MODELING RADIOCESIUM FIXATION IN UPLAND ORGANIC SOILS OF NORTHWEST ENGLAND

Two models of time-dependent Cs-137 adsorption and fixation by soil we re fitted to solution activity data from incubated organic upland soil s, over a period of 709 days. Both model fits were highly (p < 0.01) o r very highly (p < 0.001) significant. Both models differentiated betw een specifically adsorbed and nonspecifically adsorbed 'labile' radioc esium. Transfer to a nonlabile pool was either by a kinetic step (mode l I) or a diffusive process (model II). Although fits for model II wer e comparatively worse in some soils, the second model had the dual adv antage of being more mechanistic in structure and requiring fewer para meters. Parameters estimated for models I and II compared favorably wi th those derived experimentally; some dependence on soil mica was evid ent. Model I was used to predict the change in bioavailability of radi ocesium in soils following documented inputs from Windscale (1957), fa llout from weapons testing (1960s), and the Chernobyl release (1986). For all but one of the five organic soils, long-term predictions of re lative bioavailability generated by model I were similar to the limite d range of published values for Cs-137 uptake by vegetation. It is con cluded that the Cs fixation models presented account for a substantial part of the change in availability of Cs-137 observed in field studie s.