Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district
Stephen J. Harris, Diοni I. Cendón, Stuart Hankin, Mark A. Peterson, Shuang Xiao, Bryce F. J. Kelly
Abstract
The application of N fertilisers to enhance crop yield is common throughout the world. Many crops have historically been, or are still, fertilised with N in excess of the crop requirements. A portion of the excess N is transported into underlying aquifers in the form of NO3−, which is potentially discharged to surface waters. Denitrification can reduce the severity of NO3− export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO3−-rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in H2O, NO3−, DIC, DOC and SO42−) and radioactive isotope (3H and 36Cl) tracers were used to develop a conceptual N process model. Fertiliser-derived N is likely incorporated and retained in the soil organic N pool prior to its mineralisation, nitrification, and migration into aquifers. This process, alongside the near absence of other anthropogenic N sources, results in a homogenised groundwater NO3− isotopic signature that allows for denitrification trends to be distinguished. Regional-scale denitrification manifests as groundwater becomes increasingly anaerobic during flow from an upgradient basalt aquifer to a downgradient alluvial aquifer. Dilution and denitrification occurs in localised electron donor-rich suboxic hyporheic zones beneath leaking irrigation channels. Using approximated isotope enrichment factors, estimates of regional-scale NO3− removal ranges from 22 to 93% (average: 63%), and from 57 to 91% (average: 79%) beneath leaking irrigation channels. In the predominantly oxic upgradient basalt aquifer, raised groundwater tables create pathways for NO3− to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO3− is limited both physically (through groundwater-surface water disconnection) and chemically (through denitrification). These observations underscore the need to understand regional- and local-scale hydrogeological processes when assessing the impacts of groundwater NO3− on adjacent and end of system ecosystems.