Carbon source and sink investigations in a Late Quaternary-age coastal limestone aquifer using radiocarbon of dissolved inorganic and organic carbon

Abstract

This study aims to investigate the inorganic and organic aspects of the carbon cycle in groundwaters throughout a freshwater lens and mixing zone of a carbonate island aquifer and identify the sources of carbon that dissolve in the groundwater. Groundwater samples were collected from shallow (5-20 m) groundwater wells on a carbonate island in Western Australia and analysed for inorganic ions, stable water isotopes (δ18O, δ2H), 3H, 14C and 13C carbon isotope values of DIC and DOC. The composition of groundwater DOC was investigated by Liquid Chromatography-Organic Carbon Detection (LC-OCD) analysis. The presence of 3H (0.12 to 1.35 TU) in most samples indicates that groundwaters on the island are modern, however the measured 14CDIC values (8.4 to 97.2 pmc) would suggest that the carbon in most samples is older due to carbonate dissolution and recrystallisation reactions. 14CDOC values (46.6 to 105.6 pMC) were higher than 14CDIC values and were well correlated with 3H values. Deeper, saline groundwaters were characterised by an absence of 3H, and lower 14CDOC values. The DOC composition of these groundwaters was found to be different to fresher groundwaters, with higher proportions of humic substances. The 3H free, saline waters are hypothesised to be old, remnant sea water resulting from a sea level highstand that occurred between ~4.5 and 4.3 ka ago. This study shows that a combined approach utilising both DIC and DOC tracers, as well as 3H, is required to identify the sources and evolution of carbon in groundwater, and the processes that effect the application of 14C dating to groundwaters. This is important for understanding the evolution of groundwater resources and is essential for residence time calculations.