[Show abstract][Hide abstract] ABSTRACT: Hillslope vegetated buffers are recommended to prevent water pollution from agricultural runoff. However, models to predict the efficacy of different grass buffer designs are lacking. The objective of this work was to develop and test a mechanistic model of coupled surface and subsurface flow and transport of bacteria and a conservative tracer on hillslopes. The testing should indicate what level of complexity and observation density might be needed to capture essential processes in the model. We combined the three-dimensional FEMWATER model of saturated-unsaturated subsurface flow with the Saint-Venant model for runoff. The model was tested with data on rainfall-induced fecal coliforms (FC) and bromide (Br) transport from manure applied at vegetated and bare 6-m long plots. The calibration of water retention parameters was unnecessary, and the same manure release parameters could be used both for simulations of Br and FC. Surface straining rates were similar for Br and bacteria. Simulations of Br and FC concentrations were least successful for the funnels closest to the source. This could be related to the finger-like flow of the manure from the strip along the bare slopes, to the transport of Br and FC with manure colloids that became strained at the grass slope, and to the presence of micro-ponds at the grassed slope. The two-dimensional model abstraction of the actual 3D transport worked well for flux-averaged concentrations. The model developed in this work is suitable to simulate surface and subsurface transport of agricultural contaminants on hillslopes and to evaluate efficiency of grass strip buffers, especially when lateral subsurface flow is important.
[Show abstract][Hide abstract] ABSTRACT: Land application of manure is recommended to recycle organic matter and nutrients, thus enhancing the soil quality and crop productivity. However, pathogens in manure may pose a human health risk if they reach potable or recreational water resources. The objective of this study was to observe and quantify the effects of vegetated filter strips (VFS) on surface and vertical transport of fecal coliform (FC) bacteria, surrogates for bacterial pathogens, released from surface-applied bovine manure. A two-sided lysimeter with 20% slope on both sides was constructed with a sandy loam soil on one side and a clay loam soil on the other. Each side of the lysimeter was divided into two subplots (6.0 x 6.4 m), one with grass and the other with bare soil. Plots were instrumented to collect runoff samples along a 6.0 m slope at three equidistant transects. Samples of runoff were also collected in a gutter at the edge of each plot. All plots were equipped with multi-sensor capacitance moisture probes to monitor water content through the soil profile. Bovine manure was applied at the top of each plot in a 30 cm strip. Rainfall was simulated at a 61 mm h(-1) intensity using a portable rainfall simulator. Surface runoff rate was measured and water quality sampled periodically throughout the simulation. Soil samples were taken at incremental depths (0-60 cm) after each simulation. Runoff (as % of total rainfall) decreased from 93% to 12% in the bare vs. vegetated clay loam plots and from 61% to 2% in the bare vs. vegetated sandy loam plots. The reduced runoff from vegetated plots decreased the surface transport of FC while increasing its vertical transport. The amount of FC in runoff (as % of applied) decreased from 68% to 1% in the bare vs. vegetated clay loam plots and from 23% to non-detectable levels in the bare vs. vegetated sandy loam plots. These data indicate that VFS can reduce surface transport of FC, even for slopes as high as 20%, especially in soils with high infiltration (e.g., sandy loam).