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

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dc.contributor.authorHarris, SJen_AU
dc.contributor.authorCendón, DIen_AU
dc.contributor.authorHankin, SIen_AU
dc.contributor.authorPeterson, MAen_AU
dc.contributor.authorXiao, Sen_AU
dc.contributor.authorKelly, BFJen_AU
dc.date.accessioned2022-08-19T02:06:42Zen_AU
dc.date.available2022-08-19T02:06:42Zen_AU
dc.date.issued2022-04-15en_AU
dc.date.statistics2022-07-26en_AU
dc.description.abstractThe 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. © The Authors 2022, Published by Elsevier B.V. CC BY-NC-ND licenseen_AU
dc.description.sponsorshipThis research was funded by the Cotton Research and Development Corporation (CRDC) via project ANSTO1801 “Quantifying the nitrogen cycle: from farm gate to catchments, groundwater and atmosphere”. Stephen Harris is supported by PhD scholarships from the Australian Government, the Australian Institute of Nuclear Science and Engineering (AINSE), Australian Nuclear Science and Technology Organisation (ANSTO) and CRDC.en_AU
dc.identifier.articlenumber152606en_AU
dc.identifier.citationHarris, S. J., Cendón, D. I., Hankin, S. I., Peterson, M. A., Xiao, S., & Kelly, B. F. (2022). Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district. Science of The Total Environment, 817, 152606. doi:10.1016/j.scitotenv.2021.152606en_AU
dc.identifier.issn0048-9697en_AU
dc.identifier.journaltitleScience of The Total Environmenten_AU
dc.identifier.urihttps://doi.org/10.1016/j.scitotenv.2021.152606en_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/13578en_AU
dc.identifier.volume817en_AU
dc.language.isoenen_AU
dc.publisherElsevieren_AU
dc.subjectGround wateren_AU
dc.subjectNitratesen_AU
dc.subjectTracer techniquesen_AU
dc.subjectDenitrificationen_AU
dc.subjectAgricultureen_AU
dc.subjectQueenslanden_AU
dc.subjectAustraliaen_AU
dc.titleIsotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural districten_AU
dc.typeJournal Articleen_AU
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