Technetium-99 cycling in deciduous forests: Review and ecosystem model development

Abstract

This paper presents a model to summarize information on the fate of technetium-99 ( 99Tc) in a woodland site amidst an old radioactive waste disposal area in the eastern United States. Rate constants for 99Tc transfer between the biological and physical components of the forest have been derived based on field measurements and associated laboratory studies. The fate of 99Tc in the woodland occupying the drainage are immediately adjacent to the disposal site was simulated with the derived rate constants and some assumptions about relative inputs to and outputs from the surface soil available pool in the model forest. A steady state in the distribution of 99Tc among soil compartments and forest compartments was reached in simulations after ∼30 years of continuous input to the soil available pool. If the rate of 99Tc export from the soil available pool exceeded inputs to the pool by ten times, then the model forest had little impact on the cumulative loss of 99Tc from the system. Under steady state conditions (input to the soil available pool equalled export), 42% of the total 99Tc input to the system during a 34-year period was retained by the forest, mostly in the unavailable mineral soil pool. This model, along with a review of recent work by other researchers, explains the probable fate of 99Tc that might enter element cycles in humid deciduous forests typical of the eastern United States. Experience with the model points to a need for further work on reduction and reoxidation rates of 99Tc in soil, particularly studies of reoxidation rate in soils where 99Tc has been recycled from aboveground or belowground organic matter.