Browsing by Author "Spencer, RGM"
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- ItemChanges 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, AADissolved 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
- ItemCharacterisation of shallow groundwater dissolved organic matter in aeolian, alluvial and fractured rock aquifers(Elsevier, 2020-03-15) McDonough, LK; Rutlidge, H; O’Carroll, DM; Andersen, MS; Meredith, KT; Benkhe, MI; Spencer, RGM; McKenna, AM; Marjo, CE; Oudone, PP; Baker, AAGroundwater organic matter is processed within aquifers through transformations such as the adsorption of dissolved organic matter (DOM) to minerals and biodegradation. The molecular character of DOM varies according to its source and this can impact its bioavailability and reactivity. Whilst the character of DOM in riverine and oceanic environments is increasingly well understood, the sources, character and ultimately the fate of groundwater DOM remains unclear. Here we examine groundwater DOM from contrasting hydrogeological settings in New South Wales, Australia. For the first time, we identify the distinct molecular composition of three groundwater DOM end-members including a modern terrestrial input, an aged sedimentary peat source, and an aged stable by-product pool. We also identify and characterise the processing pathway of DOM in semi-arid, low sedimentary organic carbon (OC) environments. Based on size exclusion chromatography, ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), isotopic analyses (13C, 14C and 3H) and principle component analysis (PCA), we show that in higher rainfall temperate coastal peatland environments, large amounts of aged sedimentary organic carbon can leach into groundwater resulting in higher molecular weight (500 g mol−1 < molecular weight > 1000 g mol−1) and highly aromatic groundwater DOM with high O/C ratios and low H/C ratios. We show that in semi-arid environments with low rainfall rates and high groundwater residence times, groundwater dissolved organic carbon (DOC) is processed into increasingly low molecular weight (<350 g mol−1), low aromaticity DOM with low O/C ratios and high H/C ratios by subsurface processing mechanisms such as biodegradation and adsorption. We provide the first comprehensive study of groundwater DOM characterisation based on multiple analytical techniques, and highlight the impact of source inputs and processing on groundwater DOM composition at a molecular level. Crown Copyright © 2020 Published by Elsevier Ltd
- ItemLignin degradation in a coastal groundwater aquifer: a useful tracer?(American Geophysical Union, 2014-12) Howley, EM; Jex, CN; Andersen, MS; Baker, AA; Zainuddin, NS; Meredith, KT; Wells, E; McDonald, J; Kham, S; Blyth, AJ; Spencer, RGMLignin as a biomarker in soils, peat, lakes and intertidal and marine sediment cores has been widely researched in the last four decades. The biogeochemical processes controlling their distribution and composition include fractionation due to phase changes, mineral binding, and abiotic and biotic decay. However, there appears to be no studies in the literature describing the concentration and composition of lignin in groundwater aquifers, despite lignin tracing having the potential to differentiate between types of vegetation and recharge source. In the latter case aquifers could potentially be a source of old, degradation resistant lignin. In this study, we characterise the lignin composition in groundwater samples from a coastal sand aquifer, in SE Australia. We compare these data with lignin composition of fresh vegetation samples from the study site, and with lignin data from other environments in the literature. Groundwater samples were also analysed for Dissolved Organic Carbon (DOC) and inorganic chemistry (major ions, Dissolved Inorganic Carbon (DIC) and redox sensitive species) to investigate lignin fractionation processes. To achieve this, the groundwater lignin composition was compared to the total DOC and DIC in the samples and the prevailing redox reactions. The lignin composition in groundwater was found to be unrelated to the surrounding surface vegetation, indicating significant alteration by biogeochemical processes along its flow paths. We identify potential lignin degradation zones, via determination of total OC consumed through redox reactions and microbial activity. In conclusion, this study, by closely examining the correlations of lignin phenols in groundwater with lignin in surface waters, as well as biogeochemical processes in the aquifer is shedding a new light on the ability of lignin as a biomarker in these subsurface systems. To our knowledge, this is the first attempt to assess the usefulness of lignin as a tracer in groundwater. © AGU
- ItemA 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, AAGroundwater 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
- ItemA 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, AAGroundwater 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
- ItemUsing 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, AADissolved 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.