Transfer Factors for Contaminant Uptake by Fruit and Nut Trees (NUREG/CR-7174)
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Manuscript Completed: November 2013
Date Published: July 2014
B. A. Napier
R. J. Fellows
L. D. Minc
Pacific Northwest National Laboratory
P. O. Box 999
Richland, WA 99352
P. R. Reed, NRC Project Manager
NRC Job Codes N6455 and V6237
Office of Nuclear Regulatory Research
U. S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Transfer of radionuclides from soils into plants is one of the key mechanisms for longterm contamination of the human food chain. Plants absorb nutrients through their roots and transport them via the phloem to active portions of the plant. Nearly all computer models that address soil-to-plant uptake of radionuclides use empirically-derived transfer factors to address this process. Essentially all available soil-to-plant transfer factors are based on measurements in annual crops. Very few measurements are available for tree fruits.
In order to address this limitation, a sampling of various "standard" crops and fruit and nut trees from a single farm was made. This particular farm uses irrigation water from the local aquifer using surface irrigation (not overhead sprinklers) and is registered as an organic farm (no pesticides or refined fertilizers are used). Samples of alfalfa and oats (to compare with available transfer factors) and stems, leaves, and fruits and nuts of almond, apple, apricot, carob, fig, grape, nectarine, pecan, pistachio (natural and grafted), and pomegranate were collected, along with local surface soil. The samples were dried, ground, weighed, and analyzed for trace constituents through a combination of induction-coupled plasma mass spectrometry and instrumental neutron activation analysis for a wide range of naturally-occurring elements.
Analysis results are presented and converted to soil-to-plant transfer factors. These are compared to commonly used and internationally recommended values. Those determined for annual crops (e.g., alfalfa, grain (oats)) are very similar to commonly-used values; those determined for fruits and nuts differ from the generic recommendations in the literature. In most cases, the results of the transfer factors in fruits and nuts from this study are the only data available. Transfer factors for most macro- and micronutrients are slightly reduced in fruits from the generic recommendations; transfer factors for non-essential elements are reduced further. The results tend to support the use of chemical analogues for elements that are not homeostatically regulated (i.e., those for which internal levels are not regulated by the plant at optimal concentrations). These different findings may allow development of tree-fruit-specific transfer models.
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