Isotopic and chromatographic fingerprinting of the sources of dissolved organic carbon in a shallow coastal aquifer

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.

Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

“…In line with our study from the German North Sea coast, peat and/or paleosol leaching by groundwater was observed in coastal aquifers in Australia and North America (Goñi and Gardner, 2003;Couturier et al, 2016;McDonough et al, 2020b). This process can enrich coastal groundwater DOC concentrations up to tenfold compared to the global average (McDonough et al, 2020c;Meredith et al, 2020). Therefore, the transfer of ancient organic matter deposits from deep aquifers to surficial environments (with high degradation potential) could be an important process in the global coastal carbon cycle.…”

“…Little is known about vertical distributions of groundwater DOM in STEs, since spatial and temporal observations of DOC and DOM endmembers are often discussed in the context of seawatergroundwater mixing (e.g., Santos et al, 2009;Seidel et al, 2015). Most STE studies focus on the upper ∼2-3 m (e.g., Roy et al, 2010;Kim et al, 2012;Seidel et al, 2015;Yang et al, 2015;Couturier et al, 2016) although sampling of 4 m depth or more (Charette and Sholkovitz, 2006;Beck et al, 2017;Kim et al, 2019;Meredith et al, 2020) revealed, in some cases, variations in DOC of up to 100-fold between sampling depths at the same station (Charette and Sholkovitz, 2006;Meredith et al, 2020). On Spiekeroog, the upper saline plume(s) alone reach (modeled) depths of ∼20 m (Figure 2; Grünenbaum et al, 2020).…”

“…In microtidal, sheltered beaches with a high hydraulic gradient from inland groundwater, terrestrial DOM can be supplied to the STE in large proportions (McAllister et al, 2015;Couturier et al, 2016;Meredith et al, 2020). Contrastingly, in meso-to macrotidal regions, large volumes of marine, labile DOM are pumped through the STE, with models predicting seawater infiltrations up to several tens of meters depth into the beach aquifer (Robinson et al, 2006;Grünenbaum et al, 2020).…”

Tracking down carbon inputs underground from an arid zone Australian calcrete

“…In groundwater systems, carbon is replenished by diffuse recharge through the unsaturated zone and/or via direct recharge from surface waters [ 79 ]. These processes are linked to rainfall conditions (i.e.…”

Response of Nitrate Processing to Bio-labile Dissolved Organic Matter Supply Under Variable Oxygen Conditions in a Sandy Beach Seepage Face

“…In intertidal permeable sediments, the organic matter inventory includes both dissolved organic matter (DOM) and sedimentary organic matter (SOM). There, DOM shows high mobility due to seawater intrusion caused by tidal pumping, wave setup and temperature-driven porewater movement (Suryaputra et al, 2015;Meredith et al, 2020), while the SOM concentration is frequently related to adsorption and retention of particulate organic matter near the infiltration zones (Jiang et al, 2020).…”