TRACING WATER-ROCK INTERACTION AND ESTIMATING GROUNDWATER CONTRIBUTION TO STREAMFLOW WITH U-SERIES AND SR ISOTOPE MIXING ANALYSIS

Basin aquifers and surface waters in the American Southwest rely in large part on mountainous environments for water resources. Yet, the complex geology and intricate hydrologic flow paths common to those mountainous environments often complicate our understanding of their contribution to downstream systems. One such mountainous system, the Valles Caldera National Preserve in northern New Mexico is the location of the Jemez River Basin Critical Zone Observatory (JRB-CZO) where this study investigates the dynamic relationship between subsurface structure and hydrologic function and strives to develop our understanding of water routing and groundwater contribution to streamflow. The recent drilling of groundwater monitoring wells across multiple depths in three sites with different rock type revealed several distinct groundwater stores, most notably shallow groundwater in caldera-collapse breccia and deep groundwater in fractured tuff. Furthermore, shallow groundwater has distinctively higher (²³⁴U/²³⁸U) activity ratios and ⁸⁷Sr/⁸⁶Sr signatures than deeper groundwater stores in the fractured tuff aquifer system. The distinct isotope signatures of these groundwater stores enabled their use as end members in isotope mixing analysis that indicated that deep groundwater from the fractured tuff aquifer system contributed more than 90% to streamflow in the greater catchment whereas shallow groundwater contributed less than 10%. Additional U and Sr isotope analysis of surface water and springs from multiple catchments throughout the JRB-CZO demonstrated distinct isotope signatures influenced by differences in water routing and isotopic evolution along flow paths; however, streamflow from all catchments was sourced predominantly by deeper groundwater from the fractured aquifer system. We conclude that in complex geologic terrain like that of the JRB-CZO, U-series and Sr isotopes can disentangle shallow and deep flow paths demonstrating that deep fractured aquifer systems sustain streamflow across seasons while shallow groundwater contribution is minimal.