Evaluating a Proposed Farm Best Management Practice: Nitrous Oxide Emissions, In-Bed Nitrate and Carbon Monitoring, and Hydraulic Retention Times of Denitrifying Woodchip Bioreactors in Monterey County, California
Thesis (M.S.) Division of Science and Environmental Policy
Because surface waters in agricultural regions along the Central Coast of California have relatively high nitrate concentrations, there is a lot of interest to evaluate and implement various practices to improve water quality. This study assessed hydraulic retention times (HRTs), nitrate and carbon concentration gradients, and atmospheric fluxes and dissolved concentrations of nitrous oxide in three denitrification woodchip bioreactors on Salinas Valley farms that treat nitrate at 30 mg/L NO3-N to 180 mg/L NO3-N from field runoff. To evaluate HRT, we used sodium bromide tracer tests on two of the bioreactors, DBR1 and DBR 3, the former which had double the volume and flow rate. The mean HRT for DBR 1 was 41 hours and for DBR 3 was 35 hours, both of which were longer than expected. However, several system design parameters made the woodchip bioreactors non-ideal for salt tracer testing, as suggested by tracer stratification observed particularly during the DBR 3 test. Becaus e of the potential for spatial heterogeneity of carbon and nitrate within the bioreactor beds that could indicate differential nitrate removal efficiency, we analyzed water samples from DBR 2 at various depths and distances along the bioreactor. Dissolved organic carbon availability decreased by 0.9 mg/L with a 0.3-m increase in depth, while nitrate increased by 4.3 mg/L with the same depth increase, both of which indicate that a greater extent of denitrification may be occurring near the surface of the bioreactor; however, this may or may not indicate greater nitrate removal efficiency, since if a slower flow path existed near the surface then nitrate removal would be increased but volume of water treated would be decreased. The volumetric nitrate removal rate of 6.7 g NO3-N/m3 bioreactor volume/day achieved by DBR 1 was high compared to other woodchip bioreactor studies, but because tile drain nitrate concentrations were also high, the two bioreactors at the tile drain sit es only removed an average 12% to 19% of the incoming nitrat! e. Final ly, few studies have evaluated nitrous oxide production from wood chip bioreactors, particularly atmospheric fluxes. To address this weakness, we installed static chambers in two of the bioreactors to measure nitrous oxide flux, and also took water samples to measure dissolved nitrous oxide. The mean atmospheric flux for DBR 2 was 2,700±1,300 µg N2O-N/m2/hour (mean±1 SD), while export of dissolved nitrous oxide was estimated as 424,000 µg N2O-N/hour and 200,000 µg N2O-N/hour for DBR 2 and DBR 3, respectively. These values were of similar magnitude to a New Zealand woodchip-and-sawdust bed study, much higher than two other woodchip bioreactor studies, and an order of magnitude higher than typical emissions from treatment wetlands. Therefore, although woodchip bioreactors have some potential as a nitrate-removing best management practice for the Central Coast, more work needs to be done to optimize the nitrate removal and explore if it is possible to limit their production of nitrous oxide.