Abstract

 Fecal contamination of streams in grazed agricultural land in New Zealand can introduce a health risk for in-stream recreation, water supply, and downstream shellfish harvesting. We studied fecal bacterial dynamics during flood events in a stream draining land used for grazing dairy and beef cattle that had unrestricted access to the channel. The objective was to understand the pathways by which fecal bacteria contaminate stream water, so as to improve formulation of a model of fecal dynamics. Concentrations of the well accepted freshwater bacterial indicator, Escherichia coli, increased by more than two orders of magnitude over the course of natural flood hydrographs, and peaked closer to the times of maximum flow acceleration than peak flow. Experiments with artificial floods on the same stream, created by releasing water from a supply reservoir during fine weather, produced a broadly similar pattern of fecal contamination that, in the absence of wash in from the catchment, must have come from E. coli stores within the stream channel. A series of three identical floods created on three successive days displayed a pattern of declining yield, indicating progressive wash out of the sediment store and providing a means to estimate its size. Preliminary attempts to sample the sediment store suggest that there exist "hot spots" at cattle access points where E. coli concentrations are sufficiently high to account for the artificial flood yield. This and other evidence suggests that direct deposition of fecal matter by cattle in the stream channel may be of similar or greater importance to wash-in from land stores. We conclude (1) that stream bed sediment stores of fecal contamination represent a health risk in agricultural streams and (2) that exclusion of livestock from stream channels should appreciably reduce fecal contamination of streams and the total yield of fecal bacteria.