[A-HW23] Tracer Hydrology: Advances in Measurement and Modelling

[AHW23-04] Helium isotopes as groundwater source and age tracer: benefits and limitations

★Invited Papers

*Daniele Luigi Pinti¹ (1.International Association of Geochemistry and Cosmochemistry, European Association of Geochemistry)

キーワード：noble gases, groundwater, age tracer, hydrological tracer

Helium is a noble gas used in hydrological sciences for at least 50 years as a source and age tracer. It is chemically inert and rare, which is perfect for being a geochemical tracer in water, even if, strictly speaking, it is not a hydrophile element such as chlorine. Its isotopic signature, namely the ³He/⁴He ratio (or R), is contrasted for its three potential terrestrial source reservoirs: the mantle, the crust, and the atmosphere. The mantle is enriched in primordial ³He isotopes compared to ⁴He and the atmosphere (Ra = 1.384 x 10^-6), with an air normalized ratio R/Ra of 8±1. Its presence in groundwater systems is usually limited to deep brines in extensional basins or geothermal fluids in volcanic areas. Still, several works have reported them also in stable cratons or large orogens (Himalaya-Tibet system), suggesting a poorly understood role of tectonics in pervasively transporting mantle helium into crustal domains. The crust is dominated by ⁴He produced by the decay of U and Th in rocks, which is the theoretical basis for using helium as a groundwater age tracer, particularly for “fossil” waters. However, sources internal and external to aquifers, often difficult to discriminate, made this dating tool semi-quantitative: obtained U-Th/⁴He groundwater ages must be cross-calibrated with other age tracers such as ¹⁴C or ⁸¹K;r or can indicate the progressive aging of groundwater bodies along flow paths. Tritium decay-produced ³He is the other helium isotope that can be straightforwardly used to date younger waters. But ³He is also produced by neutron reactions with lithium, suggesting its future potential use for tracing Li hydrogeological cycle or as an exploration tool for lithium ores. This invited review offers several examples of the benefits and limitations of using helium as a hydrogeological tracer, from studies on shallow aquifers of southern Canada and Tibet’s geothermal spring waters, to pegmatite-related and salars brines.