DETERMINING VADOSE ZONE GEOCHEMICAL AND GROUNDWATER PROCESSES WITHIN A SOIL-WEATHERED ARGILLITE MANTLED HILLSLOPE

Understanding the relationship between rock moisture and solute transport to the saturated zone is critical to resolving fundamental hydrologic processes like chemical weathering, contaminant transport, and runoff dynamics. However, little is known about the geochemical and weathering processes that occur within this unsaturated region due in large part to the difficulty in obtaining measurements. At the Rivendell site, a hillslope underlain by weathered argillite bedrock located in the Angelo Coast Range Reserve as part of the Eel River Critical Zone Observatory (ERCZO), inclined boreholes have been drilled into the vadose zone and instrumented with water samplers and sensors to generate an unprecedented, high spatiotemporal resolution data set of hydrological and geochemical dynamics. The purpose of this project is to conduct both geochemical and isotopic analyses of groundwater and weathered-bedrock samples to elucidate the geochemical processes taking place within the vadose zone.

To quantify the extent of weathering and the associated mineralogical changes with depth in the Rivendell system , mass transfer coefficients, τ, (using Ti as the immobile element) will be determined through elemental concentration measurements of digested weathered bedrock samples collected from depths of 4.6- 24.4m. Concentration determination for a suite of elements will be accomplished through the use of an ICP-MS for weathered bedrock samples and cations as well as Ion Chromatography for anions. Preliminary anion and cation analysis of groundwater samples generated from the VMS (Vadose Monitoring System) depict relatively high sulfate and cation concentrations at the ppm level for some elements (i.e. Mg, Al, Fe, Mn, and Sr). Yet, these concentrations may be influenced by other factors outside of the system such as the recent VMS wells installation. With time and additional data sets, cation and anion concentrations should better reflect the actual composition of the groundwater. By analyzing temporal variability of solutes in unsaturated and saturated water within the hillslope as well as the elemental composition of soil and weathered rock, we can identify the controls on the development of the weathering zone, groundwater chemistry evolution, and the transport of nutrients to vegetative and aquatic ecosystems.