Using radiocarbon and organic carbon characterisation techniques to identify the sources and degradation of dissolved organic matter in groundwater

Full text: Dissolved organic matter (DOM) plays a significant role in biogeochemical processes, ecological functioning, and carbon cycling. Interactions with the environment over time results in changes to the molecular size of DOM molecules as well as the number and arrangement of their atoms, thereby changing DOM reactivity, functioning and fate. Significant advances have been made in our understanding of surface water DOM sources and degradation throughout the past decade, largely due to the development of ultra-high resolution carbon characterisation techniques such as Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) which can be linked to changes in radiocarbon (¹⁴C) dissolved organic carbon (DOC) content. The sources and degradation of DOM in groundwater, however, remains poorly studied. This is in part due to the typically low (~1 ppm) DOM concentrations in these environments, meaning that in the past, large quantities of water were required for ¹⁴C-DOC analyses. The ability to measure ¹⁴C in small mass samples on ANTARES allows for a larger throughput of ¹⁴C-DOC samples and quicker sample preparation due to lower quantities of water required. Here, we combine ¹⁴C-DOC, ¹⁴C of dissolved inorganic carbon (DIC), FT-ICR MS, liquid chromatography organic carbon detection, fluorescence and DOC concentrations to identify groundwater DOM sources at three locations in New South Wales. We then determine the changes in DOM character as it ages in groundwater. Sources identified include aged peat with an aromatic character and a young hydrophilic terrestrial source. The young terrestrial source was found to degrade into carboxylic-rich alicyclic molecules with intermediate hydrogen/carbon (H/C) and low to intermediate oxygen/carbon (O/C ratios in shallow groundwater. Sources identified include aged peat with an aromatic character and a young hydrophilic terrestrial source. The young terrestrial source was found to degrade into carboxylic-rich alicyclic molecules with intermediate hydrogen/carbon (H/C) and low to intermediate oxygen/carbon (O/C) ratios in shallow groundwater. In contrast, increasing thermodynamic constraints in deep confined aquifers resulted in an increase in ancient (up to 25,310 ± 600 years before present) low O/C DOM due to the preferential microbial decomposition of more oxidised formulae. We also identify the accumulation of high H/C and heteroatom containing microbial metabolites and biomass in deep highly aged anoxic groundwater. Our results show that the current paradigm of aged, stable DOM occurring in the centre of H/C versus O/C space may be constrained to well-mixed, oxic aquatic environments. In contrast, the oldest DOM appears instead to occur in anoxic aquifers where the most persistent formulae have high H/C and low O/C ratios. Interestingly, these formulae are typically biolabile in aerobic environments.