Carbon dynamics in a Late Quaternary-age coastal limestone aquifer system undergoing saltwater intrusion
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Date
2017-12-31
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Publisher
Elsevier
Abstract
This study investigates the inorganic and organic aspects of the carbon cycle in groundwaters throughout the freshwater lens and transition zone of a carbonate island aquifer and identifies the transformation of carbon throughout the system. We determined 14C and 13C carbon isotope values for both DIC and DOC in groundwaters, and investigated the composition of DOC throughout the aquifer. In combination with hydrochemical and 3H measurements, the chemical evolution of groundwaters was then traced from the unsaturated zone to the deeper saline zone. The data revealed three distinct water types: Fresh (F), Transition zone 1 (T1) and Transition zone 2 (T2) groundwaters. The 3H values in F and T1 samples indicate that these groundwaters are mostly modern. 14CDOC values are higher than 14CDIC values and are well correlated with 3H values. F and T1 groundwater geochemistry is dominated by carbonate mineral recrystallisation reactions that add dead carbon to the groundwater. T2 groundwaters are deeper, saline and characterised by an absence of 3H, lower 14CDOC values and a different DOC composition, namely a higher proportion of Humic Substances relative to total DOC. The T2 groundwaters are suggested to result from either the slow circulation of water within the seawater wedge, or from old remnant seawater caused by past sea level highstands. While further investigations are required to identify the origin of the T2 groundwaters, this study has identified their occurrence and shown that they did not evolve along the same pathway as fresh groundwaters. This study has also shown that a combined approach using 14C and 13C carbon isotope values for both DIC and DOC and the composition of DOC, as well as hydrochemical and 3H measurements, can provide invaluable information regarding the transformation of carbon in a groundwater system and the evolution of fresh groundwater recharge. © 2017 Elsevier B.V
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Keywords
Carbon, Dynamics, Quaternary period, Coastal regions, Limestone, Aquifers, Seawater, Recrystallization
Citation
Bryan, E., Meredith, K. T., Baker, A., Andersen, M. S., & Post, V. E. A. (2017). Carbon dynamics in a Late Quaternary-age coastal limestone aquifer system undergoing saltwater intrusion. Science of the Total Environment, 607, 771-785. doi:10.1016/j.scitotenv.2017.06.094