<p>61590 Identifying Pollutant Sources in Tidally Mixed Systems: Case Study of Fecal Indicator Bacteria from Marinas in Newport Bay, Southern California Scott Ritter Mentor: Stanley Grant</p><p>The development and implementation of Total Maximum Daily Load (TMDL) management plans requires, at a minimum, the identification of pollution sources likely to be responsible for water quality impairment. In this study we develop a field-based method for quantifying the contribution of specific geographical regions to water quality impairment in tidally mixed systems, such as estuaries, harbors, and tidal saltwater marshes. The method relies on a data analysis technique called Empirical Orthogonal Function (EOF) analysis widely used in the oceanographic community to interpret complex multi- dimensional data sets. In the case study presented here, we demonstrate that EOF analysis yields quantitative estimates for the relative contribution of two recreational marinas to water quality impairment in Newport Bay, a regionally important embayment in southern California. The analysis indicates that less than 5% of the fecal indicator bacteria signal in Newport Bay can be attributed to sources of fecal indicator bacteria originating inside the two marinas, and hence TMDL management efforts should focus on other sources of pollution. More generally, our methodologies provide a much- needed approach for establishing clean-up priorities in water quality impaired and tidally mixed water bodies.</p>