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Fecal Indicator Bacteria (FIB) Levels During Dry Weather from Southern California Reference Streams

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High fecal indicator bacteria levels are one of the most common surface water impairments in southern California.  Frequent exceedences of bacterial water quality standards have resulted in development of Total Maximum Daily Loads (TMDL) as a regulatory mechanism to address bacterial contamination in several southern California watersheds.

Current water quality standards for freshwater use fecal coliforms or E. coli as an indicator of fecal contamination, because their presence is well correlated with the many waterborne disease-causing organisms or pathogens.  However, fecal coliforms and E. coli are naturally present in the intestines of warm-blooded.  Consequently, besides human related activities, fecal contamination of surface waters can result from potential natural sources such as indigenous wildlife.  In undeveloped areas, such as small and large mammals and birds, has the potential to be a significant source of fecal bacteria to surface waters.

In recognition of the potential for natural sources to affect bacteria levels in surface waters, several TMDLs either allow or require development of numeric targets that account for natural bacteria levels. The objective of this project is to assess natural bacteria levels in numerous unimpaired streams throughout southern California in order to provide a regional characterization of background bacteria concentrations.

Bacterial indicators were measured from unimpaired streams in 12 southern California watersheds weekly between May.2006 and May.2007.  These data were used to investigate background levels, frequency of exceedences of relevant water quality standards, and spatial and temporal patterns.  The project report was completed in January 2008. Results shows:

  • Overall, the 30-day geometric mean exceedences of freshwater standards were 2% for E. coli and 14% for enterococci.
  • There were clear seasonal patterns with exceedences being most common during July and August.