A comprehensive model for the activity of radionuclides in sediments is presented. The model is based on the advection-diffusion equations for sediment solids and a radioactive tracer. Mixing, caused by deposit feeders, is taken into account by a half-Gaussian (HG), integrated Gaussian (IG), or exponential (EX) diffusion coefficient with a maximum value D O at the sediment-water interface and an effective mixing depth m, where m = •r (HG), z m (IG), or a (EX). Compaction is described by an exponential bulk sediment density. The differential equations are solved by finite differences, in particular, the Crank-Nicolson method for the time-dependent case (Cs-137). Estimation of the mixing parameters D O and m is carried out efficiently by minimizing chi-square for measured and calculated steady state Pb-210 activities. The derived mixing depths ofz m = 4.6 __ 0.1 cm (station 14) and a = 0.8 +_ 0.1 cm (station 18) for the cores from Lake Huron are in good agreement with the organism 2 ß qdistributions. The corresponding D O values are 12 _ 2 and 1.5 _ 0.5 cm/yr, respectively, where the latter number is lower than a previous lower limit estimate (-> 3.3 cm2/yr), based on 1974 data. These diffusion coefficients are in excellent agreement with those inferred from the densities of Pontoporeia and tubificid oligochaetes (9.3 and 1.6 cm2/yr, respectively). Compared with southern Lake Michigan, there appears to be less sediment focusing in northern Lake Michigan, where the ratios of measured to atmospheric Cs-137 inventories have an average value of 0.89 and range from 0.32 to 1.41 for six cores from the deep basin. When these ratios are used to correct the Pb-210 fluxes, we obtain an atmospheric Pb-210 flux of 0.99 __ 0.06 dpm/cm2/yr. Because of its more realistic activity profiles, we consider the present model to be generally better than previous models.
INTRODUCTIONRadiotracers can provide valuable information on rates of sedimentation and mixing in aquatic sediments. These parameters are necessary for the evaluation of the rate of burial of toxic chemicals such as polychlorinated biphenyls (PCB's) and for estimating the probability that these chemicals will return to the water column. The study of rates of sedimentation and mixing is also necessary in order to properly interpret historical records of particle-associated substances in sediment cores. With a knowledge of these rates, one can predict the profile in the sediments from a given input record and conversely, through deconvolution, derive the input record from a given sedimentary record.Mixing in nearshore ocean and lake sediments is usually of biological origin and is often confined to the upper 3 -8 cm. Typical organisms in the Great Lakes are worms, e.g., tubificid oligochaetes and lumbriculids, and amphipods, e.g., Pontoporeia affinis and Pontoporeia hoyi [Robbins et al., 1977; Krezoski and Robbins, 1985]. Worms are conveyor belt feeders and eddy diffusers, whereas amphipods are mainly eddy diffusers. The determination of sedimentation parameters is greatly...
