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Molecular genetic markers, such as those from fecal Bacteroides microorganisms, can link microbial pollution with its source, and have been used successfully in studies of sheltered aquatic environments. Their applicability to wave-driven, open coast environments has not been tested. We assessed the contribution of a tidal outlet to surf zone water quality in coastal Orange County, California, USA by measuring three traditional culture-based fecal indicator bacteria (FIB) as well as the human-specific Bacteroides molecular marker (HF marker) at four shoreline locations. We found that total and fecal coliform levels were higher during low tides than high tides at two of the four stations, and that this effect was strongest at the mouth of the tidal lagoon and decayed with distance from the outlet. The HF marker was detected in 23% and 47% of samples from the tidal outlet and 26% and 41% of samples from an adjacent recreational beach in 2005 and 2006 respectively. Surprisingly, the station farthest from the tidal outlet had the highest occurrence of the HF marker. We found no relationship between FIB abundance and occurrence of the HF marker for individual samples, but that when the data were considered together by year, higher FIB abundance was correlated with a higher incidence of the HF marker. DNA sequences of the HF marker recovered from this site were > 99% similar to those recovered from other states and countries, suggesting low global diversity of this marker. These data provide strong support for the idea that multiple time points and physical conditions should be considered when assessing coastal water quality. [ABSTRACT FROM AUTHOR]

The classic model of pollutant transport in the surf zone of a long, sandy beach developed by Inman et al. (J. Geophys. Res. 1971, 76, 3493) is altered to account for first-order pollutant inactivation in an effort to understand how rip cell dilution and bacterial inactivation control the length of shoreline adversely impacted by microbial pollution from a point source. A dimensionless number Γ dictates whether physical processes (dilution of microbes in the surf zone by rip cell mixing) or biological processes (microbial inactivation) control the distribution of pollution along the shoreline. Estimates Γ for beaches in Northern Orange County, California, indicate that dilution is the primary factor controlling total coliform levels surrounding two drains that release nuisance runoff directly onto the beach. It is also shown that, even when alongshore currents are fast, pollutant levels will drop e-fold at distances under 4000 m from the point source due to dilution alone. Because dilution is ultimately controlled by wave climate and shoreline morphology, the results suggest the strategic position of drains and other point sources in high dilution wave environments will reduce potential adverse effects on beach water quality. In addition, the results stress the importance of understanding hydrodynamics when conducting microbial source tracking at wave-dominated marine beaches. [ABSTRACT FROM AUTHOR]