Soil-to-Plant Concentration Ratios for Assessing Food Chain Pathways in Biosphere Models

Abstract

This report describes work performed for the U.S. Nuclear Regulatory Commission's project Assessment of Food Chain Pathway Parameters in Biosphere Models, which was established to assess and evaluate a number of key parameters used in the food-chain models used in performance assessments of radioactive waste disposal facilities. Section 2 of this report summarizes characteristics of samples of soils and groundwater from three geographical regions of the United States, the Southeast, Northwest, and Southwest, and analyses performed to characterize their physical and chemical properties. Because the uptake and behavior of radionuclides in plant roots, plant leaves, and animal products depends on the chemistry of the water and soil coming in contact with plants and animals, water and soil samples collected from these regions of the United States were used in experiments at Pacific Northwest National Laboratory to determine radionuclide soil-toplant concentration ratios. Crops and forage used in the experiments were grown in the soils, and long-lived radionuclides introduced into the groundwater provide the contaminated water used to water the grown plants. The radionuclides evaluated include 99 Tc, 238 Pu, and 241 Am. Plant varieties include alfalfa, corn, onion, and potato. The radionuclide uptake results from this research study show how regional variations in water quality and soil chemistry affect radionuclide uptake. Section 3 summarizes the procedures and results of the uptake experiments, and relates the soil-to-plant uptake factors derived. In Section 4, the results found in this study are compared with similar values found in the biosphere modeling literature; the study's results are generally in line with current literature, but soil-and plant-specific differences are noticeable. This foodchain pathway data may be used by the NRC staff to assess dose to persons in the reference biosphere (e.g., persons who live and work in an area potentially affected by radionuclide releases) of waste disposal facilities and decommissioning sites. iv FOREWORDThe food-chain pathway contributes to the potential dose received by members of the public as a result of the potential release of radionuclides to the environment from various facilities licensed by the U.S. Nuclear Regulatory Commission (NRC). To quantify the contribution from this pathway, the NRC developed performance assessment strategies involving biosphere computer modeling codes to evaluate the potential dose to humans. These biosphere codes incorporate parameters for radionuclide uptake in plant roots and leaves, as well as animal products, to aid in predicting the radionuclide concentrations that humans would ingest in the event of an environmental release from the NRC=s existing and planned licensed nuclear facilities.This report provides soil-to-plant concentration ratios for long-lived radionuclides derived from experiments performed at Pacific Northwest National Laboratory (PNNL). These experiments used water and soil samples collected from thr...

Additional Research Needs to Support the GENII Biosphere Models

“…Investigations by Napier et al (2007; have indicated that regionally-distinct soil types can have a significant difference in root uptake by equivalent vegetation. The recent IAEA compendium provides some information for sand, loam, and clay type soils, but the information is sparse and frequently unavailable.…”

“…Most measurements are for Sr, Cs, and I. Napier et al (2007; investigated Tc, Pu, Am, Np, and I in three soils of potential importance to nuclear waste disposal scenarios in an attempt to expand the available information for potatoes and onions, but no other root crops.…”

“…Additionally, corn feed in the form of greenchop or silage is commonly modeled as grain, again, probably inappropriately. Napier et al (2007; investigated Tc, Pu, Am, Np, and I in three soils of potential importance to nuclear waste disposal scenarios in an attempt to expand the available information for corn. Many of the values reported in literature compendia are actually imputed on the basis of chemical similarity.…”

“…In the years since the early 1980s, additional work reflected in IAEA (2010) and NRC studies (Napier et al 2007; has broadened the possibilities for adding additional divisions of food type. The driver for subdividing the food types would be both public acceptance (people are more comfortable with describing their diets in terms of a range of common foods) and morphological differences in plant growth and growing season, size, and leaf-to-edible-portion ratios.…”

“…A similar model is needed for uptake from surface-and groundwater pathways; the model included in GENII (Napier et al 2012) is inadequate.…”