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The terrestrial sub-surface 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 in groundwater is low compared to surface waters suggesting significant processing in the subsurface. Yet the processes that remove this dissolved organic carbon (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 inorganic (δ13CDIC) and organic (δ13CDOC) carbon isotope geochemical analyses, to fingerprint the various DOC sources that influence the concentration, carbon isotopic composition and character with distance from surface water sources, depth below surface and groundwater residence time (using ³H) in groundwater. It was found that the average groundwater DOC concentration was five times higher (5 mg L-1) than the global median concentration and it doubled with depth, but the chromatographic character did not change significantly. The anoxic saturated conditions of the aquifer have limited the rate of organic matter processing leading to enhanced preservation and storage of the sources such as peats and palaeosols. All groundwater samples are more aromatic for their molecular weight in comparison to lakes and rivers and surface marine samples. The destabilisation or changes in hydrology, whether by anthropogenic or natural processes could lead to the flux of up to ten times more unreacted organic carbon from this coastal aquifer than compared to deeper inland aquifers. [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]