ARS | Publication request: Modeling transport of Escherichia coli in a creek during and after artificial high-flow events: Three year study and analysis

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 2013
Publication Date: February 21, 2013
Repository URL: http://handle.nal.usda.gov/10113/56631
Citation: Yakirevich, A., Pachepsky, Y.A., Gish, T.J., Guber, A., Shelton, D.R., Cho, K. 2013. Modeling transport of Escherichia coli in a creek during and after artificial high-flow events: Three year study and analysis. Water Research. http://dx.doi.org/10.1016/j.watres.2013.02.011.

Interpretive Summary: E. coli is the leading indicator of microbial contamination of natural waters. Therefore, its in-stream fate and transport needs to be understood to eventually minimize surface water contamination. Bottom sediments have recently been shown to be an important reservoir of E. coli. Experiments were conducted where artificial high flow events were created in a stream, and indigenous E. coli and added tracer concentrations were monitered. Results showed that high E. coli levels in water during high flow events were caused by E. coli release from sediments during resuspension. Furthermore, we observed that E. coli, and an inert tracer, remained at elevated concentrations long after the water pulse and tracer peaks had passed. We undertook this modeling study to infer probable mechanisms causing the elevated and tracer E. coli concentrations. We concluded that the predominant mechanisms were the slow settling of E. coli or dissipation of tracer in eddies present near stream banks, and the enhanced release of E. coli from the bottom sediments due to the scouring action of the high flow pulse. Results of this work will be useful for professionals involved in environmental monitoring and management in that a knowledge of in-stream transport processes can improve monitoring scheduling and microbial water quality evaluations.

Technical Abstract: E. coli is the leading indicator of microbial contamination of natural waters, and so its in-stream fate and transport needs to be understood to eventually minimize surface water contamination by microorganisms. The objective of this work was to simulate E. coli release and transport from soil sediment in a creek 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 in four equal allotments in July 2008, 2009 and 2010. To better understand basic fluid dynamics in the creek a conservative tracer difluorobenzoic acid (DFBA) was added to the released water in 2009 and 2010. Water flow rate, E. coli and DFBA concentrations as well as water turbidity were monitored with automated samplers from in-stream weirs located at 150, 290 and 640 m from the water release point. A one-dimensional model was applied to simulate water flow, and 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, effect lateral inflow/outflow, exchange with transient storage (TS), release of bacteria by shear stress from bottom sediments and their transport in the creak. Reach-specific model parameters were estimated by evaluating observed time series of flow rates and concentrations of DFBA and E. coli at all three weir stations. The TS and dispersion parameters were obtained from DFBA BTCs, while then critical shear stress and entrainment rate were assessed by fitting computed E. coli BTCs to observations. Results show that the observed DFBA and E. coli breakthrough curves (BTC) exhibited long tails after the water pulse and tracer peaks had passed indicating that transient storage (TS) might be an important element of the in-stream transport process. Comparison of simulated and measured E. coli concentrations indicated that significant release of E. coli continued when water flow returned to the base level after the water pulse passed and bottom shear stress was small. The mechanism of bacteria continuing release from sediment could be the erosive boundary layer exchange enhanced by changes in biofilm properties by erosion and sloughing detachment.