PREFERENTIAL FLOW IN THE CRITICAL ZONE: DIRECT OBSERVATIONS OF THE SPATIAL AND TEMPORAL PATTERNS OF FLUID COMPOSITION IN VARIABLY SATURATED WEATHERED BEDROCK

Large uncertainties remain in quantifying the exchange of water at the land-atmosphere interface due to challenges associated with mapping hydrologic fluxes in the critical zone (CZ). In the weathered and fractured bedrock zone it is difficult to directly observe preferential flow paths and porosity distributions. These subsurface characteristics result from near surface weathering and play important roles in the partitioning of infiltrating rainfall between vapor and surface water fluxes. Here we present intensive stable isotope and heat tracing of the spatial and temporal patterns of preferential flow throughout an 18 m thick, variably saturated weathering profile in a steep, forested hillslope developed into the turbidite sequences of the Franciscan formation in northern California. At the seasonally dry site within the Eel River CZO we use a novel Vadose-zone Monitoring System (VMS) to sample fluids throughout the entire weathering profile – from soil to unweathered bedrock. The VMS consists of two inclined sleeves with upward facing sensors and samplers, designed to intercept fluxes within the vadose zone. Lysimeters that separately sample tightly-held or freely-draining water are distributed along the length of the VMS. The results of nearly two years of water sampling at an approximate two-week frequency provide direct evidence of deep preferential flow in the fractured bedrock vadose zone and, importantly, demonstrate that seasonal depletion of rock moisture by vegetation during the dry season influences the spatial and temporal evolution of fluid composition within the CZ, underscoring the significance geochemical evolution that can occur below shallow surface soils. Seasonal patterns of water content, temperature, and stable isotope composition of precipitation and vadose zone waters indicate that a 6-8 m thick zone of progressive mixing sits above a zone in which the stable isotope composition of waters sampled from the vadose zone and groundwater are compositionally indistinguishable. In addition to providing key constraints to hydrologic models of the CZ, these findings suggest that hydrologic studies employing stable isotopes to trace sources of water to vegetation and streamflow may not be capable of resolving differences between water derived from the saturated and unsaturated zones.