Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district

• DOI: 10.1016/j.scitotenv.2021.152606
• Publication Date: 2022-01-08T07:02:41Z
• Authors: Stephen J. Harris; Dioni I. Cendón; Stuart I. Hankin; Mark A. Peterson; Shuang Xiao; Bryce F.J. Kelly
• Keywords: 2. Zero hunger; Nitrates; 13. Climate action; Denitrification; Groundwater; 01 natural sciences; Ecosystem; Water Pollutants, Chemical; 6. Clean water; Environmental Monitoring; 0105 earth and related environmental sciences

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