Factors affecting carbon-14 activity of unsaturated zone CO2 and implications for groundwater dating

Unsaturated zone processes may influence the carbon-14 (C-14) activity of infiltrating groundwater and thus introduce error in derived groundwater residence times. However unsaturated zone C-14 activities are rarely measured and there is little understanding of how they may vary spatially in a groundwater basin. In this study we measured C-14 activity in unsaturated zone gas at five sites with different water-table depths (8.2-31.5 m) in the arid Ti Tree Basin, central Australia. We observed a relatively uniform decrease in C-14 activity of unsaturated zone gas with depth at most sites, with variation in unsaturated zone depths leading to variation in C-14 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 C-14 above the watertable was more sensitive to watertable depth and shallow and deep CO2 production rates. These findings offer insight into how unsaturated zone C-14 activities may vary spatially and provide guidance as to when C-14 depletion in unsaturated zone CO2 may become important for groundwater dating, particularly in arid settings. (C) 2014 Elsevier B.V. All rights reserved.