Investigating Event Nitrate Dynamics in Paired Vegetative Treatment Areas Receiving Silage Bunker Runoff Using a Simple Mixing Approach

Abstract

Groundwater nitrate, often found in areas containing intensive agriculture, poses a human health risk at concentrations greater than 10 mg/L NO3--N. Vegetative Treatment Areas (VTAs) are alternative treatment systems that have been proposed and implemented for the treatment of silage bunker runoff and other agricultural wastewaters on Concentrated Animal Feeding Operations (CAFOs) and other farms. The objective of this study was to temporally and spatially characterize runoff movement and nitrate dynamics on the surface and in the groundwater within paired VTAs treating silage bunker runoff following a precipitation event. A conservative tracer was applied prior to rainfall, and tracer and nitrate concentrations were then monitored in surface-water collector and monitoring well networks in the VTAs. Preferential movement of runoff was observed on the surface and to the groundwater in both VTAs. Nitrate dynamics were localized within the VTAs and did not correlate well with runoff movement. Nitrate concentrations not attributable to bunker runoff in excess of 20 mg/L NO3--N were observed on the surface and in the shallow (61 cm) groundwater during the study. Observed groundwater nitrate concentrations were all less than 10 mg/L NO3--N at a 162 cm depth.