Model of microbial transport and inactivation in the surf zone and application to field measurements of total coliform in Northern Orange County, California

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 Gamma 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 of Gamma 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.