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

Autor: Harris SJ; School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia., Cendón DI; School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia. Electronic address: dce@ansto.gov.au., Hankin SI; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia., Peterson MA; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia., Xiao S; School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia., Kelly BFJ; School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia.
Jazyk: angličtina
Zdroj: The Science of the total environment [Sci Total Environ] 2022 Apr 15; Vol. 817, pp. 152606. Date of Electronic Publication: 2022 Jan 08.
DOI: 10.1016/j.scitotenv.2021.152606
Abstrakt: 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 NO 3 - , which is potentially discharged to surface waters. Denitrification can reduce the severity of NO 3 - export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO 3 - -rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in H 2 O, NO 3 - , DIC, DOC and SO 4 2- ) and radioactive isotope ( 3 H and 36 Cl) 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 NO 3 - 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 NO 3 - 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 NO 3 - to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO 3 - 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 NO 3 - on adjacent and end of system ecosystems.
Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest.
(Crown Copyright © 2022. Published by Elsevier B.V. All rights reserved.)
Databáze: MEDLINE
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