| Summary: | Evaluating the microbial quality of irrigation water is essential for the prevention of foodborne illnesses. Generic <i>Escherichia coli</i> (<i>E. coli</i>) is used as an indicator organism to estimate the microbial quality of irrigation water. Monitoring <i>E. coli</i> concentrations in irrigation water sources is commonly performed using water samples taken from a single depth. Vertical gradients of <i>E. coli</i> concentrations are typically not measured or are ignored; however, <i>E. coli</i> concentrations in water bodies can be expected to have horizontal and vertical gradients. The objective of this work was to research 3D distributions of <i>E. coli</i> concentrations in an irrigation pond in Maryland and to estimate the dynamics of <i>E. coli</i> concentrations at the water intake during the irrigation event using hydrodynamic modeling in silico. The study pond is about 22 m wide and 200 m long, with an average depth of 1.5 m. Three transects sampled at 50-cm depth intervals, along with intensive nearshore sampling, were used to develop the initial concentration distribution for the application of the environmental fluid dynamic code (EFDC) model. An eight-hour irrigation event was simulated using on-site data on the wind speed and direction. Substantial vertical and horizontal variations in <i>E. coli</i> concentrations translated into temporally varying concentrations at the intake. Additional simulations showed that the <i>E. coli</i> concentrations at the intake reflect the 3D distribution of <i>E. coli</i> in the limited pond section close to the intake. The 3D sampling revealed <i>E. coli</i> concentration hot spots at different depths across the pond. Measured and simulated 3D <i>E. coli</i> concentrations provide improved insights into the expected microbial water quality of irrigation water compared with 1D or 2D representations of the spatial variability of the indicator concentration.
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