Environ. Sci. Technol. 1996, 30, 2735-2741
Modeling Radiocesium Fixation in Upland Organic Soils of Northwest England J. P. ABSALOM,* N. M. J. CROUT, AND S. D. YOUNG Department of Physiology and Environmental Science, University of Nottingham, Sutton Bonington, Loughborough LE12 5RD, U.K.
Two models of time-dependent 137Cs adsorption and fixation by soil were fitted to solution activity data from incubated organic upland soils, over a period of 709 days. Both model fits were highly (p < 0.01) or very highly (p < 0.001) significant. Both models differentiated between specifically adsorbed and nonspecifically adsorbed ‘labile’ radiocesium. Transfer to a nonlabile pool was either by a kinetic step (model I) or a diffusive process (model II). Although fits for model II were comparatively worse in some soils, the second model had the dual advantage of being more mechanistic in structure and requiring fewer parameters. Parameters estimated for models I and II compared favorably with those derived experimentally; some dependence on soil mica was evident. Model I was used to predict the change in bioavailability of radiocesium in soils following documented inputs from Windscale (1957), fallout from weapons testing (1960s), and the Chernobyl release (1986). For all but one of the five organic soils, long-term predictions of relative bioavailability generated by model I were similar to the limited range of published values for 137Cs uptake by vegetation. It is concluded that the Cs fixation models presented account for a substantial part of the change in availability of 137Cs observed in field studies.
Introduction After the accident at the Chernobyl nuclear power plant in 1986, several upland regions of the U.K. (Wales, Scotland, and Cumbria in northwest England) became contaminated with radiocesium. Much of the radiocesium fell on acid peat soils, which led to substantial 137/134Cs+ bioavailability. The principal land use in these regions is sheep grazing, and in some areas restrictions were placed on the movement and sale of sheep. Models describing the transfer of 137Cs from soil to plants and animals are readily found in the literature (1, 2). It has been recognized that the availability of 137Cs to plants and hence the food chain is controlled to a large degree by soil adsorption processes (3). Adsorption of 137Cs in soils has * Corresponding author e-mail address:
[email protected].
S0013-936X(95)00899-6 CCC: $12.00
1996 American Chemical Society
been strongly associated with frayed edge sites in illitic minerals (4), specifically at regions of reduced interlayer spacing, called wedge zones. Cs adsorption on illite has been kinetically modeled by Comans et al. (5), who envisaged adsorbed 137Cs phases of varying exchangeability. A similar approach has also been used to describe soil availability in models concerned with transport of 137Cs from soil to plants and animals (6, 1). In their description of Cs adsorption kinetics in soil, Kirk and Staunton (1) partitioned adsorbed Cs into three forms, which differed in their rate of exchange with solution Cs. Parameter values were derived from studies of Cs adsorption on soils and clay minerals found in the literature. Such an approach may be useful for soils with a substantial mineral content, but might be inappropriate for organic soils whose radiocesium adsorption and fixation characteristics are likely to differ from those of mineral soils (7). In this paper, two models of Cs adsorption and fixation in soil are presented. The models have been developed using experimental data presented by Absalom et al. (7), which is outlined below. Each model describes Cs in terms of solution, adsorbed labile (exchangeable), and fixed (nonexchangeable) pools. Consideration is given to which model best describes the data and which is the most soundly based mechanistically. Estimated kinetic parameters are compared with those measured experimentally in the soils studied. An attempt is made to verify the models by comparing the (model) predicted changes in Chernobyl 137Cs bioavailability with the limited amount of suitable data currently in the literature.
Materials and Methods Experimental Data. The experimental procedure and resulting data used in this study have been described in detail previously (7). The soils are referred to by their site names and include five organic soils (Wastwater, Corney 2, Corney 1, Ennerdale, and Woodend) and a mineral soil (Sellafield). The soils classification, selected characteristics, and locations are given in Table 1. After being sieved to
