Browsing by Author "Xiao, S"

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    Fertilizers rule REYs: agricultural catchments of Eastern Australia
    (Goldschmidt, 2019-08-18) Cendón, DI; Harris, SJ; Kelly, BFJ; Peterson, MA; Hankin, SI; Rowling, B; Watson, J; Xiao, S
    Lanthanides, generally named Rare Earth Elements (REE), are part of the internal transition metals forming a group of 15 elements with very similar chemical characteristics and physical properties. REEs and Yttrium (named REY) are widely used to understand geochemical processes. The increasing use of REYs in technology as well as their presence as a by-product in some fertilizers has opened new pathways for these metals to enter the water cycle, thus making REYs tracers of anthropogenic activity. In this study we investigate the concentration and distribution of REYs in two predominantly agricultural catchments of Eastern Australia: the Namoi River with a 43,000 km2 catchment, which forms part of the headwaters of the Murray- Darling Basin; and the Nogoa River with a 27,600 km2 catchment, which forms part of the Fitzroy River catchment, the largest in eastern Australia flowing into the Great Barrier Reef. Bi-monthly sampling during 18 months was conducted at seven selected sites along both rivers. The [REY] in water samples were analyzed by automated chelation pre-concentration (SeaFast, ESI), followed by ICP-MS. Samples were automatically loaded onto a loop and injected to an iminodiacetate column that chelates REY allowing matrix Na+, Cl-, Ca2+, Mg2+ and, more importantly, Ba2+ ions to be rinsed out. The pre-concentration process allowed a ~20-fold increase in concentration. Results are compared to those obtained from commonly used fertilizers in the region. REY trends suggest a link to the fertilizers used in both catchments. No regional variations were apparent, possibly due to the prevailing dry conditions during the sampling period. Stream flow was controlled by dam releases in the upper ridges for both catchments.
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    Gradient boosting for forecasting groundwater levels from sparse data sets in an alluvial aquifer subjected to heavy water pumping and flooding
    (Copernicus Publications, 2020-05-04) Xiao, S; Cendón, DI; Kelly, BFJ
    In most catchments, there is usually inadequate information to build an accurate three-dimensional representation of the sediment type and associated hydraulic properties. This makes it challenging to build a physics-based groundwater flow model that accurately replicates measured fluctuations in the groundwater level, and it also results in considerable uncertainty in forecasting the groundwater level under various climate scenarios. However, in many catchments in Australia, and around the world, there are 100 year-long rainfall and streamflow records. Good groundwater level data sets often date from mid last century, when advances in pumping technology enable high volume groundwater extractions to support irrigated agriculture. For the lower Murrumbidgee alluvial aquifer in Australia, which covers an area of 33,000 km2, we demonstrate that it is possible to train the gradient boosting algorithm to predict the annual change in the groundwater level to within a few centimetres. The lower Murrumbidgee aquifer, which is up to 300 m thick, is an important but highly stressed aquifer system in Australia. Annually the groundwater level fluctuates many metres due to groundwater withdrawals and occasional flooding. Some portions of the alluvial aquifer are unconfined and other portions semi-confined. Under current groundwater pumping conditions, groundwater levels decline in the semi-confined portions of the aquifer during extended periods of below average rainfall. In other portions of the catchment, there have been periods of groundwater level rise due to deep drainage beneath irrigated crops. Despite the catchment size, groundwater levels throughout the region are driven by four primary processes: ongoing river leakage, pumping, deep drainage and occasional flooding. Combined with knowledge of the hydrogeological setting, we successfully used just rainfall, streamflow and annual groundwater withdrawal records to build a gradient boosting model to predict where the groundwater level will rise and fall, in both space and time. Under existing annual pumping rates, the gradient boosting model forecasts that the groundwater level will fall many metres if the catchment has a period of below average rainfall as occurred from 1917 to 1949. This fall in the groundwater level will trigger groundwater access restrictions in some portions of the aquifer. © The Authors
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    Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district
    (Elsevier, 2022-04-15) Harris, SJ; Cendón, DI; Hankin, SI; Peterson, MA; Xiao, S; Kelly, BFJ
    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. © The Authors 2022, Published by Elsevier B.V. CC BY-NC-ND license
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    Nitrogen cycling dynamics in a humid subtropical climate: insights from the Nogoa River sub-catchment, central Queensland, Australia
    (Copernicus Publications, 2019-04-07) Harris, SJ; Cendón, DI; Peterson, MA; Hankin, SI; Watson, J; Xiao, S; Kelly, BFJ
    The Nogoa River sub-catchment, Queensland, Australia, supports a multimillion-dollar agricultural sector. For the last decade, efforts have been made to monitor river nitrate loads (Fitzroy partnership for River Health, 2017), which may affect sensitive ecosystems downstream, such as the World Heritage-listed Great Barrier Reef (Brodie et al. 2012). Research into nitrous oxide, which arises from both the oxidation of ammonium fertilisers and/or reduction of subsequent nitrate, is also very important due to its increasing impact on the atmosphere. An integrated approach that considers the interactions between atmosphere, river water and groundwater nitrogen compounds is thus integral to closing the nitrogen cycle in the region. Nitrogen fertiliser contributions to greenhouse gas emissions, riverine environments and aquifers remain uncertain for several reasons: (1) ad-hoc river water sampling frequency and infrequent shallow groundwater sampling; (2) a lack of isotopic evidence for attributing sources and highlighting attenuation processes; (3) poor understanding of groundwater recharge pathways, residence times, and contributions to the Nogoa River; and (4) a lack of quantification of river water and groundwater nitrous oxide concentrations and emissions. In this poster, we present hydro-geochemical data (major ions, N2O-N, δ2H-H2O and δ18O-H2O, δ15N-NO-3 and δ18O-NO- 3, and natural radioactive tracers) from seven sites along the Nogoa River that were repeatedly sampled over a 1-year period, and from 24 shallow groundwater bores sampled in October 2018. A comparison with historical major ion groundwater data reveals that nitrate concentrations have increased due to continued fertiliser input over the past ∽ 20 years, reaching up to 25 mg L-1 NO- 3 -N. Dual nitrate isotopes (δ15N and δ18O) reveal that denitrification occurs in both the shallow groundwater and Nogoa River samples, and suggest a predominant fertiliser source of nitrate. The data will be placed in the wider context of recharge pathways, residence times and contributions to the Nogoa River, and will be used to understand the interplay between the river and alluvial aquifer nitrate and nitrous oxide emissions. © Author(s) 2019. CC Attribution 4.0 license.