A simple model to predict the duration of the mercury problem in Sweden

There are about 83 000 lakes in Sweden; about 40 000 lakes have fish (here = 1 kg pike) with Hg-concentrations above the guideline of 0.5 mg Hg/kg ww; about 10 000 lakes have pike with Hg-concentrations higher than the ‘blacklisting’ limit of 1.0 mg Hg/kg ww. How long will this situation last? The answer to that question involves many complex processes and assumptions about future developments, and is, thus, a very interesting area for modelling. The basic aim of this work has been to present a dynamic model to address this question, and critically evaluate the rates and environmental factors regulating the recovery process, so that quantitative predictions concerning the duration of the mercury problem in Sweden may be obtained. This modelling approach suggests that it would take very long, till about year 2360, until the mean Hg-concentration in pike would fall from the present of about 1 below the guideline value of 0.5. This model is based on certain assumptions about the future Swedish and European Hg-emissions. Sensitivity and uncertainty tests indicate that the most important rates regulating the uncertainties in the model predictions, and hence the duration of the mercury problem, are the processes governing the transport of mercury from land to water, and not lake processes. These land to lake fluxes, are, in turn, regulated by the atmospheric emissions and the deposition of mercury via wet and dry deposition. The only way to remediate the mercury problem (i.e., high concentrations of Hg in fish used for human consumption) is to reduce the basic cause of the problem, i.e., the anthropogenic emissions related to, e.g., the burning of fossil fuels, and the industrial use of Hg in the technosphere.