Browsing by Author "Behnke, MI"

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    Changes in groundwater dissolved organic matter character in a coastal sand aquifer due to rainfall recharge
    (Elsevier, 2020-02-01) McDonough, LK; O'Carroll, DM; Meredith, KT; Andersen, MS; Brügger, C; Huang, HX; Rutlidge, H; Behnke, MI; Spencer, RGM; McKenna, AM; Marjo, CE; Oudone, PP; Baker, AA
    Dissolved organic matter (DOM) in groundwater is fundamentally important with respect to biogeochemical reactions, global carbon cycling, heavy metal transport, water treatability and potability. One source of DOM to groundwater is from the transport of organic matter from the vadose zone by rainfall recharge. Changes in precipitation patterns associated with natural climate variability and climate change are expected to alter the load and character of organic matter released from these areas, which ultimately impacts on groundwater quality and DOM treatability. In order to investigate potential changes in groundwater DOM character after rainfall recharge, we sampled shallow groundwater from a coastal peat-rich sand aquifer in New South Wales, Australia, during an extended period of low precipitation (average daily precipitation rate < 1.6 mm day−1 over the 8 months prior to sampling), and after two heavy precipitation events (84 mm day−1 and 98 mm day−1 respectively). We assess changes in DOM composition after correcting for dilution by a novel combination of two advanced analytical techniques: liquid chromatography organic carbon detection (LC-OCD) and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We also assess changes in water chemistry pre- and post-rainfall. Post-rainfall, we show that the dilution-corrected amount of highly aromatic DOM molecular formulae (i.e. those categorised into the groups polyphenolics and condensed aromatics) were 1.7 and 2.0 times higher respectively than in pre-rainfall samples. We attribute this to the flushing of peat-derived DOM from buried organic material into the groundwater. We also identify that periods of low precipitation can lead to low hydrophilic/HOC ratios in groundwater (median = 4.9, n = 14). Redundancy analysis (RDA) was used to compare the HOC fraction with FT-ICR MS compound groups. We show that HOC has a more aromatic character in pre-rainfall samples, and is less similar to the aromatic groups in post-rainfall samples. This suggests that the decline in water-borne hydrophobics observed post-rainfall could be associated with preferential adsorption of the hydrophobic aromatic DOM, making post-rainfall samples less treatable for potable water supply. Post-rainfall we also observe significant increases in arsenic (leading to concentrations greater than 3 times the World Health Organisation drinking water limit of 10 μg / L). Increases in coastal rainfall due to climate change may therefore alter the composition of groundwater DOM in coastal peatland areas in ways that may impact DOM bioavailability, and increase arsenic concentrations, reducing the ease of water treatment for human consumption. To the best of our knowledge, this is the first study to identify the chemical and molecular changes of shallow groundwater DOM pre-rainfall and post-rainfall in a sedimentary organic carbon rich environment through multiple analytical techniques. © 2019 Elsevier Ltd
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    Molecular insights into the unique degradation trajectory of natural dissolved organic matter from surface to groundwater
    (Copernicus GmbH, 2021-04-19) McDonough, LK; Behnke, MI; Spencer, R; Marjo, CE; Andersen, MS; Meredith, KT; Rutlidge, H; Oudone, PP; O'Carroll, DM; McKenna, AM; Baker, AA
    Dissolved organic matter (DOM) comprises a large and complex range of molecules with varying mass, elemental arrangements, conformation, and polarity. These diverse molecules interact with the environment resulting in changes to their molecular character and reactivity over time. Significant advances in our understanding of the molecular character of reactive and recalcitrant DOM have been made throughout the past decade, largely due to the development of ultra-high resolution techniques such as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). This understanding, however, is almost entirely based on surface water environments. Here, we investigate how the molecular properties of DOM change due to reactions occurring in a groundwater environment over time. We use FT-ICR MS combined with liquid chromatography organic carbon detection (LC-OCD), fluorescence and radiocarbon (14C) dissolved organic carbon (DOC) for a range of groundwater DOM samples, including the oldest DOC reported from a site which is not impacted by sedimentary organic carbon inputs (25,310 ± 600 years BP). Our results indicate that polarity and nominal oxidation state of carbon (NOSC) play a major role in the reactivity of groundwater DOM, with a preferential removal of hydrophilic, high oxygen to carbon (O/C) ratio molecules over time (rs = 0.91, p = 2.4 x 10-6). We also note an increase in likely bio-produced molecules containing low numbers of O atoms in deep methanogenic groundwater environments. These molecular formulae appear to accumulate due to the prolonged anoxic conditions which would not be experienced by surface water DOM. The decline in NOSC with increasing average bulk groundwater DOC age contrasts with findings from marine environments where NOSC has been reported to increase over time. Furthermore, the proportion of specific molecular formulae which are stable in marine waters, decline in groundwater as 14CDOC decreases (rs = 0.68, p = 6.9 x 10-3) suggesting that current indicators of DOM degradation state derived from marine environments are not applicable to groundwater environments. Our research shows that the molecular character of reactive DOM in groundwater differs from that of surface water due to exposure to different environments and processing mechanisms, suggesting that it is the interaction between external environmental factors and intrinsic DOM molecular properties which control DOM recalcitrance.
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    A new conceptual framework for the transformation of groundwater dissolved organic matter
    (Springer Nature Limited, 2022-04-20) McDonough, LK; Andersen, MS; Behnke, MI; Rutlidge, H; Oudone, PP; Meredith, KT; O'Carroll, DM; Santos, IR; Marjo, CE; Spencer, RGM; McKenna, AM; Baker, AA
    Groundwater comprises 95% of the liquid fresh water on Earth and contains a diverse mix of dissolved organic matter (DOM) molecules which play a significant role in the global carbon cycle. Currently, the storage times and degradation pathways of groundwater DOM are unclear, preventing an accurate estimate of groundwater carbon sources and sinks for global carbon budgets. Here we reveal the transformations of DOM in aging groundwater using ultra-high resolution mass spectrometry combined with radiocarbon dating. Long-term anoxia and a lack of photodegradation leads to the removal of oxidised DOM and a build-up of both reduced photodegradable formulae and aerobically biolabile formulae with a strong microbial signal. This contrasts with the degradation pathway of DOM in oxic marine, river, and lake systems. Our findings suggest that processes such as groundwater extraction and subterranean groundwater discharge to oceans could result in up to 13 Tg of highly photolabile and aerobically biolabile groundwater dissolved organic carbon released to surface environments per year, where it can be rapidly degraded. These findings highlight the importance of considering groundwater DOM in global carbon budgets. © The Authors, Open Access under CC 4.0
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    A new conceptual framework for the transformation of groundwater dissolved organic matter
    (Springer Nature, 2022-04-20) McDonough, LK; Andersen, MS; Behnke, MI; Rutlidge, H; Oudone, PP; Meredith, KT; O'Carroll, DM; Santos, IR; Marjo, CE; Spencer, RGM; McKenna, AM; Baker, AA
    Groundwater comprises 95% of the liquid fresh water on Earth and contains a diverse mix of dissolved organic matter (DOM) molecules which play a significant role in the global carbon cycle. Currently, the storage times and degradation pathways of groundwater DOM are unclear, preventing an accurate estimate of groundwater carbon sources and sinks for global carbon budgets. Here we reveal the transformations of DOM in aging groundwater using ultra-high resolution mass spectrometry combined with radiocarbon dating. Long-term anoxia and a lack of photodegradation leads to the removal of oxidised DOM and a build-up of both reduced photodegradable formulae and aerobically biolabile formulae with a strong microbial signal. This contrasts with the degradation pathway of DOM in oxic marine, river, and lake systems. Our findings suggest that processes such as groundwater extraction and subterranean groundwater discharge to oceans could result in up to 13 Tg of highly photolabile and aerobically biolabile groundwater dissolved organic carbon released to surface environments per year, where it can be rapidly degraded. These findings highlight the importance of considering groundwater DOM in global carbon budgets. Crown Copyright © 2022
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    Using radioactive and stable carbon isotopes, LC-OCD and FT-ICR MS to understand groundwater organic carbon sources and processing
    (Copernicus GmbH, 2019-04-07) McDonough, LK; Oudone, PP; Rutlidge, H; Meredith, KT; Andersen, MS; O'Carroll, DM; Behnke, MI; Spencer, RGM; Baker, AA
    Dissolved organic matter (DOM) concentrations typically decrease from surface to groundwater, which suggests that most groundwater DOM is sourced from the surface. DOM undergoes many removal processes in the subsurface, including sorption to mineral surfaces, biodegradation, and filtration as it moves through soils, sediment and bedrock. In addition, there is potential for subsurface sediments to act as a source of organic carbon in groundwater. However, relatively little is understood about the character of sedimentary organic carbon sources and how DOM character changes as it undergoes processing along a flow path. We obtained 21 groundwater samples and 3 surface water samples from two alluvial aquifers and one coastal sand aquifer in New South Wales, Australia. Samples were analysed for 14C and 3H to identify groundwater recharge sources, flow paths and water residence times. Radioactive (14C) and stable (13C/12C) carbon isotopes, liquid chromatography organic carbon detection (LC-OCD) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were used to characterise DOM and determine DOM source and processing mechanisms. For our coastal aquifer we identify a decrease in low molecular weight neutrals (LMW-N), peptide-like, aliphatic, highly unsaturated and phenolic groups, and an increase in condensed aromatics and polyphenolic groups, with increasing DOM age. We attribute this to the contribution of old, unprocessed sedimentary organic carbon in the form of peat associated with the dune-slack morphology of the site. However, the opposite trend was observed for LMW-N, polyphenolic, highly unsaturated and phenolic groups at both inland alluvial aquifers which is likely to be associated with processing of DOM from high to low molecular weight carbon over time at sites dominated by a surface DOM source, with comparatively less sedimentary organic carbon. This research forms part of an ongoing project which will assist in identifying the factors affecting the mobilisation, transport and sources and removal of DOM in groundwater. Importantly, quantification of the change in DOM concentration and character over time, and the relative importance of sedimentary organic carbon as a source of DOM in groundwater will help guide policy and identify the need to include groundwater resources as part of the carbon economy. © Author(s) 2019. CC Attribution 4.0 license.