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Abstract. Tile drains, while important for productive farming in certain areas, often discharge nitrate and other agricultural pollutants into surface receiving waters. In regions with prolonged dry periods, like California, drainage from early rainfall events carries a large portion of the total annual nitrate load. In these regions, edge of field practices focused on treating first flush discharge may be beneficial. This paper presents a design for connecting an existing tile drain system to a denitrifying woodchip bioreactor for first flush treatment, monitoring results, and lessons learned. The project was implemented as part of an agricultural water quality improvement project in Los Osos, California. The primary objective was to reduce tile discharge concentrations to below 10 mg/L NO₃^- - N. The woodchip bioreactor was designed with a long hydraulic retention time and small particle size to maximize denitrification in early flows when the microbial population was lowest. Monitoring results indicate a 91% - 100% reduction in nitrate concentration with discharge concentrations ranging from 0 – 4 mg/L NO₃^- - N. However, frequent wet-dry cycles led to rapid woodchip decomposition and increased maintenance requirements for the woodchip bioreactor.