Measuring and modeling the flux of fecal bacteria across the sediment-water interface in a turbulent stream

Sediments are a pervasive source of fecal indicator bacteria (FIB) in rivers, lakes, estuaries, and oceans and may constitute a long-term reservoir of human disease. Previous attempts to quantify the flux of FIB across the sediment-water interface (SWI) are limited to extreme flow events, for which the primary mechanism of bacterial release is disruption and/or erosion of the sediment substrate. Here we report measurements of FIB flux across the SWI in a turbulent stream that is not undergoing significant erosion. The stream is formed by the steady discharge of bacteria-free disinfected and highly treated wastewater effluent to an earthen channel harboring high concentrations of FIB in the sediment from in situ growth. The flux j '' of FIB across the SWI, estimated from mass balance on FIB measurements in the water column, scales linearly with the concentration of bacteria in sediment pore fluids C(pore) over a 3 decade change in both variables: j '' = k(m)(obs) C(pore). The magnitude of the observed mass transfer velocity (k(m)(obs) 5 x 10(-5) m s(-1)) is significantly larger than values predicted for either the diffusion of bacteria across a concentration boundary layer (k(m)(diff) = 8 x 10(-6) m s(-1)) or sweep and eject fluid motions at the SWI (k(m)(sweep) = 10(-6) m s(-1)) but is similar to the flux of water between the stream and its hyporheic zone estimated from dye injection experiments. These results support the hypothesis that hyporheic exchange controls the trafficking of bacteria, and perhaps other types of particulate organic matter, across the SWI in turbulent streams.