Carbon-14 in unsaturated zone gases: implications for groundwater dating

Wood, C; Cook, PG; Harrington, GA; Meredith, KT; Kipfer, K

Carbon-14 in unsaturated zone gases: implications for groundwater dating

Date

2013-09-19

Authors

Publisher

International Association of Hydrogeologists

Abstract

Quantitative interpretation of the carbon-14 activity (14C) of dissolved inorganic carbon (DIG) in groundwater requires an understanding of the various chemical and physical processes that can vary the initial 14C activity from that of the original atmospheric carbon dioxide (CO2) source. Such processes include radioactive decay, carbonate mineral dissolution, isotope exchange, decay of organic matter and molecular diffusion. Many geochemical correction models exist to account for some of these processes (e.g., Fontes and Garnier, 1979). However in most existing correction schemes, it is assumed that the 14C activity of CO2 in the unsaturated zone is in equilibrium with the atmosphere (i.e., 14C:12C is the same as the atmospheric ratio). This assumption is rarely tested and in several cases has been found to be false (eg. Bacon and Keller, 1998). Not accounting for the influence of unsaturated zone processes on "C may lead to an over-estimate of groundwater residence time, or an under-estimate of fluxes derived from measured 14C data in groundwater. In this study we examine carbon isotope processes in deep unsaturated zone proles (up to 30m in depth) in arid central Australia. At multiple sites, multi-levelled proles were sampled for both unsaturated zone gas and groundwater for “C analysis. Chlorofluorcarbon (CFC-11 and CFC-12) samples were also collected in unsaturated zone gas. At all sites we observe a decrease in the 14C activity of unsaturated zone gas with depth, from approximately 110 pmC near ground surface to 50-80 pmC immediately above the water table. The measured 14C data was reproduced in a model using Hydrus, with CFC-12 concentrations used to constrain the gas transport parameters. Modelling showed that the decrease in 14C can be explained by CO2 production from different sources at different depths in the unsaturated zone (eg. plant root respiration at shallower depths, oxidation of dead organic matter at greater depths). This work has important implications for using raw 14C results for groundwater dating projects but also improves our understanding of carbon processes within the unsaturated zone. The results of this work are particularly important in arid environments where groundwater residence time is typically long (>1,000 years old) and unsaturated zones may be deep (>1 m).

Keywords

Carbon 14, Ground water, Chemical reactions, Carbon dioxide, Isotopes, Decay, Australia, Decomposition

Citation

Wood, C., Cook, P. G., Harrington, G. A., Meredith, K., & Kipfer, R. (2013). Carbon-14 in unsaturated zone gases: implications for groundwater dating. Paper presented to the IAH 2013 Perth Australia : "Solving the groundwater challenges of the 21st century", International Association of Hydrogeologists 40th International Conference, Perth, Western Australia, 15-20 September 2013, (pp. 161).