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

Abstract

Abstract 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 ( 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.