Influence of resuspension on the fate of fecal indicator bacteria in large-scale flumes mimicking an oligotrophic river

In this study, large-scale flume systems simulating an oligotrophic river were used to explore the fate and transport of the fecal indicator bacteria (FIB) Escherichia coli and enterococci following a combined sewer overflow (CSO). Specifically, the removal pattern of FIB from the water column was examined as well as deposition onto the flume bed. Finally, the impact that a sudden increase in bed shear stress has on FIB in the water column was investigated. The large-scale flumes utilized in this study proved extremely useful for our investigations as they very closely approximated conditions within the Isar River (Munich, Germany). By using both natural substratum and fresh river water, as well as a flow velocity of nearly 1ms^-1 at a water depth of roughly 0.5m, shear stresses typical of the Isar River (9Nm^-2) were achieved. As a result, scaling effects were appreciably reduced. In our flume system, UV inactivation played only a minimal role in overall FIB removal. Therefore, we were able to more precisely investigate other mechanisms which result in FIB removal from the water column. From the two standard FIB removal experiments following a CSO, the removal rate coefficient (k) of 0.2h^-1 was identified for both E.coli and enterococci in the water column. An increase in the bed shear stress led to more than a 150% rise in total suspended solid (TSS) levels in the water column. These elevated TSS levels (≈50mgl^-1) increased the persistence of suspended FIB in the water column by 20h (k=0.05h^-1). This indicates that higher TSS loads resulting from resuspended bed sediments can significantly expand the area that is impacted by a CSO event. At lower TSS loads (<20mgl^-1) deposition onto the flume bed did not contribute significantly to FIB removal from the water column. Any deposition which did occur did not result in a net accumulation of culturable FIB in the benthic biofilm.