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• We found no sorption of bromide to woodchips in laboratory column experiments. • Actual hydraulic retention time is more accurate using effective porosity. • Field tracer tests showed an average 82.1% bromide recovery from seven beds. • There is a need to ascertain the cause of low bromide recovery in beds. One biotechnology that can reduce nitrate concentration in subsurface drainage water is called a denitrification bed. Bromide tracer testing has been used to determine the hydraulic properties of these systems to be used in design and modeling, but efficacy of bromide tracers has not been investigated. One important hydraulic property of woodchip beds is its in-situ effective porosity. The objectives of this study were to (1) determine if bromide is a suitable tracer for evaluating the hydraulics of woodchip beds based on laboratory sorption experiments and field tracer testing, and (2) to determine the in-situ effective porosity of denitrification beds. To achieve this goal, we conducted bromide sorption experiments in the laboratory, and conducted bromide tracer testing in seven denitrification beds near Willmar, Minnesota, USA. We did not find sorption (neither adsorption nor absorption) of bromide to woodchips in our laboratory sorption experiments. In contrast, bromide tracer tests showed an average bromide recovery of 82%, revealing that bromide was retarded in the woodchip denitrification beds. To the best of our knowledge, our experiment is the first study to estimate the in-situ effective porosity (average of 0.61) of a typical field-scale denitrification bed using a tracer test. A more accurate estimate of the actual hydraulic retention time (HRT) can be obtained by using the in-situ effective porosity rather than total porosity. Our laboratory experiment did not show any sorption of bromide to woodchips, but in contrast and for unclear reasons, our field tracer testing showed bromide retardation. [ABSTRACT FROM AUTHOR]

Denitrification beds are being used to reduce the transport of water-soluble nitrate via subsurface drainage systems to surface water. Only recently has the non-linearity of water flow through woodchips been ascertained. To successfully design and model denitrification beds with optimum nitrate removal, a better understanding of flow in denitrification beds is needed. The main objectives of this study were to characterize the hydraulic properties of old degraded woodchips and provide a better understanding of the factors affecting flow. To achieve this goal, we conducted constant-head column experiments using old woodchips that were excavated from a four-year old denitrification bed near Willmar, Minnesota, USA. For Izbash’s equation, the non-Darcy exponent ( n ) ranged from 0.76 to 0.87 that indicates post-linear regime, and the permeability coefficient ( M 10 ) at 10°C ranged from 0.9 to 2.6 cm s −1 . For Forchheimer’s equation, the intrinsic permeability of 5.6 × 10 −5 cm 2 and ω constant of 0.40 (at drainable porosity of 0.41) closely resembled the in-situ properties found in a previous study. Forchheimer’s equation was better than that of Izbash’s for describing water flow through old woodchips, and the coefficients of the former provided stronger correlations with drainable porosity. The strong correlation between intrinsic permeability and drainable porosity showed that woodchip compaction is an important factor affecting water flow through woodchips. Furthermore, we demonstrated the importance of temperature effects on woodchip hydraulics. In conclusion, the hydraulic properties of old woodchips should be characterized using a non-Darcy equation to help design efficient systems with optimum nitrate removal. [ABSTRACT FROM AUTHOR]

Cold temperatures limit nitrate-N load reductions of woodchip bioreactors in higher-latitude climates. This two-year, on-farm (Willmar, Minnesota, USA) study was conducted to determine whether field-scale nitrate-N removal of woodchip bioreactors can be improved by the addition of cold-adapted, locally isolated bacterial denitrifying strains (bioaugmentation) or dosing with a carbon (C) source (biostimulation). In Spring 2017, biostimulation removed 66% of the nitrate-N load, compared to 21% and 18% for bioaugmentation and control, respectively. The biostimulation nitrate-N removal rate (NRR) was also significantly greater, 15.0 g N m−3 d−1, versus 5.8 and 4.4 g N m−3 d−1, for bioaugmentation and control, respectively. After five weeks of operation, bioclogging of the biostimulation beds limited dosing for the remainder of the experiment; NRR was greater for biostimulation in Fall 2017, but in Spring 2018 there were no differences among treatments. Carbon dosing did not increase outflow of dissolved organic C concentration. The abundance of one of the inoculated strains, Cellulomonas cellacea strain WB94, increased over time, while another, Microvirgula aerodenitrificans strain BE2.4, increased briefly, returning to background levels after 42 d. Eleven days after inoculation in Spring 2017, outflow nitrate-N concentrations of bioaugmentation were sporadically reduced compared to the control for two weeks, but the effects were insignificant over the study period. The study suggests that biostimulation and bioaugmentation are promising technologies to enhance nitrate removal during cold conditions. A means of controlling bioclogging is needed for biostimulation, and improved means of inoculation and maintaining abundance of introduced strains is needed for bioaugmentation. In conclusion, biostimulation showed greater potential than bioaugmentation for increasing nitrate removal in a woodchip bioreactor, whereas both methods need improvement before implementation at the field scale. [ABSTRACT FROM AUTHOR]