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() Soil-to-plant bio-concentration factor (BCF) estimates: Experimental variability versus model uncertainty.

McKone, T.1, 2, Maddalena, R.2, 1 University of California, Berkeley, California, USA2 Lawrence Berkeley National Laboratory, Berkeley, California, USA

ABSTRACT- Plant/soil bioconcentration factors (BCFs) are used to relate chemical concentrations measured in different vegetation tissues to concentrations in the soil supporting that vegetation. Much has been written about the uncertainty of plant/soil BCF estimates from empirical models. But these uncertainty assessments focus primarily on the ability of the empirical models to reproduce observed BCF values among a set of different chemicals. In this paper, we consider a single chemical and focus on the variability of BCF measurements obtained from a number of experiments with different plant species, different plant tissues, different experimental conditions, and different methods for reporting concentrations in the soil and plant tissues. We selected hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a compound that has been studied extensively. We consider 82 different experimental observations of plant-to-soil concentration ratios for RDX. This set includes different plant types--alfalfa, beans, corn, lettuce, tomato, carrot, cucumber, nutsedge, radish, and spinach; different plant tissues--roots, stem, leaves, pods, fruits, tassels, and seeds; both dry- and fresh-mass based measures of plant concentrations; measurements of chemical concentration and of radio-labeled tracers; and both dry- and solution-based measures of soil concentrations. We use these observations to evaluate both the magnitude and primary sources of experimental variability in BCF and compare this to estimated model uncertainty. Among these 82 observations, we find that coefficient of variability (CV) and geometric standard deviation (GSD) of the plant/soil BCF are on the order of 4.0 and 3.5, respectively. These values are significant relative to empirical model uncertainties--which have an estimated CV of 10 and GSD of 8.6. However, because contributions to overall uncertainty in fate models depend on the square of the CV, our estimated experimental variability for a single chemical only contributes about one twentieth of the uncertainty that is attributable to current empirical models for BCF.

Key words: terretrial vegetation, bioconcentration, plant uptake, uncertainty

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