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Abstract:The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980-2010)

HighlightsDenitrifying woodchip bioreactors treat nitrate-N in a variety of applications and geographies.This review focuses on subsurface drainage bioreactors and bed-style designs (including in-ditch).Monitoring and reporting recommendations are provided to advance bioreactor science and engineering.Abstract. Denitrifying bioreactors enhance the natural process of denitrification in a practical way to treat nitrate-nitrogen (N) in a variety of N-laden water matrices. The design and construction of bioreactors for treatment of subsurface drainage in the U.S. is guided by USDA-NRCS Conservation Practice Standard 605. This review consolidates the state of the science for denitrifying bioreactors using case studies from across the globe with an emphasis on full-size bioreactor nitrate-N removal and cost-effectiveness. The focus is on bed-style bioreactors (including in-ditch modifications), although there is mention of denitrifying walls, which broaden the applicability of bioreactor technology in some areas. Subsurface drainage denitrifying bioreactors have been assessed as removing 20% to 40% of annual nitrate-N loss in the Midwest, and an evaluation across the peer-reviewed literature published over the past three years showed that bioreactors around the world have been generally consistent with that (N load reduction median: 46%; mean ±SD: 40% ±26%; n = 15). Reported N removal rates were on the order of 5.1 g N m-3 d-1 (median; mean ±SD: 7.2 ±9.6 g N m-3 d-1; n = 27). Subsurface drainage bioreactor installation costs have ranged from less than $5,000 to $27,000, with estimated cost efficiencies ranging from less than $2.50 kg-1 N year-1 to roughly $20 kg-1 N year-1 (although they can be as high as $48 kg-1 N year-1). A suggested monitoring setup is described primarily for the context of conservation practitioners and watershed groups for assessing annual nitrate-N load removal performance of subsurface drainage denitrifying bioreactors. Recommended minimum reporting measures for assessing and comparing annual N removal performance include: bioreactor dimensions and installation date; fill media size, porosity, and type; nitrate-N concentrations and water temperatures; bioreactor flow treatment details; basic drainage system and bioreactor design characteristics; and N removal rate and efficiency. Keywords: Groundwater, Nitrate, Nonpoint-source pollution, Subsurface drainage, Tile.

Core Ideas Bioreactors can be designed to remove nitrate from drainage ditches. Designing bioreactors for ditch drainage requires site‐specific flexibility. All mid‐Atlantic ditch bioreactors tested removed nitrate from drainage water. Practical concerns will require adjustments to design and installation. There is strong interest in adapting denitrifying bioreactors to mid‐Atlantic drainage systems to help address Chesapeake Bay water quality goals. Three ditch drainage‐oriented bioreactors were constructed in 2015 in Maryland to evaluate site‐specific design and installation concerns and nitrate (NO3–N) removal. All three bioreactor types removed NO3–N, as measured by load and/or concentration reduction, showing promise for denitrifying bioreactors in the mid‐Atlantic's low gradient Coastal Plain landscape. The ditch diversion bioreactor (25% NO3–N load reduction; 0.97 g NO3–N removed m−3 d−1) and the sawdust denitrification wall adjacent to a ditch (>90% NO3–N concentration reduction; 1.9–2.9 g NO3–N removed m−3 d−1) had removal rates within range of the literature. The in‐ditch bioreactor averaged 65% NO3–N concentration reduction, but sedimentation is expected to be one of the biggest challenges. A robust water balance is critical for future assessment of bioreactors’ contribution to water quality improvement in low gradient mid‐Atlantic landscapes.

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