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

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 14Cuz to MRTs derived from 14C data.

Analytical Expressions of Radiocarbon Distribution in Transient State Unconfined Aquifers and Their Application to Determination of Past and Present Recharges of North Africa Aquifers

“…The method by Han and Plummer (2016) to identify A 0 could not be unequivocally applied for three out of the 25 selected data preventing obtaining a radiocarbon‐based recharge estimate for the Djeffara plain (see Section 2.3). However, an empirical approach to estimate the initial radiocarbon activity based on the relation between vadose zone radiocarbon data and depths to the water table (Irvine et al., 2021) can be alternatively used. For the Djeffara samples, long‐term natural values for the unsaturated zone thickness can be derived using piezometric levels in 2006 and the cumulative drawdown from the steady state (OSS, 2006; Trabelsi et al., 2009), and ground elevation giving 30 and 60 m for samples 24 and 25, respectively.…”

“…Introducing these values into the expression provided by Irvine et al. (2021) gives A 0 at 63 and 37.9 pmc for samples 24 and 25. Using these values for A 0 in the PSS model leads to the mean recharge at 3.5 mm yr −1 indicated in Table 2.…”

“…For the Djeffara samples, long-term natural values for the unsaturated zone thickness can be derived using piezometric levels in 2006 and the cumulative drawdown from the steady state (OSS, 2006;Trabelsi et al, 2009), and ground elevation giving 30 and 60 m for samples 24 and 25, respectively. Introducing these values into the expression provided by Irvine et al (2021) gives A 0 at 63 and 37.9 pmc for samples 24 and 25. Using these values for A 0 in the PSS model leads to the mean recharge at 3.5 mm yr −1 indicated in Table 2.…”

CMBEAR: Python‐Based Recharge Estimator Using the Chloride Mass Balance Method in Australia

“…Determination of residence times and recharge rates using radioactive tracers (e.g., tritium, carbon‐14, chlorine‐36) require knowledge of the tracer input function, water‐rock interaction or biogeochemical processes, and the aquifer geometry (e.g., Vogel 1967; Clark and Fritz 1997; Han et al 2012). Aquifer heterogeneities, depth to the water table, and diffusion also have an impact (e.g., Sudicky and Frind 1981; Walker and Cook 1991; McCallum et al 2015; Cartwright et al 2020; Irvine et al 2021).…”