U-series and Sr isotopes as tracers of mineral weathering and water routing from the deep Critical Zone to streamflow in a high-elevation volcanic catchment

Tracing water routing and groundwater contribution to streamflow is a key tool for better understanding limited water resources. Measurements of U-series activity ratios and radiogenic Sr isotope ratios in drill cores, streams, and groundwater samples were employed to investigate weathering controls and better understand hydrologic flow paths and evolution through the heterogenous subsurface of a montane catchment. Situated within the semi-arid geologically complex volcanic setting of the Valles Caldera National Preserve in northern NM, this study focuses on streams draining the tallest (3432 masl) resurgent dome (Redondo Peak). Recent drilling of groundwater monitoring wells allowed for the collection and isotopic analysis of intact continuous cores and groundwater collected from similar depths, a unique and valuable contribution to U-series studies, within a high elevation headwater catchment focusing on two hillslopes with contrasting Critical Zone (CZ) structure and several distinct groundwater stores. U-series composition of core samples identified U–Th fractionation from recent (< 1.25 Ma) disturbance in both hillslopes; however, (²³⁴U/²³⁸U) and (²³⁰Th/²³⁸U) disequilibria and ⁸⁷Sr/⁸⁶Sr values indicated more intense weathering and lithologic complexity in the hillslope underlain by volcanic breccia over vesicular tuff compared to the hillslope underlain by fractured tuff. The weathering profile of the site composed of breccia overlying vesicular tuff was influenced by the presence of shallow groundwater situated above deeper groundwater stores, which have distinctively higher (²³⁴U/²³⁸U) activity ratios and ⁸⁷Sr/⁸⁶Sr signatures than those of the deep groundwater stores in the fractured tuff site. The combination of U-series and Sr isotopes across time and in isotope mixing analysis highlights the utility of pairing these two isotope tracers that pinpoint distinct groundwater stores and help to isolate controls on U-series isotopes. Time series of U and Sr isotope signatures of groundwater and surface water suggest small seasonal changes in composition of streamflow while U–Sr isotope mixing analysis suggests that deep groundwater from the fractured tuff aquifer system generates more than 90% of streamflow to the greater catchment whereas shallow groundwater and soil water contribute less than 10% to streamflow, primarily following spring snowmelt. Importantly, constraining streamflow sources using isotope mixing analysis highlights that deep groundwater from fractured bedrock within the CZ sustains streamflow here and emphasizes the need to consider deep groundwater in future studies of fractured bedrock systems.