Preprints
https://doi.org/10.5194/hess-2021-276
https://doi.org/10.5194/hess-2021-276

  31 May 2021

31 May 2021

Review status: a revised version of this preprint was accepted for the journal HESS and is expected to appear here in due course.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Dylan Irvine1,2, Cameron Wood3, Ian Cartwright4, and Tanya Oliver2 Dylan Irvine et al.
  • 1Research Institute for the Environment and Livelihoods, Charles Darwin University, Casuarina, 0810, Australia
  • 2National Centre for Groundwater Research and Training, and College of Science and Engineering, Flinders University, Bedford Park, 5042, Australia
  • 3Department of Environment and Water, Adelaide, 5000, Australia
  • 4School of Earth, Atmosphere and Environment, Monash University, Clayton, 3800, Australia

Abstract. Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C-depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling, or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C-depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of 14Cuz to MRTs derived from 14C data.

Dylan Irvine et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-276', Marina GILLON, 24 Jun 2021
    • AC1: 'Reply on RC1', Dylan Irvine, 09 Aug 2021
  • RC2: 'Comment on hess-2021-276', Anonymous Referee #2, 23 Jul 2021
    • AC2: 'Reply on RC2', Dylan Irvine, 09 Aug 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-276', Marina GILLON, 24 Jun 2021
    • AC1: 'Reply on RC1', Dylan Irvine, 09 Aug 2021
  • RC2: 'Comment on hess-2021-276', Anonymous Referee #2, 23 Jul 2021
    • AC2: 'Reply on RC2', Dylan Irvine, 09 Aug 2021

Dylan Irvine et al.

Dylan Irvine et al.

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Short summary
It is widely assumed that 14C is in contact with the atmosphere until recharging water reaches the water table. Unsaturated zone (UZ) studies have shown that 14C decreases with depth below the land surface. We produce a relationship between UZ 14C and depth to the water table to estimate input 14C activities for groundwater age estimation. Application of the new relationship shows that it is important for UZ processes to be considered in groundwater mean residence time estimation.