the distribution to target organs. Furthermore, plutonium solubility in lung tissue after inhalation affects the exposure time, with less-soluble compounds (e.g., Pu-239 dioxide) being retained longer than the more soluble forms (e.g., Pu-239 nitrate). Soluble compounds, however, are more available for absorption into the cytoplasm of a cell. This decreases the distance between the ionizing radiation and sensitive cellular structures such as DNA, which results in an increased chance of mutations. With dermal contact, the penetration of plutonium through the tissue is generally slight, which limits the exposure to cells near the site of contact and reduces systemic effects.

Deposition of fallout plutonium onto plant surfaces, soils, and surface waters can cause increased human uptake through ingestion of contaminated food and water. Whereas the total plutonium intake by ingestion is significantly higher than by inhalation, the fraction of ingested plutonium that is absorbed into the blood and distributed to target organs is relatively low (0.000 5). Inhaled material thus contributes to over 90% of fallout plutonium retained in the human body.

In a terrestrial environment, the fate and transport of plutonium are related to the chemical speciation and the partitioning of plutonium between soil and porewater. This determines the potential for removal from soil by processes such as plant uptake, water percolation through soil, or groundwater flow. Plant uptake of plutonium in terrestrial systems is relatively limited, due to the formation of insoluble Pu-hydrolysis products in soil solutions, and the fact that most plants can discriminate against and

Table 3 Examples of plutonium concentration limits used by regulatory agencies

US Environmental Protection Agency Air: maximum dose of radionuclides to humans Drinking water: max. contam. level for gross alpha emitters Example for soil action levels6 Rocky Flats, USA Enewetak Atoll, Pacific

European Union - Council Directive 98/83/EC Drinking water: total indicative dose

World Health Organization - Guidelines for Drinking Water Qualitye

Recommended RDL for drinking waterf

Guidance level for individual Pu-238, Pu-239, Pu-240 conc.

10mrema (0.1 mSv)

i5pCi r1s

35-600 pCig~1c 40-60 pCig~1c

0.1 mSvyr1d

0.1 mSvyr1 1 BqT1

ahttp://www.epa.gov/

bSoil action levels are determined individually for contaminated sites based on site-specific risks.

chttp://www.pacificislandtravel.com

dhttp://www.lenntech.com

ehttp://www.who.int/en/

fRDL = Reference dose level from a year's consumption of drinking water.

Table 4 Plutonium bioconcentration factors

Sample description

Bioconcentration factor

Flesh of freshwater fish

350a

Fish in water of low mineral content

50b

Fish in water of high mineral content

10b

Piscivores fish

5-250c,d

Planktivores fish

25d

Bottom-feeding fish

250d

Plants

2E-03e

aNational Research Council of Canada (NRCC) (1982) Data sheets on selected toxic elements, no. 19252. Ottawa: National Research Council. bCanadian Standards Association (CSA) (1987) Guidelines for Calculating Derived Release Limits for Radioactive Material in Airborne and Liquid Effluents for Normal Operation of Nuclear Facilities. National Standard of Canada, CAM/CSA-N288.1-M8. Rexdale, Toronto: Canadian Standards Association.

cMyers DK (1989) The general principles and consequences of environmental radiation exposure in relation to Canada's nuclear fuel waste management concept. AECL 9917. Chalk River, ON: Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories. dPoston TM and Klopfer DC (1985) A literature review of the concentration factors of selected radionuclides in freshwater and marine fish. PNL-5484. Richland, WA: Pacific Northwest Laboratory.

eGarten CT, Jr., Bondietti JR, Trabalka RL, et al. (1987) Field studies of terrestrial behavior of actinide elements in East Tennessee. In: Pinder JE, Alberts JJ, McLeod KW, and Schreckhise RG (eds.) Environmental Research on Actinide Elements, pp. 109-119. Washington, DC: US Department of Energy.

Primary sources

Environmental media

Biological receptors

Primary sources

Biological receptors

Ecological Receptors
Figure 1 Plutonium in the environment: sources of release, potential distribution, and pathways of exposure to biological receptors. 'Ad./inc.' indicates 'adsorption or incorporation'.

exclude plutonium at the root membrane level. However, if the plant absorbs the contaminant, its distribution in the plant tissue is minimized due to Pu stabilization by natural ligands and metabolites, causing most plutonium to be retained in the roots. As a consequence, the exposure to and accumulation by organisms feeding on the above-ground foliage is minimal, resulting in low plutonium concentrations in terrestrial food webs.

Up to 98% of the Pu in contaminated soils is immobile and is concentrated in the top few meters or centimeters. Recent field studies show, however, that plutonium can penetrate at least 2 m into the ground, and be transported with mineral or organic colloids in groundwater flow. Once plutonium reaches aquatic systems, by either direct release or mobilization from contaminated soils, it is distributed in the ecosystem over various pathways, as depicted in Figure 1. In freshwater and marine systems, plutonium has been shown to adsorb to and/or bioaccumulate and biocon-centrate in plants (e.g., seaweed), protists (e.g., algae), invertebrates (e.g., zooplankton, crayfish, snails, clams, mussels), and vertebrates (e.g., fish, fish eggs, and sharks), with benthic organisms and submerged plants accumulating the greatest concentrations. Uptake of plutonium is typically greatest from sediment and food sources, although assimilation efficiency is typically low. To a lesser extent, water can also provide a bioavailable source of plutonium depending upon environmental conditions. Typically, biomagnification does not occur. The majority of accumulated plutonium is deposited and sequestered in shell and bone, as opposed to the soft tissue of vertebrates and invertebrates.

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