Abstract
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 unsaturated zone 14C to MRTs derived from 14C data.
Highlights
Environmental tracers are widely used to estimate both groundwater residence times (e.g. Love et al, 1994; Plummer and Sprinkle, 2001; Cartwright and Morgenstern, 2012; Jurgens et al, 2012) and recharge rates (e.g. Leaney and Allison, 1986; Cartwright et al, 2007; Gillon et al, 2009; Wood et al, 2015) that are important to effectively manage groundwater resources
We provide a demonstration of the newly presented relationship using datasets from the Limestone Coast and Ovens/Goulburn–Broken catchments in Australia to estimate mean residence times (MRTs)
For the Ovens/Goulburn– Broken catchments dataset, we present tritium data to provide supporting information for 14C-based calculations of MRTs
Summary
Environmental tracers are widely used to estimate both groundwater residence times (e.g. Love et al, 1994; Plummer and Sprinkle, 2001; Cartwright and Morgenstern, 2012; Jurgens et al, 2012) and recharge rates (e.g. Leaney and Allison, 1986; Cartwright et al, 2007; Gillon et al, 2009; Wood et al, 2015) that are important to effectively manage groundwater resources. Environmental tracers are widely used to estimate both groundwater residence times Groundwater tracers can be used to determine mean residence times (MRTs) from tens to hundreds of years (e.g. tritium or CFCs), thousands to tens of thousands of years (e.g. carbon-14) to millions of years (e.g. helium-4 or chlorine-36). Clark and Fritz, 1997; Cartwright et al, 2017), most studies of regional groundwater assume that prior to the atmospheric bomb testing in the 1960s they were constant at 100 % modern carbon (pmC). This approach yields so-called conventional radiocarbon ages in years before present (BP)
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