Assessing the evolution of nitrate removal and hydraulic performances of woodchip bioreactors

Abstract

Widespread agricultural activity, poor crop nitrogen-use efficiency, and extensive agricultural subsurface drainage systems in the Upper Mississippi River Basin combine to create an accelerated flux of nitrate to aquatic environments. High concentrations of nitrate in aquatic systems can create areas known as hypoxic zones, or dead zones, where dissolved oxygen levels become too depleted to support life. Woodchip bioreactors are an effective edge-of-field conservation practice that are designed to remove nitrate from agricultural water before it is released into the environment. Although their estimated lifespan is about 10 years, little is known about the performance of bioreactors as they age.

The first study (chapter two) assesses the nitrate removal and hydraulic performance changes over time in two bioreactors. Performance monitoring methods included routine water sampling during the drainage year and tracer testing. Original performance monitoring was completed when the bioreactors were two years old. Later, when the bioreactors were a decade old, the same performance monitoring was repeated to evaluate changes over time. The nitrate data showed no significant changes in median annual nitrate removal efficiency for both bioreactors (p-value > 0.05), meaning their effectiveness at removing nitrate has not changed over time. Tracer test results varied with time for both sites. The first bioreactor (DV) showed unideal increases in short circuiting and mixing over time, but remained effective at utilizing its entire pore volume and had a satisfactory hydraulic efficiency. The concerning element of this bioreactor is its poor ability to remove nitrate (median removal efficiency of 13.94%), though this isn’t suspected to be due to its age or hydraulic performance since historical data also showed poor nitrate removal. The second bioreactor (SC) showed improvements in its hydraulic performance over time with decreasing mixing and short circuiting. The use of the entire pore volume and hydraulic efficiency were observed to be satisfactory as well. A unique hydraulic complication at the SC bioreactor is its extended hydraulic residence time (which contributes to the high nitrate removal) and sedimentation. Active stoplog management and vigilance for sediment build-up can mitigate these issues and extend the longevity of the bioreactor. Regardless of the individual complications, the overall performances of these decade-old conservation practices did not significantly change over time. These findings suggest that bioreactors can exceed their estimated lifespans with good management practices.

The next study (chapter three) investigates a bioreactor (NERF) that has been operating for 14 years. Routine water sampling has occurred at this site since 2012, and tracer tests were attempted in 2011, 2018, and 2023. A significant increase in the median annual nitrate removal efficiency was observed at the NERF bioreactor (p-value < 0.05). Nitrate removal efficiencies had natural variations from 2012 to 2019 as environmental factors fluctuated, but removal efficiencies became consistently near 100% after 2019. Tracer results at this site reported strong increases in short circuiting from 2011 to 2018, but overall performances remained satisfactory. A failed tracer test in 2023 prevented further hydraulic comparisons, but still provided insight on the current conditions. The 2023 failed tracer test is suspected to be from extreme clogging at the site which prevented the bromide plug from eluding the bioreactor. Clogging is apparent as bypass flow simultaneously occurs with minimal effluent flow, contradicting the expected gravitational flow pattern. Moreover, the consistently high nitrate removal efficiency throughout the year suggests a sustained high hydraulic residence time, likely from the clogging restricting the flow. This prolonged residence time implies that the limited water passing through the clogged chamber remains in the chamber for an extended amount of time. Together, these characteristics suggest that the NERF bioreactor struggles to effectively conduct flow through the chamber, minimizing its effectiveness at treating agricultural subsurface drainage. After 14 years of operation, the overall performance of the NERF bioreactor is considered to be failing, not due to declines in nitrate removal, but rather due to declines in flow conduction.

All three bioreactors in this study showed varying performances at extended ages with unique complications. This implies that generally estimating a lifespan for all bioreactors is difficult as it will vary with individual design, environmental factors, and management strategies. Further investigation into aged woodchip bioreactor could identify complications that decrease lifespans and could uncover solutions or management strategies that could extend the longevity of bioreactors.