When cattle have direct access to streams, fecal bacteria concentrations in stream sediments increase. If these bacteria persist, and if the sediments are resuspended, fecal bacteria may also appear in surrounding water for extended periods. Why do fecal bacteria persist, since dry conditions, high acidity or alkalinity, sunlight, competition from native microbes, and extreme temperatures all diminish their populations in soil? The effects of these environmental factors are much reduced in sediment. Water protects fecal bacteria from desiccation and ultraviolet light. High temperatures can promote their regrowth in wet environments. Fecal bacteria also survive on fine-sized sediments in streams because the sediments have a high surface area. These factors may help explain our observations that streams flowing through pastures typically exceed Kentucky standards for primary contact water (200 fecal coliforms/100 ml) long after cattle depart.

The fecal coliform/fecal streptococci ratio (FC/FS), is a tool in water quality assessment that diagnoses the source of fecal contamination, whether from people (FC/FS > 4) or animals (FC/FS < 0.1). The ratio is extremely variable and sensitive to the persistence of the indicator bacteria used in it. For example, we observed in central Kentucky streams that as the temperature increased during spring, the FC/FS ratio also increased. Fecal coliform growth shortly after manure deposition might explain some of the variability we have observed in our water monitoring studies. In this study we tried to account for the seasonal variability of FC/FS ratios in agricultural watersheds, and determine whether sediment particle size and water temperature interacted to influence fecal bacteria persistence and the FC/FS ratio.

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