Objectives
Artificial subsurface drainage controlling high seasonal groundwater has been found to be a substantial pathway for nutrients from agricultural lands into surface waters. Thus, mitigating the impacts of agriculture on surface water quality needs to address nutrient transport via subsurface drainage. Woodchip bioreactors are a promising mitigation option as demonstrated in arable agriculture in the US. However, research is needed to understand their efficiency in removing nutrients from very flashy drainage flows from NZ pastoral agriculture and any possible pollution swapping.
Methods
A lined 60-m3 woodchip bioreactor was constructed on a dairy farm in the Hauraki Plains (Waikato, NZ). Rainfall, flow, hydrochemistry and dissolved gases in the inflow and outflow were monitored for two drainage seasons (part of 2017, 2018).
Results
The mean nitrate-N concentrations in the inflow and outflow respectively, were 5.59 and 0.01 mg/L in 2017, and 13.72 and 7.45 mg/L in 2018. Based on the nitrate-N fluxes, the estimated nitrate removal efficiency of the bioreactor was 99 and 48% in 2017 and 2018, respectively. The higher removal efficiency in 2017 could be attributed to; the much longer residence time of the water in the bioreactor (mean=22 days vs 5 days in 2018) allowing more opportunity for microorganisms to reduce the nitrate in the water; and the availability of electron donor (DOC) to support denitrification. In 2017, greater DOC available within the bioreactor was indicated by the higher DOC flux from the bioreactor (17.9 kg vs 9.3 kg in 2018).
Very long residence times in 2017 promoted strongly reduced conditions, resulting in the production of hydrogen sulphide and methane. However, short residence times constrained complete reduction of nitrate resulting in higher nitrous oxide concentrations in the outflow vs inflow in 2018. Elevated discharges of DOC and DRP were evident during the start-up phase of the bioreactor in 2017. Significant removal (89%) of DRP was observed in 2018.
Conclusion
Woodchip bioreactors are a useful tool in removing nitrate, and possibly DRP, from subsurface drainage water. Enhancing their efficiency may require a combination of; adding another electron donor (e.g. methanol) to promote complete denitrification during flow and N load peaks, and preventing very long residence times to minimise the production of odorous or greenhouse gases.
Acknowledgement
This work is part of the SSIF-funded ‘Enhanced Mitigation of Nitrate in Groundwater’ programme led by ESR. We gratefully acknowledge the co-funding by Waikato Regional Council and the co-operation of the Mourits family.