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<strong class="journal-contentHeaderColor">Abstract.</strong> Contamination of surface water is a concern for public health. Lands used for animal production are sources of fecal microorganisms that can reach water bodies, impact their quality, and adversely affect their potential uses. Understanding the mechanisms of microbial transport through surface/subsurface flow is imperative to predict surface water contamination and to assign management strategies for enhanced water quality. The aim of this work to develop and test a mechanistic numerical model to simulate watershed-scale surface/subsurface water flow, bacteria release from cow manure, and their fate, as well as transport to a cattle pond. The integrated surface-subsurface hydrological platform HydroGeoSphere (HGS) was the basis for the site-specific model. The pond and its environs were monitored for 15 months for <em>E. coli</em> concentrations, which remained relatively high throughout the study The model was applied to simulate <em>Escherichia coli</em> (<em>E. coli</em>) bacteria transport in a grassed drainage basin grazed by a permanent herd of approximately 50 cattle. Most model parameter values were adopted from the literature. The model explicitly accounted for cow excretion to the pond as a source of microbial contamination. The latter was estimated from the time spent by cows in the pond, which in turn was estimated from imagery obtained with eight trail cameras installed to cover the pond surface. Images were obtained every 15 min. Simulations for two years showed that the non-calibrated model replicated spatiotemporal patterns and peak <em>E. coli</em> concentration reasonably well. The <em>E. coli</em> cumulative flux loaded by cattle excretion directly to the pond was around two orders of magnitude greater than that with the surface flow. The results of this work indicate the opportunity and show the approach to obtaining a moderately accurate forecast of microbes in cattle ponds using only readily available data.