The curved 14C vs. δ13C relationship in dissolved inorganic carbon:A useful tool for groundwater age-and geochemical interpretations

Determination of the C-14 content of dissolved inorganic carbon (DIC) is useful for dating of groundwater. However, in addition to radioactive decay, the C-14 content in DIC (C-14(DIC)) can be affected by many geochemical and physical processes and numerous models have been proposed to refine radiocarbon ages of DIC in groundwater systems.Changes in the delta C-13 content of DIC (delta C-13(DIC)) often can be used to deduce the processes that affect the carbon isotopic composition of DIC and the C-14 value during the chemical evolution of groundwater. This paper shows that a curved relationship of C-14(DIC) vs.delta C-13(DIC) will be observed for groundwater systems if (1) the change in delta C-13 value in DIC is caused by a first-order or pseudo-first-order process, e.g.isotopic exchange between DIC and solid carbonate, (2) the reaction/process progresses with the ageing of the groundwater, i.e.with decay of C-14 in DIC, and (3) the magnitude of the rate of change in delta C-13 of DIC is comparable with that of C-14 decay.In this paper,we use a lumped parameter method to derive a model based on the curved relationship between C-14(DIC) and delta C-13(DIC).The derived model, if used for isotopic exchange between DIC and solid carbonate, is identical to that derived by Gonfiantini and Zuppi (2003).The curved relationship of C-14(DIC) vs. delta C-13(DIC) can be applied to interpret the age of the DIC in groundwater.Results of age calculations using the Method discussed in this paper are compared with those obtained by using other methods that calculate the age of DIC based on adjusted initial radiocarbon values for individual samples.This paper shows that in addition to groundwater age interpretation,the lumped parameter method presented here also provides a useful tool for geochemical interpretations, e.g.estimation of apparent rates of geochemical reactions and revealing the complexity of the geochemical environment. (C) 2014 Elsevier B.V.All rights reserved.