Tracer movement through paired vegetative treatment areas receiving silage bunker runoff

Abstract

The need for less resource-intensive agricultural waste treatment alternatives has lately increased. Vegetative treatment areas (VTAs) are considered a low-cost alternative to the collection and storage of various agricultural wastewaters. As VTAs become more widespread, the need for design guidance in varying climates and landscapes increases. The purposes of this study were to investigate runoff movement and nitrate-nitrogen concentrations within two VTAs and to use the results to improve VTA design and recommendations for management. Silage bunker runoff movement through the selected VTAs following a 7.8 mm (0.31 in) rainfall event was characterized using a chloride tracer. Both surface and subsurface runoff movement was analyzed using tracer concentrations and a simple binary mixing model. Results show that concentrated surface flow paths existed within both VTAs, and surface flow in general was more prevalent in the VTA that received a higher hydraulic loading. Rapid preferential flow to shallow monitoring wells was also observed. A shallow restrictive soil layer likely exacerbated surface flow but restricted runoff water and nitrate-nitrogen from leaching to deeper groundwater. The nitrate-nitrogen did not appear to be directly linked to runoff movement, but concentrations as high as 28 mg L⁻¹ were observed in downslope surface flow in the wetter VTA. A more comprehensive VTA design process is called for that accounts for shallow soils and antecedent moisture conditions. Regular maintenance and design measures to prevent the formation of concentrated flow paths are also critical to the prevention of surface discharge.