Modeling E. coli Release And Transport In A Creek During Artificial High-Flow Events

In-stream fate and transport of E. coli, is a leading indicator of microbial contamination of natural waters, and so needs to be understood to eventually minimize surface water contamination by microbial organisms. The objective of this work was to simulate E. coli release and transport from soil sediment in a creek bed both during and after high water flow events. The artificial high-water flow events were created by releasing 60-80 m3 of city water on a tarp-covered stream bank at a rate of 60 L/s in four equal allotments in July of 2008, 2009 and 2010. The small first-order creek used in this study is part of the Beaver Dam Creek Tributary and is located at the USDA Optimizing Production inputs for Economic and Environmental Enhancement (OPE3) research site, in Beltsville, Maryland. In 2009 and 2010 a conservative tracer difluorobenzoic acid (DFBA) was added to the released water. Specifically, water flow rates, E. coli and DFBA concentrations as well as water turbidity were monitored with automated samplers at the ends of the three in-stream weirs reaching a total length of 630 m. Sediment particle size distributions and the streambed E. coli concentrations were measured along a creek before and after experiment. The observed DFBA breakthrough curves (BTCs) exhibited long tails after the water pulse and tracer peaks indicating that transient storage might be an important element of the in-stream transport process. Turbidity and E. coli BTCs also exhibited long tails indicative of transient storage and low rates of settling caused by re-entrainment. Typically, turbidity peaked prior to E. coli and returned to lower base-line levels more rapidly. A one-dimensional model was applied to simulate water flow, E. coli and DFBA transport during these experiments. The Saint-Venant equations were used to calculate water depth and discharge while a stream solute transport model accounted for advection-dispersion, lateral inflow/outflow, exchange with the transient storage, and resuspension of bacteria by shear stress from stream bottom sediments. Reach-specific model parameters were estimated by using observed time series of flow rates and concentrations at three weir stations. Transient storage and dispersion parameters were obtained with DFBA BTCs, then critical shear stress and resuspension rate were assessed by fitting computed E. coli BTCs to observations. To obtain a good model fit for E. coli, we generally had to make the transient storage for E. coli larger than for DFBA. Comparison of simulated and measured E. coli concentrations indicated that significant resuspension of E. coli continued when water flow returned to the base level after the water pulse passed and bottom shear stress was small. The hypothetical mechanism of this extended release could be the enhanced boundary layer (water-streambed) exchange due to changes in biofilm properties by erosion and sloughing detachment.