THE IMPACT OF IN SITU RADIOGENIC PRODUCTION ON TRACER-BASED HYDROCHRONOLOGY

Short-lived isotopes (e.g. ³H, ¹⁴C, ⁸¹Kr) produced via a combination of anthropogenic and natural processes are introduced into groundwater systems at the point of hydrogeologic recharge. The initial concentrations of these are highly dependent on the environmental and geographic setting and decrease over time through radioactive decay. This allows constraints on the last time a surface-recharged groundwater was in contact with the atmosphere. However, processes and reactions occurring in the subsurface (e.g. mixing, dissolution, and precipitation), can introduce complexity. The effects can be significant and result in apparent temporal discrepancies between different tracers and with other related radiogenic tracers (e.g. noble gases) affecting their role as tracers of geologic processes and their respective dates and rates. Although less commonly considered, in situ subsurface production through subsurface nucleogenic reactions involving U and Th decay may add additional complexity to the application of these tracers. With new and emerging tracers, coupled with the advent of more refined analytical techniques, we are at a stage where subsurface production should be revisited.

In the subsurface, neutron capture reactions produce ³H, ¹⁴C, ³⁶Cl, ³⁹Ar, and ⁸¹Kr, in both the host matrix and in any associated fluids^e.g.1-3. Despite this, the dependence of geochemistry, host rocks and geologic settings on such tracer production and the corresponding baseline production remains poorly constrained. We address this through numerical modelling to evaluate and quantify how the geochemistry of the host rock and fluids control in situ production of ³H, ¹⁴C, ³⁶Cl ³⁹Ar, and ⁸¹Kr via neutron capture of parent elements. Using this approach, the rates and production routes are modelled for a variety of host rock lithologies and fluids to evaluate the impact baseline fluid production may have on apparent residence times in a wide range of geologic setting. This model demonstrates how combined fluid and rock geochemistry produce measurable in situ concentrations of ³⁶Cl, ³⁹Ar, and potentially ¹⁴C, while ³H and ⁸¹Kr remain principally related to surficial recharge.

¹Andrews et al. (1989) GCA 53, 1803–1815.

²Lehmann et al. (1993) WRR 29, 2027–2040.

³Purtschert et al. (2021) GCA 295, 65–79.