Modeling episodes in pollutant dispersion in Lake Michigan

Numerical simulations of several important episodes in the horizontal dispersion of a conservative substance in thermally-stratified (summer) Lake Michigan were performed. Lake-wide dispersion of chloride during the summer of 1970 was modeled for 125 days using U.S. Weather Service and USEPA data. The results obtained indicated that more useful information might be obtained more easily by simulating local short-term episodes of pollutant movement from single sources. Accordingly, best and worst case 33-day simulations of the dispersion of identical slugs of pollutant from Calument Harbor were performed. The results for these extreme cases suggested that the disposition of pollutant from any given source is highly dependent upon short-term wind conditions and that such best and worst case modeling might easily provide some of the detailed insights required for water management purposes. The dispersion model's applicability to unstratified (winter) Lakes Michigan and Ontario is demonstrated for some simple cases. The numerical model used is readily adaptable to a wide range of episodes' of interest. Actual time-varying lake circulations are efficiently approximated by sequences of quasi-steady-state circulations produced by a separate flow model, and time-varying pollutant sources are easily introduced at any point in the lake. A strictly mass-conservative finite-difference formulation ensures numerical stability, even for (potentially) inconsistent circulation data.U.S. Department of the InteriorU.S. Geological SurveyOpe