The triple argon isotope composition of groundwater on ten-thousand-year timescales

Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas ¹⁴C of dissolved inorganic carbon and dissolved ⁴He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic ⁴⁰Ar in groundwater acquired from decay of ⁴⁰K in aquifer minerals should be insensitive to some processes that impact ¹⁴C and ⁴He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic ⁴⁰Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (<1‰) measurements of triple Ar isotopes (⁴⁰Ar, ³⁸Ar, ³⁶Ar) in groundwater. We introduce a model that distinguishes radiogenic ⁴⁰Ar from atmospheric ⁴⁰Ar by using the non-radiogenic Ar isotopes (³⁶Ar, ³⁸Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic ⁴⁰Ar excess (Δ⁴⁰Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ⁴⁰Ar ranges from ~0‰ (the expected minimum value) to +4.2‰ across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ⁴⁰Ar on aquifer lithology. We suggest that Δ⁴⁰Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret ¹⁴C- and ⁴He-based constraints on groundwater age and flow.