Effect of streambed bacteria release on E. Coli concentrations: Monitoring and Modeling with the Modified Soil and Water Assessment Tool (SWAT)

Authors: Kim, Jung Woo; Pachepsky, Yakov; Shelton, Daniel; Coppock, Cary

Submitted to: Ecological Modeling
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2010
Publication Date: 4/12/2010
Citation: Kim, J., Pachepsky, Y.A., Shelton, D.R., Coppock, C.R. 2010. Effect of streambed bacteria release on E. Coli concentrations: Monitoring and Modeling with the Modified Soil and Water Assessment Tool (SWAT). Ecological Modeling. 221:1592:1604.

Interpretive Summary: E. coli concentrations in stream water are commonly used to evaluate microbiological water quality and to assess the degree of the microbial contamination or impairment. Streambed sediments are known to be reservoirs of E. coli bacteria; however, the effects of the E. coli release due to sediment resuspension on microbiological quality of surface waters has not been addressed in water quality prediction and assessment tools. We developed the bacterial release and deposition module for the ARS-USDA model SWAT and applied it to model bacterial water quality in the rural Little Cove Creek watershed in Southern Pennsylvania. Streamflow, E. coli in water and sediments, and weather were monitored for three years and used in the model calibration. Simulations indicated that streambed bacterial release controls microbiological water quality in the watershed throughout the year, whereas runoff from pastures contributes E. coli only during sporadic periods during high flow events. Results of this work show that the effect of the direct bacterial input from pasturelands to surface water on microbiological water quality can be significantly overrated if the streambed bacterial contribution is ignored. These results are important for design, evaluation and implementation of conservation and best management practices nationwide in that the work has identified important source of water quality indicator bacteria in freshwater streams that have largely been overlooked.

Technical Abstract: Streambed sediment has been increasingly attracting attention as the reservoir of bacteria, including pathogenic strains. Soil and Water Assessment Tool (SWAT) has been augmented with bacteria transport subroutine in SWAT2005 in which bacteria die-off is the only in-stream process. The purpose of this study was to evaluate the potential significance of streambed E. coli release and deposition for the SWAT microbial water quality simulations. SWAT bacteria module was modified in terms of streambed E. coli release and deposition which were computed based on the sediment resuspension and deposition modules in SWAT. The modified SWAT was applied to the Little Cove Creek watershed, Pennsylvania, with forestry and dairy pasture land uses. Both sensitivity analyses and calibrations were separately conducted for hydrologic parameters and bacteria parameters. Hydrologic calibration characterized soils in the watershed as pervious, and thus the surface runoff was only moderately contributing to the streamflow. However, the surface runoff carried large numbers of E. coli to the stream, and sediment resuspension contributed to the persistent concentration of E. coli in stream water. Although the uncertainty of E. coli concentrations in streambed sediments and from wildlife probably affected the performance of the modified SWAT model, this study qualitatively confirmed that improvements can be expected from including E. coli release from streambed and deposition in the SWAT microbial water quality simulations