Author of Response:Thomas Harter, Cooperative Extension Specialist, Groundwater Hydrology, University of California, Davis, thharter@ucdavis.edu

No research data are currently available that provide general levels of E. coli emissions to surface water or groundwater from animal feeding operations. We are only beginning to understand the occurrence and fate of E. coli and other pathogens in animal feeding operations. A recent study of 500 cows on 60 dairy farms in Switzerland showed that Shiga toxigenic E. coli occur on approximately one-quarter of the farms, regardless of whether these are conventional or organic farms. Consistent with other studies, approximately 5% of cattle feces samples were found to contain the O157 strain of E. coli. These are occurrence rates in fresh feces. It is unknown to which degree E. coli subsequently is entrained in surface runoff and what percentage typically reaches surface water streams or groundwater.
The amount of E. coli and other manure-borne pathogens discharging into surface water from confined animal feeding operations varies from nearly zero to over 1 million bacteria per 100 ml, depending on the management practices and infrastructure in place at the operation. For example, a recent study showed that the mean concentration of fecal coliforms, of which E. coli is a constituent, was 153,000 bacteria per 100 ml downstream of coastal dairies along the central California coastline, with some water samples having in excess of 3 million bacteria per 100 ml. Transport is most likely to occur during high intensity rainfall events that create significant on-farm runoff. Best management practices that reduce erosion and runoff (e.g., reduced tillage and grass filter strips) significantly reduce the emission of E. coli to surface water or groundwater (see BMPs). Systems that limit animal access to surface water or capture runoff from earthen lots are important for reducing pathogen risk.

Additional reading:
Lewis, D.J., E.R. Atwill, M.S. Lennox, L.Hous, B. Karle, and K.W. Tate. 2005. Linking on-farm dairy management practices to storm-flow fecal coliform loading for California coastal watersheds. J. Environ. Monitor. Assess. 107:407-425.