John A. Krantz Jessica Ng Wesley R. Danskin David Bekaert Peter H. Barry David L. Kimbrough Justin T. Kulongoski Jeffrey P. Severinghaus Alan Seltzer 2021 <p><span>Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform&nbsp;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&nbsp;last glacial&nbsp;period, there is a strong need for improving geochemical dating tools on this timescale. Whereas&nbsp;</span>¹⁴<span>C of&nbsp;dissolved inorganic carbon&nbsp;and dissolved&nbsp;</span>⁴<span>He are common age tracers for&nbsp;Late Pleistocene&nbsp;groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic&nbsp;</span>⁴⁰<span>Ar in groundwater acquired from decay of&nbsp;</span>⁴⁰<span>K in aquifer minerals should be insensitive to some processes that impact&nbsp;</span>¹⁴<span>C and&nbsp;</span>⁴<span>He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic&nbsp;</span>⁴⁰<span>Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (&gt;100&nbsp;ka). Here we present the first high-precision (&lt;1‰) measurements of triple Ar isotopes (</span>⁴⁰<span>Ar,&nbsp;</span>³⁸<span>Ar,&nbsp;</span>³⁶<span>Ar) in groundwater. We introduce a model that distinguishes radiogenic&nbsp;</span>⁴⁰<span>Ar from atmospheric&nbsp;</span>⁴⁰<span>Ar by using the non-radiogenic Ar isotopes (</span>³⁶<span>Ar,&nbsp;</span>³⁸<span>Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic&nbsp;</span>⁴⁰<span>Ar excess (Δ</span>⁴⁰<span>Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ</span>⁴⁰<span>Ar 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 Δ</span>⁴⁰<span>Ar on aquifer lithology. We suggest that Δ</span>⁴⁰<span>Ar 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&nbsp;</span>¹⁴<span>C- and&nbsp;</span>⁴<span>He-based constraints on groundwater age and flow.</span></p> application/pdf 10.1016/j.chemgeo.2021.120458 en Elsevier The triple argon isotope composition of groundwater on ten-thousand-year timescales article