Linking watershed modeling and bacterial source tracking to better assess E. coli sources Jaehak Jeong a, ⁎, Kevin Wagner b , Jaime J. Flores c , Tim Cawthon d , Younggu Her e , Javier Osorio a , Haw Yen a a Blackland Research Center, Texas A&M AgriLife Research, Texas A&M University, 720 East Blackland Road, Temple, TX 76502, USA b Oklahoma Water Resources Center, Oklahoma State University, 139 Ag Hall, Stillwater, OK 74078, USA c Texas Water Resources Institute, 2260 TAMU, College Station, TX 77843, USA d Texas Commission on Environmental Quality, 2100 Park 35 Circle, Austin, TX 78753, USA e Agricultural and Biological Engineering Department/Tropical Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Homestead, FL 33031, USA HIGHLIGHTS • Diffuse sources of E. coli identified by a BST assessment were successfully incor- porated into SWAT for a watershed scale assessment. • SWAT was implemented to identify crit- ical source areas of E. coli in mixed land uses in south Texas. • Wildlife contribution is the major source of E. coli in streamflow and may remain to be significant after land use change with urbanization. • A combined effort that implements land managements and advanced treatment is needed to restore water quality for recreational and aquatic life uses. GRAPHICAL ABSTRACT abstract article info Article history: Received 9 March 2018 Received in revised form 27 June 2018 Accepted 6 August 2018 Available online 07 August 2018 Editor: Ouyang Wei Terrestrial fate and transport processes of E. coli can be complicated by human activities like urbanization or livestock grazing. There is a critical need to address contributing sources of bacterial contamination, properly assess the manage- ment of critical sources, and ultimately reduce E. coli concentrations in impaired water bodies. In particular, character- ization of wildlife animal contributions and other “background” input sources of microbial pollution are highly uncertain and data are scarce. This study attempts to identify critical sources of E. coli and the efficacy of conservation practices for mitigating E. coli concentrations in the Arroyo Colorado watershed, Texas, using a process-based hydro- logic and water quality model. We propose to incorporate a bacterial source tracking assessment into the modeling framework to fill the gap in data on wildlife and human contribution. In addition, other sources identified through a GIS survey, national census, and local expert knowledge were incorporated into the model as E. coli sources. Results suggest that simulated distribution of E. coli sources significantly improved after incorporating this enhanced data on E. coli sources into the model (R 2 = 0.90) compared to the SWAT result without BST (R 2 = 0.59). Scenario assessments indicate that wildlife contributions may remain significant despite land use change and urbanization, expected to mostly occur in agricultural and range lands. A combination of nonpoint source management measures, voluntary im- plementation of advanced treatment by wastewater plants where possible, and installation of aerators in the zone of impairment were demonstrated to be effective measures for restoring the recreation and aquatic life uses of the Arroyo Colorado. © 2018 Elsevier B.V. All rights reserved. Keywords: E. coli FIB SWAT Modeling Water quality Watershed Science of the Total Environment 648 (2019) 164–175 ⁎ Corresponding author. E-mail address: jeongj@tamu.edu (J. Jeong). https://doi.org/10.1016/j.scitotenv.2018.08.097 0048-9697/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv