Browsing by Author "Harrington, GA"

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    Carbon-14 in unsaturated zone gases: implications for groundwater dating
    (International Association of Hydrogeologists, 2013-09-19) Wood, C; Cook, PG; Harrington, GA; Meredith, KT; Kipfer, K
    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).
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    Factors affecting carbon-14 activity of unsaturated zone CO2 and implications for groundwater dating
    (Elsevier, 2014-11-27) Wood, C; Cook, PG; Harrington, GA; Meredith, KT; Kipfer, R
    Unsaturated zone processes may influence the carbon-14 (14C) activity of infiltrating groundwater and thus introduce error in derived groundwater residence times. However unsaturated zone 14C activities are rarely measured and there is little understanding of how they may vary spatially in a groundwater basin. In this study we measured 14C activity in unsaturated zone gas at five sites with different watertable depths (8.2–31.5 m) in the arid Ti Tree Basin, central Australia. We observed a relatively uniform decrease in 14C activity of unsaturated zone gas with depth at most sites, with variation in unsaturated zone depths leading to variation in 14C activities directly above the watertable at each site (ranging from 54 to 106 percent Modern Carbon (pMC)). Through modelling we show that the profiles are influenced by CO2 production at different depths from sources with different isotopic ratios, including production of ‘modern’ CO2 in the root zone and production of ‘old’ CO2 above the watertable. Scenario modelling showed that these processes are independent of recharge when recharge is low (0–10 mm y−1) but that higher recharge rates (>100 mm y−1) result in more advective transport of atmospheric CO2 to the watertable. The variation in 14C above the watertable was more sensitive to watertable depth and shallow and deep CO2 production rates. These findings offer insight into how unsaturated zone 14C activities may vary spatially and provide guidance as to when 14C depletion in unsaturated zone CO2 may become important for groundwater dating, particularly in arid settings. © 2014, Elsevier B.V.