The Evolution of Water Stable Isotopes Through the Critical Zone: Direct Observations from a Vadose-Zone Monitoring System at the Eel River Critical Zone Observatory

Abstract

Interpretation of water flowpaths through the critical zone is aided by stable isotope analysis of waters accessed from soils, groundwater, and streamflow. However, little is known about how water isotope composition evolves along flowpaths from soil to stream, particularly when passing through weathered bedrock. Here, we use novel sampling capabilities to reveal that waters transiting through a deeply weathered, forested hillslope in northern California show seasonal dynamics many meters beneath soils. During the dry season, trees extract approximately 270 mm of moisture from weathered bedrock, inducing progressive drying to depths of 12 m. Wet season storms lead to rapid wetting of the thin soils and thick bedrock to a field-capacity like state. Following storms are transmitted to a groundwater system at the base of the 15 m thick weathering profile, that supplies runoff to streams. To capture the evolution of water stable isotopes, precipitation inputs since 2012 have been sampled at daily frequency and we sample waters from discrete ~0.75 m intervals within the weathering profile via a vadose zone monitoring system (VMS). Annual variations in precipitation composition are transmitted within root zone waters to approximately 8 m over a seasonal timescale, but lag input by two or more years. We detect the signal of individual storm events to 4.5 m depth. Even an extreme, atmospheric river event, that delivered exceptionally light water, did not perturb the deeper rock moisture composition, meaning water delivered to groundwater is isotopically invariant. This indicates that the weathered and fractured bedrock has a large mixing volume or rock moisture that dampens the isotopic variability of arriving waters. A storage-selection model is applied to the data to estimate the transit time distribution of water in the root-zone. Waters cryogenically extracted from weathered bedrock samples are consistently more isotopically depleted than waters sampled from the VMS, except for some saprolite samples during wet periods. We conclude that seasonal dynamics in isotopic inputs extend many meters beyond the soil into the deeper rock moisture zone, and propose that mixing of dynamic and non-dynamic water storage in the root-zone determines the water isotopic composition passed to the deeper vadose zone and groundwater.