Nitrogen (N) used in excess in agricultural systems can leach to shallow groundwater and reach the ocean via submarine groundwater discharge. Excess N inputs can also be lost into the environment as greenhouse gas (GHG) emissions, contributing to anthropic climate change via nitrous oxide (N2O) production. The Great Barrier Reef (GBR) receives increasing harmful nutrient loads including nitrate (NO3-), linked to the dramatic growth of corallivores. The GBR is also subject to coral bleaching as a response to elevated sea surface temperatures as a direct consequence of GHG emissions induced by global warming.
Denitrification walls are a low cost technology for NO3- remediation and consist of organic carbon (OC) media-filled permeable trenches able to intercept NO3- polluted groundwater and catalyse denitrification. Denitrification progressively reduces NO3- to dinitrogen with N2O as an intermediary. The process is performed by microbes that use OC as an electron donor to perform their respiration under anaerobic conditions.
Two denitrification walls (30 m3 volume) filled with different OC sources were installed on a sandy aquifer in Southeast Queensland. The water table height in the bioreactors and aquifer was monitored using a dipmeter and pressure transducers. Water samples were collected and analysed photometrically to evaluate NO3- concentrations. Chemo-physical parameters were collected with portable instruments. Dissolved N2O in groundwater was determined using gas chromatographic techniques. The hydraulic characteristics of the bioreactors were tested using saline tracing tests.
Both the walls supported full removal of NO3- and N2O and resulted suitable to reduce NO3- load to the GBR whilst reducing N2O emissions. The results of this study will provide the Queensland Government with a technical tool to improve the use of this cost-limited technology.
The study was funded by the Queensland Government Office of the Great Barrier Reef in collaboration with the Queensland Department of Agriculture and Fisheries.