Escherichia coli Release from Streambed to Water Column during Baseflow Periods: A Modeling Study

Streambed sediments can harbor large Escherichia coli populations that are released into the water column during high-flow events. Few studies have been conducted on the rates of E. coli transfer from streambed sediment to water column in low-flow conditions in natural streams. The aim of this work was to apply the watershed-scale model SWAT (Soil and Water Assessment Tool) to a natural stream to evaluate the need to account for the E. coli release from streambed sediments during baseflow periods and to compare the results of simulating such a release by assuming predominantly passive transport, driven by groundwater influx, against simulations assuming predominantly active transport of random or chemotaxis-driven bacteria movement. Escherichia coli concentrations in water during baseflow periods were substantially underestimated when E. coli release from the streambed was attributed only to streambed sediment resuspension. When considered in addition to the release due to sediment resuspension at high flows, the active and passive release assumptions provided 42 and 4% improvement, respectively, in the RMSE of logarithms of E. coli concentrations. Estimated E. coli fluxes to water column during the baseflow periods from June to November ranged from 3.3 x 105 colony-forming units (CFU) m(-2) d(-1) in the game land area to 1.4 x 10(6) CFU m(-2) d(-1) in the mixed pasture and cropland. Results demonstrate that release of E. coli from streambed sediments during baseflow periods is substantial and that water column E. coli concentrations are dependent on not only land management practices but also on in-stream processes.