Denitrifying bioreactor surface subsidence varies with age and cover

Authors: CHRISTIANSON, L - University Of Minnesota; CHRISTIANSON, R - Minnesota Department Of Agriculture; HAY, CHRISTOPHER - Iowa Soybean Association; SEEMAN, ANTHONY - Iowa Soybean Association; DIAZ-GARCIA, CAROLINA - University Of Illinois; Feyereisen, Gary; PEASE, LINDSAY - University Of Minnesota; KJAERSGAARD, JEPPE - Minnesota Department Of Agriculture; HELMERS, MATTHEW - Iowa State University; SOUPIR, MICHELLE - Iowa State University

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2024
Publication Date: 1/1/2025
Citation: Christianson, L.E., Christianson, R.D., Hay, C.H., Seeman, A., Diaz-Garcia, C., Feyereisen, G.W., Pease, L., Kjaersgaard, J., Helmers, M.J., Soupir, M. 2025. Denitrifying bioreactor surface subsidence varies with age and cover. Ecological Engineering. 211. Article 107461. https://doi.org/10.1016/j.ecoleng.2024.107461.
DOI: https://doi.org/10.1016/j.ecoleng.2024.107461

Interpretive Summary: Woodchip bioreactors are a conservation practice designed to remove nitrate from tile drainage water at the edge of agricultural fields. Subsidence of the top of the woodchip bed—which is usually covered with soil—has been observed. This report quantifies subsidence on 17 bioreactor beds of various ages in three US states. A 3d LiDAR scanning method was used to determine median and maximum subsidence depths. Median subsidence over the entire bed averaged 1.0 cm per year for soil-covered beds and 7.3 cm per year for beds without soil covers. Subsidence was non-uniform, and typically the maximum subsidence occurred at the inlet or outlet; the average maximum subsidence was 40 ± 14 cm across all sites. Subsidence was greater for bioreactors that were closer to the tile outlet than for those further away. The findings indicated that subsidence is a natural process that does not necessarily indicate end of design life, but managers were encouraged to verify over time that flow was not being restricted. This research is pertinent to those who maintain the bioreactor conservation practice standard, design engineers and practitioners, researchers and academics, and agency personnel with interest in using denitrifying bioreactors to remove nitrate-N from water.

Technical Abstract: Surface subsidence at denitrifying woodchip bioreactors has been anecdotally noted but has not been consistently documented at full-size bioreactors and is thus poorly understood. Subsidence is of concern due to safety and potential exacerbation of ponding within the bioreactor but could also indicate flow restrictions within the woodchip bed. This study used 3D Light Detecting And Ranging (LiDAR) surveying on handheld devices (iPhone 12 Pro, iPad Pro) to provide minimum estimates of surface subsidence at 17 full-size bioreactors across a range of ages (0.1 to 14 years). Bioreactors with woodchips extending to the surface subsided faster than bioreactors with soil covers with median subsidence rates over the entire surface of 7.3 vs. 1.0 cm/y, respectively). Maximum subsidence averaged 40 ± 14 cm across all sites and tended to be near the inflow manifold where subsidence could disproportionately impact hydraulic performance. Bioreactors located relatively near the drainage system outlet exhibited more subsidence than bioreactors further from the outlet. Bioreactors that had documented extended dry periods (e.g., most of a year, multiple years) did not subside notably more than bioreactors with more consistent flow. Although these findings are limited to the bioreactors in the present evaluation and other sites may show different trends, it may be that subsidence is not reducible to aerobicity alone. Subsidence is a natural process to be expected in bioreactors and is not necessarily the best individual indicator of the end of design life. In practice, checking for a consistent reduction in the amount of outflow over time is the best way to assess the need for a recharge.