Evaluation of Nitrogen and Phosphorus Removal in Alternative Management Practices for Dairy Farm Production Area Runoff: Bioretention Cells and a Woodchip Bioreactor Treatment System

Non-point source pollution from agricultural areas can lead to the degradation of downstream water bodies, including eutrophication and harmful algal blooms, due to high concentrations of nitrogen (N) and phosphorus (P) emanating from these areas. One source of agricultural runoff that is often overlooked, originates from agricultural production areas, which have impervious surfaces, such as paved and compacted areas, barnyards, cow paths, and silage bunker storage; these areas generate stormwater runoff and contribute to pollution during storm events. This research evaluates two built stormwater runoff treatment systems designed to treat high concentrations of nutrients in runoff from a dairy farm. The first chapter provides a review of literature related to agricultural runoff and the ecologically designed solutions we propose: bioretention cells and denitrifying woodchip bioreactors. The second chapter assesses the performance of three bioretention cells for their ability to reduce N and P from dairy farm production area runoff. During two years of monitoring the established system, we evaluate the effects of two treatments across the three bioretention cells: a vegetation treatment using Switchgrass (Panicum virgatum) and a soil amendment treatment using low-phosphorus compost (derived from leaf litter). In the second year, we modified the bioretention cell hydrology to create an internal storage zone and increase the hydraulic retention time targeting improved nitrate (NO3--N) removal. Our results indicate that for bioretention designs in a similar context, we recommend planting with vegetation, which in both years had significantly lower NO3--N effluent concentrations compared to the design without vegetation. We suggest excluding compost due to nutrient leaching, especially in nutrient-sensitive areas, or using low-P compost if required for water holding capacity in the soil media. The third chapter evaluates components of a novel system to treat runoff from silage bunkers, consisting of pre-treatment tanks with a moving bed biofilm reactor (MBBR) and paired denitrifying woodchip bioreactors. The system was designed to reduce nutrient concentrations and mass loads. While overall the system effectively reduced N and P concentrations and mass loads, very low NO3--N concentrations in the silage bunker runoff entered the bioreactors, which may have inhibited denitrification performance. Other pre-treatment options should be considered prior to runoff entering the bioreactor so N is in the form of NO3-. In other water quality contexts, where N is already in the form of NO3-, this combined MBBR and woodchip bioreactor system may also offer a promising solution