Activities and geochronology of 137Cs in lake sediments resulting from sediment resuspension

In lakes with a large surface area to watershed ratio Cs-137 delivery is primarily by direct atmospheric fallout to the lake surface, where its activity in the sediments has been used to estimate the exposure to organisms and sediment mass deposition rates. Comparison of Cs-137 in the historical atmospheric fallout record with Cs-137 activity profiles in sediment cores reveals that although the general features of a maxima in the fallout deposition can be matched to activity peaks in the core, the general shape of the Cs-137 profile is not an exact replica of the fallout history. Instead, the sediment reflects post-depositional processes such as resuspension, bioturbation, partitioning of Cs-137 between the sediment solids and the pore fluids, and molecular diffusion of Cs-137 through the pore fluids. Presented here is a model that couples these processes to a system time averaging (STA) model that accounts for the time history of Cs-137 fallout and the particle residence time in the water column or in the 'active' surface sediment subject to resuspension. Sediment profiles are examined by comparing reasonable ranges of each of the coefficients of each of these major processes and by applying the model to cores collected from two large, shallow lakes, Lake Erie (USA/Canada) and Lake Winnipeg (Canada). The results indicate that the STA model with molecular diffusion and sediment resuspension best describes the data from these large, shallow lakes. (C) 2016 Elsevier Ltd. All rights reserved.