Hydrogeochronology: Resetting the timestamp for subsurface groundwaters

Waters recharging the subsurface contain a variety of naturally occurring geochemical components, the concentrations of which are principally a function of the atmosphere and surface environment. Over time, water-rock interactions and radiogenic processes in the subsurface control a complex geochemical evo-lution, impacting the initial surface-related signatures associated with recharged groundwaters. The rates of these geochemical changes are considerable in many important geologic settings, requiring character-isation and quantification of the dominant subsurface processes and evolving fluid characteristics. Constraining this is critically important for the discipline of fluid 'age' dating -hydrogeochronology. Age dating, or quantification of fluid residence times is typically evaluated through two categories of trac-ers; those that are incorporated at the point of recharge and decrease over time through radioactive decay (e.g. 3H, 14C, 81Kr) and those that increase over time (e.g. radiogenic noble gases) due to subsurface pro-duction and accumulation in fluids. As these fluids, elements, and tracers have different provenance, the calculated 'age' of any fluid represents the average, or mean residence time, of all components. Likewise, additional mechanisms of subsurface tracer production or loss can have significant bearing on any element-specific calculated residence times. The inevitable temporal discrepancies between tracers can be generated or accentuated in the subsurface which can prove difficult to reconcile in hydrogeochrono-logic modelling approaches.By incorporating geochemical data into existing neutron flux-based models, this study evaluates specif-ically how subsurface production of 'diminishing' tracers in the fluid phase, via neutron capture of parent elements, can produce complexity in the dating of host fluids. The in situ rates and production routes of 3H, 14C, 36Cl 39Ar, and 81Kr in different subsurface fluids are modelled for a variety of host rock lithologies to evaluate the effect such baseline crustal production in different geochemical settings may have on cal-culated residence times within naturally occurring systems. In particular this model provides a quantita-tive evaluation for how site-specific subsurface radiogeochemistry may result in enhanced baselines of 36Cl, 39Ar, (and potentially 14C) relative to canonical assumptions, and substantially impact residence time estimates calculated from these tracers for a range of subsurface environments. Additionally, we evaluate the role geologic environments can have on low levels of baseline production and calculated residence times for 3H and 81Kr. This study quantitatively evaluates the dependence of geologic environments on these subsurface processes and the impacts this may have on hydrogeochronologic studies. The ability to quantify production of tracers in the subsurface and recalibrate associated residence times in a wide range of geologic settings is of direct relevance to areas such as groundwater resource management, Carbon Capture and Storage, hydrogen storage and nuclear waste management strategies, the hydrocar-bon and energy industry, and investigations of the timing and evolution of subsurface microbial life.(c) 2023 Elsevier Ltd. All rights reserved.