Mitigation of manure-borne contaminants by prairie strips: Physical model system, overland flow simulations, and numerical model development

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2021-08
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Craig, Andrew J
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Soupir, Michelle L
Howe, Adina
Helmers, Matthew J
Mickelson, Steven K
Moore, Peter L
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Agricultural and Biosystems Engineering
Abstract
The use of manure as fertilizer on crop fields is a common practice due to the valuable nutrients it contains. Manure can also act as a reservoir for microbial contaminants, including pathogens, antibiotic-resistant bacteria, and antibiotic-resistant genes, that pose a risk to human health. Vegetative filter strips (VFS) have shown promising results in reducing runoff volume, sediment, nutrients, and manure-borne contaminants in runoff from agricultural fields. A recently developed type of VFS, prairie strips, has been shown to significantly decrease the impact of corn and soybean production systems on water quality in terms of sediment, nitrogen, and phosphorus losses. While these vegetative buffer systems have been extensively tested in field and plot-scale studies that utilize either natural or simulated rainfall, studies of such systems under highly controlled conditions in the laboratory have been limited. The studies described in this dissertation include the development of a physical model system of prairie strips in the laboratory for water quality research, experiments that utilized the physical model system and demonstrated the impact of prairie strips on mitigating the downstream transport of manure-borne contaminants, and the subsequent development of mathematical relationships of contaminant removal by prairie strips to aid in optimization and design of field applications. The first study includes the extraction of prairie strip sections from the field and their integration into an existing laboratory flume facility with specific auxiliary features to facilitate overland flow experimentation. As a proof of concept run, a potassium chloride (KCl) tracer study was conducted to verify system functionality and inform future work. The tracer pulse was injected under saturated conditions, and the response was monitored through surface water (upstream and downstream of the prairie strip model) and subsurface water (infiltrated) sampling with continuous flow rate monitoring at the sampling locations. The tracer test provided highly resolved breakthrough curves (BTCs) with 93.5% of the injected tracer mass recovered, and provided useful information on flow partitioning, velocities, and dispersion characteristics along the surface and through the subsurface profile of the model. This model prairie strip system is expected to be useful in optimizing the performance of prairie strips under highly controlled flow and contaminant source conditions. The second study sought to extend the benefits of prairie strips to include the reduction of manure-borne contaminants in agricultural runoff such as fecal organisms and pathogens. The physical model system was utilized to assess the impact of prairie strips on manure-laden agricultural runoff through highly controlled overland flow experiments. E. coli and Enterococcus concentration reductions of up to 45.3% and 65.5% were observed for runoff and infiltration flows, respectively, while mass load reductions of up to 65.0% were observed for surficial runoff flows. The degree of concentration or mass load reductions was dependent on the residence time of the flow within the strip and the partitioning of runon flow to infiltration and runoff flows. These data and other data from the literature were used to develop a design method to provide guidance on the required width of prairie strip to achieve a user-defined reduction of contaminant concentration that is based on specified influent microbial concentrations and their residence times within the prairie strip. The specific objective of the third study was to develop a simplistic steady-state model of manure-borne contaminant removal within VFS and prairie strips. Discrepancies were identified in the literature on the effectiveness of VFS in significantly reducing manure-borne bacteria concentrations. We found that the cases where VFS did not perform well generally trend toward a lack of flow residence time within the VFS. The available design tools for estimation and prediction of manure-borne contaminants through vegetative filter strips are complex, and their level of detail may not be necessary to reliably estimate the required width of a VFS to provide runoff with a contaminant concentration at some user-defined concentration or water quality standard. The model was carefully developed to include only the key parameters that function at time scales near the advective time scale or travel time within the VFS. By fitting the model to experimental data, we have shown that surficial filtration of manure-borne contaminants is highly predictable and increases logarithmically with increasing residence time or decreasing flow velocity.
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