T22 Groundwater in Mountain Systems I

Monday, 26 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

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

Paper number: T22-15
Program: Technical Sessions
Time: 5:05 PM – 5:20 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-360063

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.