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Groundwater 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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., (Copyright © 2019 Elsevier Ltd. All rights reserved.)