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 (