Browsing by Author "O'Carroll, DM"

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    Changes in global groundwater organic carbon driven by climate change and urbanization
    (Springer Nature, 2020-03-09) McDonough, LK; Santos, IR; Andersen, MS; O'Carroll, DM; Rutlidge, H; Meredith, KT; Oudone, PP; Bridgeman, J; Gooddy, DC; Sorensen, JPR; Lapworth, DJ; MacDonald, AM; Ward, J; Baker, AA
    Climate change and urbanization can increase pressures on groundwater resources, but little is known about how groundwater quality will change. Here, we use a global synthesis (n = 9,404) to reveal the drivers of dissolved organic carbon (DOC), which is an important component of water chemistry and substrate for microorganisms that control biogeochemical reactions. Dissolved inorganic chemistry, local climate and land use explained ~ 31% of observed variability in groundwater DOC, whilst aquifer age explained an additional 16%. We identify a 19% increase in DOC associated with urban land cover. We predict major groundwater DOC increases following changes in precipitation and temperature in key areas relying on groundwater. Climate change and conversion of natural or agricultural areas to urban areas will decrease groundwater quality and increase water treatment costs, compounding existing constraints on groundwater resources. © 2020, The Author(s)
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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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    Characterisation and controls on mineral-sorbed organic matter from a variety of groundwater environments
    (EarthArXiv, 2019-12-13) Oudone, PP; Rutlidge, H; Andersen, MS; O'Carroll, DM; Cheong, S; Meredith, KT; McDonough, LK; Marjo, CE; Baker, AA
    Detailed investigations into natural groundwater organic matter (OM) as carbon sources or sinks in the natural carbon cycle are generally limited. Groundwater OM concentration and composition is altered by biodegradation and sorption to minerals. In the saturated zone of an aquifer, dissolved organic matter (DOM) may represent a significant fraction of the natural groundwater dissolved organic carbon (DOC) pool, therefore understanding how mineral sorption influences OM will contribute to our understanding of how DOC is processed in groundwater. In this study we investigate the dominant fractions of natural DOC in groundwater and the extent of sorption on three common minerals found in the environment: iron-oxide coated sand, calcite and quartz sand. DOM sorption on these minerals was studied using groundwaters from three different geological environments in New South Wales, Australia: Anna Bay (quartz-sand coastal aquifer); Maules Creek (alluvial gravel and clay aquifer); and Wellington (alluvial karst limestone aquifer). Each groundwater and surface sample were characterised before and after sorption using size exclusion liquid chromatography with organic carbon detection (LC-OCD). Analysis revealed that humic substances (HS) are the dominant (13 – 70%) fraction of natural groundwater DOC. HS sorption on iron-oxide coated sand was higher than that on calcite and quartz sand, respectively while sorption on the calcite was also higher than on quartz sand. In shallow-sandy aquifer groundwater, due to less DOC sorption in sandy environment (Anna Bay), DOC concentration was found to be the highest compared to that from karst and other alluvial boreholes from Maules Creek and Wellington. HS sorption increases with the mineral mass and DOC concentration indicating that DOC sorption to the mineral surface did not reach saturation under the study conditions. Only the high-DOC alluvial groundwater produced significant sorption to each mineral phase and of the chemical fractions present (85% of 72 batch systems that HS sorption was found). Multiple linear regression showed that mineral mass, mineral type, depth of groundwater sample, DOC concentration, aqueous Fe2+ concentration and DOM aromaticity are the controlling factors of DOC sorption in the various groundwater environments. The regression analysis showed sorption decreases with depth, which could be because of DOC sorption along the groundwater flow path, resulting in less DOC at depth. The multiple linear regression predicts less DOC (HS) sorption in quartz sand system, agreeing with laboratory sorption results. HS sorption also correlated with aromaticity suggesting the chemical character of HS will control the degree of mineral sorption. The model also indicated that DOC sorption is negatively correlated with dissolved Fe2+ concentration in water samples presumably due to redox condition which is under anoxic environment iron oxide became electron acceptors under the process of DOC biodegradation leaving higher Fe2+ concentration and less available DOC for sorption.
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    Dissolved organic matter (DOM) concentration and quality in a coastal aquifer
    (Copernicus Publications, 2015-04-14) Zainuddin, NS; Andersen, MS; Baker, AA; Howley, EM; O'Carroll, DM; Jex, CN; Meredith, KT; Wells, E
    This study investigates the range of fluorescence properties of natural occurring DOM in a coastal aquifer at Anna Bay, NSW, Australia. The determination of the extent to which DOM varies in coastal groundwater has been distinguished through fluorescence spectroscopy by excitation-emission matrices (EEM), and the application of parallel factor analysis (PARAFAC). In addition, DOM was characterised by a combination of DOC-LABOR Liquid Chromatography - Trace Organic Carbon Detector (LC-OCD) method and PHREEQC modelling. In general, results show an anoxic aquifer featuring calcite dissolution in the upper regions of the aquifer and organic matter degradation with redox zonation dominated by iron and sulphate reduction as well as methanogenesis. Several fluorescence EEM patterns were identified. DOM in coastal environment was variable, but mainly composed of low molecular weight compounds. On overall system two humic-like substances (C1, C2) and one fulvic-like substance (C3) were identified by the PARAFAC model. C1 and C2 exhibited same trends and were very similar. Measurement of the fluorescence excitation-emission matrices (EEM) and subsequent PARAFAC reveal different fluorescent DOM fractions and hence variable contributions by DOM to the reduction process in the coastal aquifer zones. © 2015 The Author(s).
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    Dissolved organic matter in the unsaturated zone: the view from the cave
    (American Geophysical Union (AGU), 2017-12-14) Baker, AA; Duan, W; Rutlidge, H; McDonough, LK; Oudone, PP; Meredith, KT; Andersen, MS; O'Carroll, DM; Coleborn, K; Treble, PC
    Soil organic matter content is typically a few percent of the total soil composition. Diffuse recharge can mobilise some of this soil-derived organic matter. While soil pore water dissolved organic matter (DOM) concentrations are up to 100 ppm, the resulting groundwater dissolved organic matter concentration is typically less than 2ppm. Dissolved organic matter transported from the soil can be both biodegraded and sorbed to minerals, and the relative importance of these two processes in the unsaturated zone is poorly understood. Caves in karstified limestone uniquely provide direct access to water percolating from the soil to the groundwater. Cave percolation waters can be analysed for their DOM concentration and character. This provides insights into the extent and type of biological and chemical processing of DOM during transport from the soil to the groundwater. We determine the concentration and characteristics of DOM in cave percolation waters using liquid chromatography (LC-OCD) and optical spectrophotometry (fluorescence and absorbance). We sample DOM from multiple caves in SE Australia (Cathedral Cave, Wellington; South Glory and Harrie Wood Caves, Yarrangobilly), permitting comparison of unsaturated zone DOM properties at different depths (up to 30m below land surface) and different climate zones (montane and temperate). We use caves with long-term hydrological monitoring programs so that DOM in waters of contrasting residence times can be compared. Additionally, we compare these cave percolation water DOM characteristics to those from local and regional groundwater, sampled from nearby wells. Our results will help improve our understanding of how DOM is processed from soil to groundwater, and is also relevant to speleothem scientists interested in using organic matter preserved in speleothems as a paleoclimate or paleoenvironmental proxy. Plain Language Summary When plants die, they break down to organic matter, which forms part of the soil. When this organic matter is washed out of the soil and into the subsurface, we know very little about what happens next. Partly it is because we can't see and measure what is happening. There is a solution. We can use caves as observatories. We can collect the organic matter in the water which enters the caves, and analyse it back in the laboratory. There, we can determine not only the cocntration of organic matter, but also its chemical composition. Why is this important? There's lot of organic matter in soil. But only one or two organic molecues per million water molecules are present in groundwater. Where does it all go? One idea is that it is used as food by subterranean microbes. Another is that is sorbed to minerals. By measuring the chemical composition of organic matter in cave drip waters, we can work out which is more important, and help understand why there is so little organic matter in groundwater.
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    The effect of microbial activity and adsorption processes on groundwater dissolved organic carbon character and concentration
    (American Geophysical Union (AGU), 2017-12-14) McDonough, LK; Oudone, PP; Rutlidge, H; Meredith, KT; O'Carroll, DM; Andersen, MS; Baker, AA
    Balancing the terrestrial global carbon budget has proven to be a significant challenge. Whilst the movement of carbon in the atmosphere, rivers and oceans has been extensively studied, the potential for groundwater to act as a carbon source or sink through both microbial activity and sorption to and from mineral surfaces, is poorly understood. To investigate the biodegradable component of groundwater dissolved organic carbon (DOC), groundwater samples were collected from multiple coastal and inland sites. Water quality parameters such as pH, electrical conductivity, temperature, dissolved oxygen were measured in the field. Samples were analysed and characterised for their biodegradable DOC content using spectrofluorometric and Liquid Chromatography-Organic Carbon Detection (LC-OCD) techniques at set intervals within a 28 day period. Further to this, we performed laboratory sorption experiments on our groundwater samples using different minerals to examine the effect of adsorption processes on DOC character and concentration. Calcium carbonate, quartz and iron coated quartz were heated to 400ºC to remove potential carbon contamination, and then added at various known masses (0 mg to 10 g) to 50 mL of groundwater. Samples were then rotated for two hours, filtered at 0.2 μm and analysed by LC-OCD. This research forms part of an ongoing project which will assist in identifying the factors affecting the mobilisation, transport and removal of DOC in uncontaminated groundwater. By quantifying the relative importance of these processes, we can then determine whether the groundwater is a carbon source or sink. Importantly, this information will help guide policy and identify the need to include groundwater resources as part of the carbon economy.
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    Extent and characterisation of natural groundwater organic matter sorption onto minerals
    (Copernicus GmbH, 2019-04-08) Oudone, PP; Mustonen, O; Marjo, CE; Meredith, KT; McDonough, LK; Rutlidge, H; Andersen, MS; O'Carroll, DM; Baker, AA
    Rivers and aquifers have been proven to be physiochemically connected. Despite their interaction, organic matter (OM) concentration in groundwater is much lower than rivers. One might ask where it goes. For example, is sorption responsible for this missing fraction? If so, what components of OM are utilised by groundwater sorption to the minerals? This research aims to quantify the sorption of natural groundwater DOC over a range of groundwater and surface water environments in South East Australia. Batch experiments were set up by adding 40 ml of filtered (0.22 m) sample to a range of masses of three types of sterilised minerals: iron coated sand, quartz sand and calcium carbonate. The systems were rotated for 1 hour under controlled conditions before analysis by Liquid Chromatography-Organic Carbon Detection (LC-OCD) for the aqueous phase and X-ray Photoelectron Spectroscopy (XPS) for the solid phase. Size-exclusion chromatography using LC-OCD is used to determine the remaining chemical fractions in solution. LC-OCD separates DOC into biopolymers (»20,000 g/mol), humic substances ( 1000 g/mol), building blocks (300-500 g/mol), low molecular weight neutrals (<350 g/mol) and low molecular weight acids (350 g/mol). The technique also provides measures of humic substances aromaticity and relative molecular weight. XPS is used to characterise the surface chemistry of the adsorbed organic layer in terms of the relative carbon, nitrogen, and oxygen content, and the types of chemical bonding. The results of solid-phase XPS is compared with the remaining chemical fractions in solution characterised by LC-OCD. LC-OCD results showed that humic substances were highest in concentration in the groundwater DOC compared to other fractions (13-65%) and was the significant sorbing fraction for all mineral types and water samples. The sorption extent ranges between 4-61%. This sorption was highest for iron coated sand (8-61%) followed by calcium carbonate (10-35%) and then quartz sand (4-22%). XPS showed that more sorbed organics (O, C and N) were found on iron coated sand and calcium carbonate compared to quartz sand. The extent of humic substance sorption was found proportional to its aromaticity and molecular weight for all mineral types and water samples. In conclusion, even though sediment types influence groundwater DOC sorption, the result suggests that groundwater DOM sorption plays an important role in the missing fraction of groundwater DOC (31-9/4%). © Author(s) 2019. CC Attribution 4.0 license.
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    Factors affecting dissolved organic carbon in coastal groundwater systems
    (National Centre for Groundwater Research And Training, 2017-07-11) McDonough, LK; O'Carroll, DM; Andersen, MS; Meredith, KT; Rutlidge, H; Oudone, PP; Marjo, CE; Baker, AA
    Balancing the terrestrial global carbon budget has proven to be a significant challenge. Whilst the movement of carbon in the atmosphere and riverine waters has been extensively studied, the potential for organic carbon to desorb/adsorb from mineral surfaces and act as a groundwater organic carbon source/sink, is poorly understood. To investigate the biodegradable component of groundwater dissolved organic carbon (DOC), groundwater samples were collected from six wells located on Rottnest Island, WA. Wells were selected to cover a range of DOC ages and concentrations in a carbonate aquifer. Water quality parameters such as pH, electrical conductivity, temperature, dissolved oxygen were measured in the field. Samples were analysed for their biodegradable DOC content using spectrofluorometric techniques at set intervals within a 28 day period. Further to this, we examined the conditions and processes affecting DOC at a coastal wetland in Anna Bay, NSW. Four multilevel samplers (MLS’s) were installed in a transect with 1m spacing, with a distance of up to 3 m from the wetland edge. Two samples were taken from each MLS and analysed for DOC, dissolved inorganic carbon (DIC), anions and cations using LC-OCD, spectrofluorometry, UV-Vis and FIA techniques. This research forms part of an ongoing project which will assist in identifying the factors affecting the mobilisation, transport and removal of DOC in uncontaminated groundwater. By quantifying the processes, we can then determine whether the groundwater is a carbon source or sink. Importantly, this information will help guide policy and identify the need to include groundwater resources as part of the carbon economy.
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    Groundwater organic matter: carbon source or sink?
    (National Centre for Groundwater Research And Training, 2017-07-11) Rutlidge, H; Andersen, MS; O'Carroll, DM; Oudone, PP; McDonough, LK; Meredith, KT; Marjo, CE; Baker, AA
    The natural environment plays a critical role in offsetting the anthropogenic carbon emissions. Despite the size of the global groundwater store the processes controlling the concentration and characteristics of organic matter in groundwater are poorly understood. Through a survey of global carbon concentrations, it is apparent that groundwater carbon concentrations are significantly lower than terrestrial (soil, sediment and river) concentrations. This indicates that terrestrial OM is biologically processed (and a potential source of inorganic carbon) or sorbed to mineral surfaces (a sink of carbon). This will be explored through an ARC Discovery research project which will investigate factors that determine groundwater organic matter concentration, how important is groundwater to the terrestrial carbon budget and under what conditions where groundwater is a carbon source or sink. This project is bringing together geochemists, ecologists, hydrologists and anyone with an interest in organic matter in groundwater. Specifically, the amount of colloid and dissolved organic matter present will be quantified, the rate and extent of biological processing, desorption and sorption will be investigated and the relative importance of each process to be determined. The processes that control organic matter in groundwater will be investigated at a range of field sites with differing surface soil, land cover, recharge type and hydrological properties. Preliminary results from various field sites has shown that sedimentary organic matter is mobilised as water flows through the hyporheic zone. The results from this project will provide guidelines for the management of groundwater resource as part of the carbon economy.
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    Insights in groundwater organic matter from liquid chromatography-organic carbon detection
    (American Geophysical Union (AGU), 2017-12-14) Rutlidge, H; Oudone, PP; McDonough, LK; Andersen, MS; Baker, AA; Meredith, KT; O'Carroll, DM
    Understanding the processes that control the concentration and characteristics of organic matter in groundwater has important implications for the terrestrial global carbon budget. Liquid Chromatography – Organic Carbon Detection (LC-OCD) is a size-exclusion based chromatography technique that separates the organic carbon into molecular weight size fractions of biopolymers, humic substances, building blocks (degradation products of humic substances), low molecular weight acids and low molecular weight neutrals. Groundwater and surface water samples were collected from a range of locations in Australia representing different surface soil, land cover, recharge type and hydrological properties. At one site hyporheic zone samples were also collected from beneath a stream. The results showed a general decrease in the aromaticity and molecular weight indices going from surface water, hyporheic downwelling and groundwater samples. The aquifer substrate also affected the organic composition. For example, groundwater samples collected from a zone of fractured rock showed a relative decrease in the proportion of humic substances, suggestive of sorption or degradation of humic substances. This work demonstrates the potential for using LC-OCD in elucidating the processes that control the concentration and characteristics of organic matter in groundwater.
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    Isotopic and chromatographic fingerprinting of the sources of dissolved organic carbon in a shallow coastal aquifer
    (European Geosciences Union, 2020-04-30) Meredith, KT; Baker, AA; Andersen, MS; O'Carroll, DM; Rutlidge, H; McDonough, LK; Oudone, PP; Bryan, E; Zainuddin, NS
    The terrestrial subsurface is the largest source of freshwater globally. The organic carbon contained within it and processes controlling its concentration remain largely unknown. The global median concentration of dissolved organic carbon (DOC) in groundwater is low compared to surface waters, suggesting significant processing in the subsurface. Yet the processes that remove this DOC in groundwater are not fully understood. The purpose of this study was to investigate the different sources and processes influencing DOC in a shallow anoxic coastal aquifer. Uniquely, this study combines liquid chromatography organic carbon detection with organic (δ13CDOC) carbon isotope geochemical analyses to fingerprint the various DOC sources that influence the concentration, carbon isotopic composition, and character with respect to distance from surface water sources, depth below surface, and inferred groundwater residence time (using 3H activities) in groundwater. It was found that the average groundwater DOC concentration was 5 times higher (5 mg L−1) than the global median concentration and that the concentration doubled with depth at our site, but the chromatographic character did not change significantly. The anoxic saturated conditions of the aquifer limited the rate of organic matter processing, leading to enhanced preservation and storage of the DOC sources from peats and palaeosols contained within the aquifer. All groundwater samples were more aromatic for their molecular weight in comparison to other lakes, rivers and surface marine samples studied. The destabilization or changes in hydrology, whether by anthropogenic or natural processes, could lead to the flux of up to 10 times more unreacted organic carbon from this coastal aquifer compared to deeper inland aquifers. © Author(s) 2020.
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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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    Natural organic matter in goundwater: carbon source or sink?
    (European Geosciences Union, 2019-04-07) Rutlidge, H; McDonough, LK; Oudone, PP; Andersen, MS; Baker, AA; Meredith, KT; O'Carroll, DM; Marjo, CE; Mustonen, O
    Carbon plays an essential role in all biological processes on the earth and hence it is important to Mustin the environment. The concentration of organic matter in groundwater, with a global median of 1.0 mg C/L, is often significantly lower than in adjacent soil and surface waters. The likely processes that are responsible for this decrease are sorption to mineral surfaces and biological processing by microbes as water travels through sediments. While these processes have been quantified individually at different sites, they have not been investigated concurrently, and hence the relative importance of each process is unknown. Therefore, the role of organic matter processes in groundwater and in the terrestrial global carbon budget is unknown. To investigate this a series of laboratory-based experiments were conducted, in conjunction with the organic matter characterization of field samples by Liquid Chromatography-Organic Carbon Detection (LC-OCD). LC-OCD is a size-exclusion based chromatography technique that separates dissolved organic carbon into five fractions based on their mass, plus a hydrophobic fraction, which remains in the column. For the laboratory-based experiments, the amount of sorption onto pure mineral surfaces (quartz sand, iron-coated sand, and calcium carbonate), desorption from natural sediments and biological degradation was investigated at a range of different locations in New South Wales, Australia. The sites covered a range of different aquifer materials (coastal sands, river alluvium and fractured meta-basalts), land cover and recharge type. At each site, groundwater samples were collected from wells located with varying distance from surface water bodies for the subsequent laboratory experiments. The results showed that predominately the humics fraction was adsorbing onto the mineral surfaces and the low-molecular weight neutrals were being biologically degraded. For the desorption experiments several fractions desorbed with the humics and hydrophobic fraction being dominant. The amount of desorption increased with increasing salinity and increasing number of cycles of drying and wetting. The LC-OCD results of field samples indicated that proportionally, sorption is more dominant than biological degradation. Hence changing environmental conditions, such as increasing salinity and/or drier conditions, could lead to a release of sorbed carbon.
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    Natural organic matter in groundwater: carbon source or sink ?
    (Copernicus GmbH, 2019-04-08) Rutlidge, H; McDonough, LK; Oudone, PP; Andersen, MS; Baker, AA; Meredith, KT; O'Carroll, DM; Marjo, CM; Mustonen, O
    Carbon plays an essential role in all biological processes on the earth and hence it is important to Mustin the environment. The concentration of organic matter in groundwater, with a global median of 1.0 mg C/L, is often significantly lower than in adjacent soil and surface waters. The likely processes that are responsible for this decrease are sorption to mineral surfaces and biological processing by microbes as water travels through sediments. While these processes have been quantified individually at different sites, they have not been investigated concurrently, and hence the relative importance of each process is unknown. Therefore, the role of organic matter processes in groundwater and in the terrestrial global carbon budget is unknown. To investigate this a series of laboratory-based experiments were conducted, in conjunction with the organic matter characterization of field samples by Liquid Chromatography-Organic Carbon Detection (LC-OCD). LC-OCD is a size-exclusion based chromatography technique that separates dissolved organic carbon into five fractions based on their mass, plus a hydrophobic fraction, which remains in the column. For the laboratory-based experiments, the amount of sorption onto pure mineral surfaces (quartz sand, iron-coated sand, and calcium carbonate), desorption from natural sediments and biological degradation was investigated at a range of different locations in New South Wales, Australia. The sites covered a range of different aquifer materials (coastal sands, river alluvium and fractured meta-basalts), land cover and recharge type. At each site, groundwater samples were collected from wells located with varying distance from surface water bodies for the subsequent laboratory experiments. The results showed that predominately the humics fraction was adsorbing onto the mineral surfaces and the low-molecular weight neutrals were being biologically degraded. For the desorption experiments several fractions desorbed with the humics and hydrophobic fraction being dominant. The amount of desorption increased with increasing salinity and increasing number of cycles of drying and wetting. The LC-OCD results of field samples indicated that proportionally, sorption is more dominant than biological degradation. Hence changing environmental conditions, such as increasing salinity and/or drier conditions, could lead to a release of sorbed carbon.
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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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    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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    The role of dissolved organic matter and groundwater biogeophysical processes in the carbon budget
    (National Centre for Groundwater Research And Training, 2017-07-11) Oudone, PP; Baker, AA; O'Carroll, DM; Andersen, MM; Rutlidge, H; McDonough, LL; Marjo, CE; Meredith, KT
    Atmospheric CO2 concentration is acknowledged to play an important role in climate change. However, quantifying more accurate predictions requires a sound understanding of the cycle and process of carbon especially in the environment. There has been extensive research on terrestrial carbon and the different conditions where it is a source or sink. However, the knowledge on whether groundwater organic matter is a carbon source or sink is limited. This work will explore the dynamic of groundwater organic matter including both its concentration and its rate and extent of biological processing and sorption. The UNSW Wellington Research Station was selected for groundwater sampling as it represents a fractured rock aquifer and alluvial aquifer for groundwater property and interaction investigation. Samples were collected from a transect of boreholes perpendicular from the river. Literature procedures were adopted for determining biological dissolved organic carbon and investigation of organic matter sorption on pure minerals (quartz sand, iron-coated quartz sand and calcium carbonate). For each sample collected total dissolved organic concentration was measured and the organic matter present was characterized by fluorescence spectroscopy and size-exclusion chromatographic technique, LC-OCD. There was greater sorption of organic matter with calcium carbonate, followed by iron-coated sand, while minimum sorption was observed with sand. This research in conjunction with similar studies in different environments will allow conclusions to be drawn groundwater organic matter and whether it is a carbon source or sink and thereby the findings can eventually have some policy application which will enable the management of the groundwater resources as part of the carbon economy.
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    Sorption of groundwater dissolved organic carbon onto minerals
    (American Geophysical Union (AGU), 2017-12-14) Oudone, PP; McDonough, LK; Meredith, KT; Rutlidge, H; Andersen, MS; O'Carroll, DM; Baker, AA
    Our understanding of groundwater organic matter (OM) as a carbon source or sink in the environmental carbon cycle is limited. The dynamics of groundwater OM is mainly governed by biological processing and its sorption to minerals. In saturated groundwaters, dissolved OM (DOM) represents one part of the groundwater organic carbon pool. Without consideration of the DOM sorption, it is not possible to quantify governing groundwater OM processes. This research explores the rate and extent of DOM sorption on different minerals. Groundwater DOM samples, and International Humic Substances Society (IHSS) standard solutions, were analysed. Each was mixed with a range of masses of iron coated quartz, clean quartz, and calcium carbonate, and shaken for 2 hours to reach equilibrium before being filtered through 0.2 μm for total dissolved organic carbon (DOC) and composition analysis by size-exclusion chromatography-organic carbon detection (LC-OCD). Sorption isotherms were constructed and groundwater DOM sorption were compared to the sorption of IHSS standards. Initial results suggest that for the IHSS standards, the operationally-defined humic substances fraction had the strongest sorption compared to the other LC-OCD fractions and total DOC. Some samples exhibited a small increase in the low molecular weight neutral (LMW-N) aqueous concentration with increasing humic substances sorption. This gradual increase observed could be the result of humic substances desorbing or their breakdown during the experiment. Further results comparing these IHSS standards with groundwater samples will be presented. In conjunction with complementary studies, these results can help provide more accurate prediction of whether groundwater OM is a carbon source or sink, which will enable the management of the groundwater resources as part of the carbon economy.
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    Tracing organic carbon processes in a shallow coastal sandy aquifer
    (American Geophysical Union (AGU), 2017-12-14) Meredith, KT; Andersen, MS; Baker, AA; O'Carroll, DM; Bryan, E; Zainuddin, NS; Rutlidge, H; McDonough, LK
    Coastal groundwater resources are likely to be impacted by climate change due to changes in recharge patterns, surface water flow and sea-level rise, which all have the potential to change how carbon is transported and stored within a catchment. Large quantities of carbon are currently stored within coastal wetland systems, so understanding carbon dynamics is important for climate change predictions into the future. Furthermore, dissolved organic carbon (DOC) can play a major role in weathering processes and deterioration of water quality, therefore understanding the sources, degradation pathways and its reactivity is important. Groundwater samples were collected from five nested sites (15 wells) from a shallow (0-20m) coastal sandy aquifer system located at Anna Bay, New South Wales, Australia. Surface water samples were also collected from the adjacent wetland. Waters were measured for major ion chemistry, carbon isotopes (δ13CDIC, δ13CDOC and 14CDIC) and tritium (3H). The dissolved organic matter (DOM) character was determined using optical spectroscopy and liquid chromatography. DOC was found to be elevated in the wetland (18 ppm) and had the lowest δ13CDOC value (-30.3 ‰). The shallow (3.5 m) groundwater located closest to but downgradient of the wetland (5 m) had similar characteristics to the wetland sample but contained significantly lower DOC concentrations (5 ppm) and were 1 ‰ more enriched in δ13CDOC values. This suggests that the aquifer is a sink for organic matter and the process fractionates the carbon isotopes. Higher resolution studies are underway to characterise and constrain timescales for the DOC transformation processes.
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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.