Evaluating the Role Of Geochemistry on Baseline Tracer Production in the Subsurface and its Effects on Hydrochronology

Fluids entering the subsurface contain a variety of naturally occurring geochemical components, the concentrations of which are highly dependent on the environmental setting. Over time, this complex geochemistry evolves due to water-rock interaction and radiogenic processes. By constraining the rates of these geochemical changes, it is possible to characterise and quantify dominant subsurface processes and fluid characteristics. One prime example is fluid age. This is typically evaluated through two categories of tracers; those that decrease over time through radioactive decay (e.g. 3H, 14C, 81Kr) and those that increase over time (e.g. radiogenic noble gases). As these fluids, elements and tracers have different complex provenance the calculated age of any fluid actually represents the average, or mean residence time, of all components. Likewise, additional mechanisms of subsurface production or loss for these tracers can have significant bearing on element-specific calculated residence times. Consequently, temporal discrepancies between tracers can be generated in the subsurface which can prove difficult to reconcile in hydrochronologic modelling approaches. By incorporating geochemical data into existing neutron flux-based models this study investigates how subsurface production of diminishing tracers, via neutron capture of parent elements, can produce such complexity in the dating of in host fluids. The 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 baseline production may have on calculated residence times within naturally occurring systems. In particular this model provides a quantitative evaluation for how site-specific subsurface radiogeochemistry may result in non-negligible effects for 36Cl, 39Ar, (and potentially 14C)-derived residence times for a range of subsurface environments. Additionally we highlight the significant 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 this subsurface process and the impacts this may have on hydrochronologic studies.