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This paper provides a simple method for estimating the annual rate of direct atmospheric deposition of pollutants to an estuary when little or no local monitoring data is available. The approach we describe uses relevant regional data as well as literature values. The Casco Bay Estuary, located in the southwestern Maine, is used as a case study. The paper also includes the results of a field air deposition monitoring program conducted in Casco Bay (1998 - 2000) and favorably compares the estimates developed for rate of deposition of nitrogen, mercury and PAHs to the field monitoring results. The estimation approach is a useful starting point for understanding the sources of pollutants entering water bodies that cannot be accounted for through run-off or point source discharges.