Nitrate Removal and Woodchip Properties across a Paired Denitrifying Bioreactor Treating Centralized Agricultural Ditch Flows

Abstract

Treatment of nitrate loads by denitrifying bioreactors in centralized drainage ditches that receive subsurface tile drainage may offer a more effective alternative to end-of-pipe bioreactors. A paired denitrifying bioreactor design, consisting of an in-ditch bioreactor (18.3 × 2.1 × 0.2 m) treating ditch base flow and a diversion bioreactor (4.6 × 9.1 × 0.9 m) designed to treat high-flow events, was designed and constructed in an agricultural watershed (3.2 km2 drainage area) in Illinois, USA. Flow and water chemistry were monitored for three years and the woodchip and bioreactor-associated soil were analyzed for denitrification potential and chemical properties after 25 months. The in-ditch bioreactor did not significantly reduce nitrate concentrations in the ditch, likely due to low hydraulic connectivity with stream water and sedimentation. The diversion bioreactor significantly reduced nitrate concentrations (58% average reduction) but treated only ~2% of annual ditch flow. Denitrification potential was significantly higher in the in-ditch bioreactor woodchips versus the diversion bioreactor after 25 months (2950 ± 580 vs. 620 ± 310 ng N g−1 dry media h−1). The passive flow design was simple to construct and did not restrict flow in the drainage ditch but resulted in low hydraulic exchange, limiting nitrate removal.

Quantifying Nitrogen Treatment by an In-stream Bioreactor in a Watershed Served by Septic Systems

“…Pfannerstill et al (2016) found nitrate concentration reductions to be more variable ranging from 7 to 41%, with an overall mean nitrate concentration reduction of 28% (8.87 to 6.43 mg L − 1 ). Maxwell et al (2022) reported the lowest mean reductions of nitrate concentrations at -1% ± 16%. Mean nitrate concentrations in the inlet and outlet were 5.9 ± 2.4 mg L − 1 and 6.1 ± 2.2 mg L − 1 , respectively.…”

“…Most of these studies focused on quantifying nitrate treatment in agricultural settings. Overall, IBRs (or in-ditch bioreactors) exhibited variability in nitrate concentration reductions ranging from -1 to 78% (Robertson and Merkley 2009;Elgood et al 2010;Pfannerstill et al 2016;Sarris and Burbery 2018;Christianson et al 2017;Maxwell et al 2022). Three of these studies (Robertson and Merkley 2009;Elgood et al 2010;Christianson et al 2017) found that nitrate concentration reductions tended to be > 50%.…”

“…Species of woodchips were not identified, but common trees in the study area include Pinus spp., Acer spp., Quercus spp., and Liquidambar spp. A check-dam was installed immediately upgradient of the IBR to abate energy and facilitate sedimentation of fine-grained sediments upstream of the IBR, which was an issue noted by Robertson and Merkley (2009), Christianson et al (2017), andMaxwell et al (2022). Two monitoring ports (P-1 and P-2) were installed in July 2017 within the woodchip layer of the IBR to monitor water levels within the IBR and collect samples.…”

“…There is growing interest in evaluating techniques designed to promote or enhance 1 3 denitrification within streams or drainage ditches. One type of practice is the in-ditch or in-stream bioreactor (IBR), which is an engineered technology that typically includes installation of carbonaceous media in the hyporheic zone of a stream (Robertson and Merkley 2009;Elgood et al 2010;Christianson et al 2017;Maxwell et al 2022) or construction of a bioreactor cell filled with carbonaceous media in the stream/ditch anchored by bulkheads, a wire mesh, and/or rocks (Pfannerstill et al 2016;Sarris and Burbery 2018;Burbery and Abraham 2022). These studies found that IBRs can be effective strategies to reduce nitrogen concentrations and/or masses in surface waters.…”

Can Woodchip Bioreactors Be Used at a Catchment Scale? Nitrate Performance and Sediment Considerations

“…In particular, surface inlets, which connect ponded overland flow with the subsurface drain system via a vertical pipe, provide a direct path for surface runoff constituents (i.e., sediment) to enter the subsurface drainage system and, as a result, any linked edge-of-field practice. Sediment has been shown to shorten the lifespans of woodchip bioreactors (Christianson, et al, 2020;Duncan, 2022) and reduce the effectiveness of a woodchip bioreactor situated in an agricultural ditch (Maxwell et al, 2022a).…”