Sediment and Fecal Indicator Bacterial Loading in a Mixed Land Use Watershed: Contributions from Suspended and Bed Load Transport

Abstract:

Water quality studies that quantify sediment and fecal bacteria loading commonly focus on suspended contaminants transported during high flows. Fecal contaminants in bed sediments are typically ignored and need to be considered because of their potential to increase pathogen loadings during high flows that produce bed load transport. Water (containing suspended sediment) and bed load samples (n = 145 & n = 28, respectively) were collected in a 122 km² watershed located in N.E. Georgia, USA during the 2013 water year. Continuous discharge was calculated from level data using USGS rating curves. This study quantifies total sediment and fecal indicator bacteria loads (FIB) in an alluvial stream in a mixed land use watershed and provides valuable data for parameterization of watershed models that simulate sediment and bacteria transport. Annual loads for sediment and FIB were determined using linear relationships between the following: turbidity and total suspended solids (R² = 0.96); turbidity and suspended Escherichia coli and enterococci (R² = 0.54 and 0.67, respectively); discharge and bed load (R² = 0.60), and discharge and sediment associated E. coli and enterococci (R² = 0.55 and 0.50, respectively). The annual sediment load was 3660 t with approximately 95% attributed to suspended sediment load and 5% attributed to bed load. The annual E. coli load was 3.7 x 10¹⁴ colonies with approximately 99.5% attributed to suspended transport and 0.50% attributed to bed load sediment-associated transport. The annual enterococci load was 1.1 x 10¹⁵ colonies with approximately 100% attributed to suspended transport. Bed load sediment transport ranged from 2% to 8% of the total storm sediment load in several high flow events (Q = 5 to 40 m³ s^-1). Bed sediment-associated E. coli load was 0.4% to 0.5% of the total storm E. coli load. Suspended transport is the predominant mechanism describing contaminant loading in this stream and load estimates can be determined using simple relationships between discharge and suspended concentrations.