A dynamic simulation based on a simple box model was made to predict Pb transport in spodosols of the Hubbard Brook Experimental Forest. Simulated results suggest that labile Pb in the forest floor may be undergoing a rapid loss, and that Pb content may reach an equilibrium within apprx 100 years with a steady-state level of approximately 0.2 kg ha-1 (concentration = 1.3 mu-g g-1). The predicted Pb loss from the forest floor is much higher than the observed Pb export based on zero-tension lysimeters, which are designed to optimize measurement of dissolved substances. It is suspected that lysimeters might have failed to effectively collect particles and colloids. The dissolved Pb-2+ loss from the forest floor, which is governed by nonlinear retardation, is insignificant relative to total Pb losses, so linear rate removal of particles and colloids from the forest floor is an adequate approximation of Pb transport. The mineral soil is currently acting as a net sink for the Pb released from the forest floor. The model suggests that Pb content in and Pb output from the mineral soil has been increasing since the 1970s. This increase should continue until a steady-state is reached in about 100 years. Unlike the forest floor, the mineral soil loses its Pb via dissolved forms that are regulated by nonlinear adsorption/desorption retardation.
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