Identifying pollutant sources in tidally mixed systems: Case study of fecal indicator bacteria in Newport Bay, southern California

Youngsul Jeong¹, Stanley B. Grant¹, Scott Ritter¹, Abhishek Pednekar¹, Linda Candelaria², and Clinton Winant³

¹ Henry Samueli School of Engineering University of California, Irvine

² Santa Ana Regional Water Quality Control Board Riverside, California

³Scripps Institution of Oceanography University of California, San Diego

Abstract

The development and implementation of fecal indicator bacteria 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 and test a field-based method for assessing the contribution of specific geographical regions to bacterial water quality impairment in tidally mixed systems, such as estuaries, harbors, and tidal saltwater marshes. The results of this field effort collectively implicate runoff--both dry weather runoff at sampling sites located near some storm drains, and wet weather runoff at all sites--as the primary source of fecal indicator bacteria in the water column and sediments of Newport Bay, a regionally important embayment in southern California. Fecal indicator bacteria associated with the runoff are transported laterally by the tides (to a greater or lesser extent, depending on location), and transported vertically into the sediments. Tidal forcing, perhaps together with sunlight-induced mortality, is evidenced at some sites by semi-diurnal and diurnal cycling of fecal indicator bacteria in the water column. Sub-tidal variability is manifest by a slow increase or decrease of fecal indicator bacteria concentration over the course of multi-day experiments. Significant month-to-month variability in the water column and sediment concentration of fecal indicator bacteria is also apparent; the highest concentrations are measured during studies with the shortest antecedent dry periods. Analysis of water column data using empirical orthogonal function analysis reveals that when the water quality changes at one site, it changes in a similar manner at all other sites as well, suggesting that the fecal indicator bacteria impairment in Newport Bay is not a geographically localized problem. The results presented here reinforce the growing body of evidence that management of fecal indicator bacteria pollution in coastal southern California will require developing long-term strategies for treating non-point sources of both dry weather and storm water runoff.

Key words: Total Maximum Daily Loads (TMDLs), estuaries, tidal salt water marshes, harbors,fecal indicator bacteria, Empirical Orthogonal Function (EOF)