Microbiological Water Quality at Non-Human Impacted Reference Beaches in Southern California During wet Weather

Author： Griffith John F. Schiff Kenneth C. Lyon Gregory S. Fuhrman Jed A.

ISSN： 1938-6478

Source： Proceedings of the Water Environment Federation, Vol.2009, Iss.6, 2009-01, pp. : 1193-1212

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Abstract

Although wet weather discharges from urban watersheds may have elevated concentrations of fecal indicator bacteria that impact water quality at swimming beaches, not all of these bacteria may arise from human sources. In this study, the contribution of fecal indicator bacteria was quantified at coastal reference beaches in southern California having minimal human impact. Operationally, reference beaches were defined as open beaches with breaking waves that receive runoff from undeveloped (>93% open space) watersheds and were selected to represent a range of geographical conditions and watershed sizes. Samples were collected during wet season storm events and analyzed for total coliform E. coli, and enterococci in the discharge from the watershed and at the confluence of the discharge and surf. Samples collected during wet weather exceeded water quality thresholds greater than 10 times more frequently during wet weather than during recent dry weather in summer or winter, although the frequency differed by beach. Exceedences were greatest < 24 hrs following recorded rainfall, then steadily declined on successive days. Early season storms exceeded thresholds more frequently than late season storms, often for multiple bacterial indicators, while most late season storms only exceeded thresholds for a single bacterial indicator. Large storms exceeded thresholds more frequently than smaller-sized storms, partly due to the breaching of sand berms. Small storms could not breach sand berms, restricting watershed discharges from entering the surf zone. When discharges did reach the surf zone, bacterial concentrations in the wave wash were correlated with watershed bacterial flux.