The Influence of Critical Zone Structure on its Hydrologic Function: Insights into Water Routing, Water-Rock Interaction, and Groundwater Contribution to Streamflow from Physical Hydrologic Approaches, Ion Chemistry, and U-Series and Sr Isotope Tracers

Abstract

Mountainous environments contribute substantially to basin aquifers and surface waters in the American Southwest and are an important component of water resources for large population centers downgradient. Many of those mountainous environments are composed of complex geology and intricate hydrologic flow paths. 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). There, this dissertation investigates the dynamic relationship between critical zone (CZ) structure and hydrologic function and strives to develop our understanding of water routing and groundwater contribution to streamflow within the CZ. The recent drilling of groundwater monitoring wells across multiple depths in three sites with different rock type situated in a high elevation zero order basin (ZOB) revealed several distinct groundwater stores, most notably shallow groundwater in caldera-collapse breccia and deep groundwater in fractured tuff. Hydrograph analysis of shallow and deep groundwater and surface water over a full water year indicated hydrologic connection between deep groundwater and surface water, which was reinforced by the physical characterization of the ZOB subsurface and differences in fracture density between the two hillslopes. Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry suggests that deep groundwater from the highly fractured aquifer system is more representative of groundwater contributing to streamflow year-round. Furthermore, U-series isotope composition of intact continuous core samples collected during drilling identified U-Th fractionation from recent (< 1.25 Ma) disturbance in both hillslopes; however, 234U and 230Th excesses and deficiencies, relative to 238U, and 87Sr/86Sr values indicated more intense weathering and lithologic complexity in the caldera-collapse breccia overlying welded tuff compared to the fractured tuff site. The weathering profile of the lithologically complex site was influenced by the presence of shallow groundwater above deeper groundwater stores that have distinctively higher (234U/238U) activity ratios and 87Sr/86Sr signatures than deep groundwater in the fractured tuff aquifer system. The distinct isotope signatures of groundwater were used as end members in isotope mixing analysis and indicated that deep groundwater from the fractured tuff aquifer system contributed more than 90% to streamflow in the greater catchment draining the ZOB whereas shallow groundwater contributed less than 10%. Further U and Sr isotope analysis of surface water and springs from several 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 deep groundwater. 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 shallow groundwater does not contribute significantly to streams, while deep fractured tuff aquifer systems sustain streamflow across seasons.