Abstract
Abstract. 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.
Highlights
Organic matter (OM) in aquatic systems forms a fundamental part of the global carbon cycle
Common at coastal sites, a fresh–saline water interface was not identified in the wells, even though Site 5 is www.hydrol-earth-syst-sci.net/24/2167/2020/
Our results showed that groundwater dissolved organic carbon (DOC) varied in concentration and isotopic character along a 500 m groundwater flow path
Summary
Organic matter (OM) in aquatic systems forms a fundamental part of the global carbon cycle. The soil and unsaturated zone has the potential to store at least 3 times more organic carbon than in the atmosphere or in living plants (Schmidt et al, 2011; Fontaine et al, 2007) and represents the largest source of carbon within terrestrial ecosystems (Keiluweit et al, 2017). The below-ground environment forms the largest source of freshwater, and the organic carbon within these systems remains largely unaccounted for in the global carbon budget. Despite the many sources of OM, groundwater dissolved organic carbon (DOC) concentrations are typically low (∼ 1 mg L−1 for the global median DOC concentration; McDonough et al, 2020a), suggesting significant processing in the subsurface
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