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 C-14 of dissolved inorganic carbon and dissolved He-4 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 Ar-40 in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact C-14 and He-4 and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic Ar-40 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 parts per thousand) measurements of triple Ar isotopes (Ar-40, Ar-38, Ar-36) in groundwater. We introduce a model that distinguishes radiogenic Ar-40 from atmospheric Ar-40 by using the non-radiogenic Ar isotopes (Ar-36, Ar-38) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic Ar-40 excess (Delta Ar-40) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Delta Ar-40 ranges from similar to 0 parts per thousand (the expected minimum value) to +4.2 parts per thousand 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 Delta Ar-40 on aquifer lithology. We suggest that Delta Ar-40 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 C-14- and He-4-based constraints on groundwater age and flow.

Automated summary

Understanding the age and movement of groundwater is crucial for predicting well vulnerability to contamination, informing groundwater management models, and interpreting past climate signals. Improvements in geochemical dating tools for Late Pleistocene groundwater are necessary due to the importance of groundwater in climate reconstruction. Radiogenic Ar-40, acquired from decay of 40K in aquifer minerals, could serve as a useful complementary tracer for groundwater age, but its detection in significant amounts has been limited. High-precision triple Ar isotope measurements may offer a new tool for interpreting constraints on groundwater age and flow, particularly in Late Pleistocene groundwater studies.