INVESTIGATING COUPLED HYDROLOGIC AND BIOGEOCHEMICAL FLUXES IN THE CRITICAL ZONE VIA DISTRIBUTED SENSING AND SAMPLING IN VARIABLY SATURATED, WEATHERED BEDROCK (Invited Presentation)

Within the Critical Zone (CZ), complex interactions between biota, minerals, and fluids control how the composition of infiltrating rainfall evolves on its transit to groundwater and streams and consequently, how weathering profiles develop. However, there are few observations made of vadose zone moisture dynamics coupled to solute and gas evolution within the weathered bedrock of the CZ to constrain geochemical and hydrological models. Here, we exploit an intensively instrumented, forested hillslope within the Eel River Critical Zone Observatory to explore how hydrologic fluxes in the CZ are linked to the biogeochemical processes responsible for altering the composition of water and bedrock in the CZ. Long-term monitoring of 12 deep wells, including borehole nuclear magnetic resonance (NMR), indicate that the upper 4-12 m of weathered bedrock exhibits significant seasonal changes in water content (rock moisture), below which water content changes are minimal despite seasonal saturation of bedrock fractures within groundwater perched at the base of the weathering profile. Field- and lab-scale NMR surveys indicate that the dynamic unsaturated water storage occurs in both large and small pores, while at deeper depths, exclusively large pores exhibit saturation changes. To understand the implications of dynamic water storage on biogeochemical processes, we directly sample water and gas throughout the entire weathering profile using a Vadose-zone Monitoring System (VMS). The VMS is comprised of upward-facing flexible lysimeters and gas ports distributed along two inclined boreholes. We sample water and measure CO₂ and O₂ concentrations at a nearly two-week frequency and water samples are analyzed for major cations, anions, O and H stable isotope composition, and dissolved organic and inorganic carbon. Our nearly two-year dataset indicates that both gas and water composition shows strong seasonal and vertical variability that broadly correspond to variations in rock moisture. These seasonally dynamic fluxes and stores of water in weathered bedrock beneath the soil layer significantly influence biogeochemical processes in the CZ, and suggest that these types of observations are needed for a process-based understanding of what controls weathering, chemistry and nutrient dynamics for the biota inhabiting the CZ.