T. B. Moorman

Simulating Woodchip Bioreactor Performance Using a Dual-Porosity Model.

There is a general understanding in the scientific community as to how denitrifying bioreactors operate, but we lack a quantitative understanding of the details of the denitrification process acting within them and comprehensive models for simulating their performance. We hypothesized that nitrate transport through woodchip bioreactors would be best described by a dual-porosity transport model where the bioreactor water is divided into a mobile domain (i.e., the water between the woodchips where it is free to flow and solute movement is by advection and dispersion) and an immobile domain of water (i.e., the water mostly within the woodchips that is stagnant and where solute movement is by diffusion alone). We calibrated the dual-porosity model contained in the HYDRUS model for a woodchip bioreactor using the results of a Br breakthrough experiment where we treated Br as a conservative nonadsorbing tracer. We then used the resulting model parameters to describe 2 yr of NO transport and denitrification within a bioreactor supplied by tile drainage. The only model parameters fitted to the NO data were either the zero- or first-order denitrification rate and its temperature dependence. The bioreactor denitrified 2.23 kg N (38%) of the NO entering it in 2013 and 3.73 kg N (49%) of the NO that entered it in 2014. The dual-porosity model fit the NO data very well, with fitted zero-order reaction rates of 8.7 and 6.8 mg N L d in 2013 and 2014, respectively, and corresponding first-order reaction rates of 0.99 and 1.02 d. For the 2-yr data set, both reaction rate models fit the data equally well. Consistent model parameters fitted for the 2 yr indicated that the model used was robust and a promising approach for modeling fate and transport of NO in woodchip bioreactors.

Subsurface Drain Modifications to Reduce Nitrate Losses in Drainage.

Nitrate in water leaving subsurface drain (‘tile’) systems often exceeds the 10 mg-N L-1 maximum contaminant level (MCL) set by the U.S. EPA for drinking water and has been implicated in contributing to the hypoxia problem within the Gulf of Mexico. Much of the NO3 - present in surface waters within the Midwest cornbelt is from subsurface field drainage. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO3 - concentrations in subsurface drainage below the MCL, additional approaches need to be devised. We are comparing the efficacy of several tile modifications for reducing NO3 - in tile drainage versus the nitrate concentration in drainage from a control treatment consisting of a free-flowing tile installed at 1.2 m below the surface. The modifications being tested include a) a deep tile - a tile installed 0.6 m deeper than control tile depth, but with the outlet maintained at 1.2 m; b) denitrification walls - trenches excavated parallel to the tile and filled with wood chips as an additional carbon source to increase denitrification; and c) phyto remediation - eastern gama grass (Tripsacum dactyloides L.) grown in 3.81 m wide strips above the tile with the plant roots capable of developing below the water table and serving as a renewable carbon source for increasing denitrification. Four replicate 30.5 x 42.7 m field plots were installed for each treatment in 1999 and a corn/soybean rotation initiated in 2000. In 2001, only the tile flow from the denitrification wall treatment had NO3 - concentrations significantly lower than the control. Poor establishment of the eastern gama grass and lack of time for roots to proliferate below the water table probably limited the effectiveness of the phyto remediation treatment. Average NO3 - concentration in tile drainage from the control was about 25 mg-N L-1 but less than 10 mg-N L-1 for the treatment with the denitrification walls. This represented an annual reduction in NO3 - mass loss of from 70 kg-N ha-1 for the control to 20 kg-N ha-1 for the denitrification walls treatment.