Transport and deposition of sediment-associated Escherichia coli in natural streams

The association of microorganisms with sediment particles is one of the primary complicating factors in assessing microbial fate in aquatic systems. The literature indicates that the majority of enteric bacteria in aquatic systems are associated with sediments and that these associations influence their survival and transport characteristics. Yet, the nature of these associations has not been fully characterized. In this study, a combination of field experiments and mathematical modeling were used to better understand the processes which control the fate and transport of enteric bacteria in alluvial streams. An experimental procedure, involving the use of a tracer-bacteria, was developed to simulate the transport and deposition of bacteria-laden bed sediments in a small alluvial stream during steady flow conditions. The experimental data and mathematical model were used to determine dispersion coefficients, deposition rates, and partitioning coefficients for sediment-associated bacteria in two natural streams. The results provided evidence that bacterial adsorption can be modeled as an irreversible process in freshwater environments. Net settling velocities of fine sediments and associated bacteria were typically two orders of magnitude lower than those predicted from Stokes equation, due to re-entrainment of settled particles. The information presented in this study will further the development of representative microbial water quality models.