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 D0 at the sediment-water interface and an effective mixing depth m, where m=&sgr;(HG), zm(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 D0 and m is carried out eficiently by minimizing chi-square for measured and calculated steady state Pb-210 activities. The derived mixing depths of zm=4.6¿0.1cm (station 14) and a=0.8¿0.1cm (station 18) for the cores from Lake Huron are in good agreement with the organism distributions. The corresponding D0 values are 12¿2 and 1.5¿0.5 cm2/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.