Using radiocarbon and organic carbon characterisation techniques to identify the sources and degradation of dissolved organic matter in groundwater
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Date
2021-11-17
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Nuclear Science and Technology Organisation
Abstract
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 (¹⁴ C) 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 ¹⁴ C-DOC analyses. The ability to measure ¹⁴ C in small mass samples on
ANTARES allows for a larger throughput of ¹⁴ C-DOC samples and quicker sample preparation due to lower
quantities of water required. Here, we combine ¹⁴ C-DOC, ¹⁴ C 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. 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.
Description
Keywords
Carbon, Monte Carlo Method, Organic matter, Ground water, Geochemistry, Atoms, Environment, New South Wales, Metabolites
Citation
McDonough, K., Merefith, K., Rutlidge, H., Andersen, M., O'Carroll, D., Oudone, P., Behnke, M., Marjo, C., Santos, I., Spencer, R., McKenna, A., & Baker, A. (2021). Using radiocarbon and organic carbon characterisation techniques to identify the sources and degradation of dissolved organic matter in groundwater. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 68). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf