In preparation of remedial action plans for areas of environmental concern in the Great Lakes Basin, the magnitude of pollutant contributions from point as well as nonpoint sources needs to be assessed. For screening evaluations of urban nonpoint source pollution, a statistically-based method was applied in one of the areas of concern. This method computes the annual pollutant load as a product of the annual runoff and the mean pollutant concentration derived from a lognormal distribution of concentrations. Approximate confidence intervals can be determined for the mean concentration and used to compute confidence intervals of the loads. For the conditions studied, the probabilistic method produced load estimates which were sufficiently accurate for comparisons of pollution sources and formulation of the remedial strategy.

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