Groundwater tracing with nucleogenic 36Cl in West Canning Basin, Western Australia

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European Geosciences Union
Chlorine-36 has been used over the past 20-30 years as a groundwater tracer in many hydrological studies and is a well-established dating technique. Given the half-life of 301 kyr it is well suited for dating of ‘old’ groundwater between 50 kyr - 1 Myr. A challenge associated with utilising 36Cl as a tracer is that it can be produced via three different pathways that will influence the result based on the unique hydrogeological setting of a study area. Typically the dominant source of 36Cl in groundwater is atmospheric 36Cl that is produced at troposphere and stratosphere via interaction of cosmic-ray protons and secondary neutrons with Ar. However, the secondary cosmic-ray neutrons can similarly produce 36Cl in surface rocks particularly at high elevations. Also nucleogenic production of 36Cl at subsurface environments can become significant, especially if U and/or Th concentrations are high. Delineating and quantifying these processes is essential when using 36Cl as a groundwater dating tool. In contrast to a conservative situation where atmospheric 36Cl dominates, we present a study in the West Canning Basin located in the Pilbara region of Western Australia, where the 36Cl/Cl ratio increases from 30 10-15 near the recharge zone to 100 10-15 over a 60 km of flow path within a confined aquifer. Additional isotopic evidence (14C and 87Sr/86Sr) in groundwater, mineralogy (X-Ray diffraction) and elemental analysis (Neutron Activation) of whole-rock powder samples from the aquifer and overlying geological units, is used to establish an interpretation that nucleogenic 36Cl production is effectively the only potential process to explain the data. Nucleogenic production can influence the groundwater 36Cl content in two different ways: (1) as an additional input of Cl with a 36Cl/Cl ratio that reflects the neutron flux within the particular mineralogy; or (2) via “in-situ” production of 36Cl directly in the groundwater from the dissolved 35Cl where the rate is dictated by the neutron flux in the aquifer and the residence time. For this study, the whole-rock elemental composition is used to calculate the nucleogenic production within the aquifer and confining units of the groundwater system. This effectively ruled out both the Wallal sandstone aquifer and the overlying confining unit of the Jarlemai clay aquitard as the sources of 36Cl. Here we explore the idea that the underlying granite is the most likely source of 36Cl. © Author(s) 2017. CC Attribution 3.0 License.
Ground water, Tracer techniques, Western Australia, Australia, Carbon 14, Troposphere
Wilcken, K., Cendón, D. I., Meredith, K., Simon, K., Stopic, A., Peterson, M., & Hankin, S. (2017). Groundwater tracing with nucleogenic 36Cl in West Canning Basin, Western Australia. Paper presented at the European Geosciences Union General Assembly 2017, Vienna, Austria, 23–28 April 2017. Retrieved from: