Assessing the performance of aged field-scale woodchip bioreactors in Iowa

Woodchip bioreactors are widely implemented to reduce nitrate-nitrogen (NO₃-N) losses from agricultural drainage, yet long-term field performance data are underreported. This study evaluated long-term NO₃-N removal efficiency (NRE) across 10 field-scale bioreactors in Iowa operating for 10-16 years and used conservative tracer tests at five sites to assess internal hydraulics. Median annual NRE varied widely among sites (13.94%-99.66%), but no significant long-term trends were detected at nine of the 10 sites, indicating general stability in performance beyond a decade of operation. One site exhibited a significant increase in NRE; however, tracer results and field observations showed that restricted flow and frequent bypass led to elevated NRE under low hydraulic loading conditions rather than improved treatment performance. Across the remaining sites, mean tracer retention times were mostly similar to theoretical values, suggesting effective volume utilization. Morrill dispersion index (3.31-3.92), short circuiting (0.60-0.70), and hydraulic efficiency (0.45-0.61) values generally indicated unideal but acceptable flow conditions typical of field-scale systems. Sites with lower hydraulic efficiency, volumetric efficiency, and effective porosity corresponded with poor NO₃-N removal, demonstrating that hydraulic limitations can reduce treatment performance over time. In general, the results highlight that most aged bioreactors can maintain stable NO₃-N removal beyond 10 years, but performance is increasingly governed by internal hydraulics. Understanding links between internal hydraulics and NO₃-N removal can help support lifespan predictions for denitrifying bioreactors in agricultural landscapes.