THE IMPORTANCE OF GROUNDWATER-DEPENDENT BEDROCK WEATHERING PROCESSES IN DRIVING THE HILLSLOPE EXPORT OF BIOLOGICALLY CRITICAL ELEMENTS (Invited Presentation)

Major components of the hydrologic and elemental cycles reside below ground, where their complex dynamics and relation to surface water processes are challenging to quantify. As part of Berkeley Lab’s Watershed Function Scientific Focus Area, the seasonal dynamics of subsurface flow and transport were documented along a shale bedrock hillslope in the Rocky Mountains (Colorado, USA) along with associated discharge into the East River, a tributary of the Gunnison River. Our findings have demonstrated that seasonal fluctuations in groundwater elevation along the hillslope and the interval where this fluctuation occurs define both the active weathering zone and its vertical extent, with three hydro-biogeochemically distinct zones identified: the soil zone, weathering zone, and unweathered bedrock zone. Using solute concentrations measured in each zone and their discrete water fluxes, we estimated seasonal export rates of aqueous carbon, nitrogen, and sulfur species. The shale weathering zone unexpectedly yielded quantitatively similar amounts of organic carbon to that of the soil zone while at the same time serving as a critical region for seasonal nitrogen cycling including tight coupling between ammonium desorption tied to cation exchange and nitrification. Such detailed characterization of hillslope soils and underlying bedrock have enabled the more rigorous interpretation of seasonably variable riverine concentrations of dissolved carbon, nitrogen, and sulfur through concentration-discharge (C-Q) relationships. Maximum dissolved organic carbon (DOC) export is associated the annual spring freshet displaying a positive C-Q relationship with clockwise hysteresis, indicating mobilization and depletion of DOC from the soil and weathering zones and emphasizing the importance of shallow to medium depth flow paths during snowmelt. Correspondingly low levels of riverine nitrogen export suggest a combination of retention and gaseous loss along the hillslope-to-river transect. High sulfate concentrations within the weathering zone are inferred to derive from pyrite oxidation in shale bedrock, with its riverine export exhibiting dilutionary behavior indicative of a groundwater source and further emphasizing the importance the weathering zone to solute export to inland waters.