T256 Radiogenic Isotopes as Tracers of Geologic Processes: Dates, Rates, and Proxies II

Wednesday, 12 October 2022
1:30 PM - 5:30 PM
502

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

Paper number: T256-17
Program: Technical Sessions
Time: 5:15 PM – 5:30 PM

Authors

Abstract

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 fluidse.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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379574

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