Abstract
Lakes act as natural integrators. Metal loads from different sources are collected and integrated in water and sediments. This may be used as a basis for a dynamic model to calculate heavy metal inflow to lakes without main tributary inflows. In this paper, a mass‒balance model for calculation of annual metal loads to lakes dominated by diffuse inflow and urban runoff is presented. Metal concentrations in urban runoff varies very much, a phenomenon generally called the ‵first flush′. In the lake, the variations are smoothed, which makes it possible to get a more stable flow assessment. Ten lakes in Stockholm (the capital of Sweden) have been investigated for water and surface sediment concentrations of heavy metals (Cd, Cr, Cu, Hg, Ni, Pb and Zn). The model accounts for inflow, outflow and sediment interactions (sediment resuspension is modelled using new approaches). It is meant to describe water and sediment concentrations as accurately as possible. An uncertainty test was carried out to rank the influence of the uncertainty of different parts of the model for the total uncertainty in modelled water and sediment concentrations. The most important factors for the uncertainty of model predictions were the settling velocity of particulate matter, the percentage A-areas (accumulating areas for fine paricles) and the dissolved fraction of the total metal concentration. Metals accumulating in the deeper (geological) sediments may be considereed as removed from the biosphere and a flux to the geosphere. Hg was found to accumulate the most in the sediments followed by Pb; least accumulating were Ni and Cr. High correlations between metal accumulation and phosphorous and nitrogen concentration indicated that the autochthonous lake production was a crucial factor for the accumulation of metals in the sediments. Significant influences of lake morphometry (resuspension) and water pH (metal particle association) on the sediment accumulation could also be noted.
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