Estimating Bacterial Loadings to Surface Waters from Agricultural Watersheds
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The bacterial model simulates die-off, bacterial partitioning between soil and water, and bacterial transport to surface waters in free (in solution) and sediment-adsorbed forms. Bacterial die-off was modeled using Chick's Law, bacterial partitioning was modeled with a linear isotherm equation, and bacterial transport was modeled using continuity and flow equations. The bacterial model was incorporated into the ANSWERS-2000 model, a continuous, distributed, nonpoint source pollution model. The model was tested using data from two plot studies. Calibration was required to improve runoff and sediment predictions. Bacterial model predictions underpredicted bacterial concentrations in runoff with a maximum underprediction error of 92.9%, but predictions were within an order of magnitude in all cases. Further model evaluation, on a larger watershed with predominantly overland flow, over a longer time period, is recommended, but such data were not available at the time of this assessment. The overall conclusions of this research were 1) FC and EC die-off or diminution under the examined field conditions followed Chick's Law, 2) measured die-off rate constants in the field were much less than those cited in literature for laboratory experiments, and 3) for the conditions simulated for two plot studies, the bacterial model predicted bacterial concentrations in runoff within an order of magnitude.
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