Integrating Geophysics and Geochemistry to Characterise Geothermal Basalt Critical Zone Evolution, Johnston Draw, Idaho

ABSTRACT The critical zone (CZ) evolves through coupled physical and chemical processes that drive subsurface weathering and mass loss. Quantifying these processes is commonly based on borehole geochemistry, which provides robust but spatially limited constraints, while near‐surface geophysical methods offer broader coverage but are often interpreted qualitatively. In this study we integrate borehole and surface geophysical data with the bulk mass transfer coefficient (bulk τ ) to develop Geophysical Tau (), a geophysics‐based proxy for subsurface weathering that better constrains weathering patterns than directly converting ERT to bulk τ . The method is demonstrated using coincident borehole geochemical, borehole geophysical and surface resistivity datasets collected in volcanic rock in Johnston Draw, a subwatershed of the Reynolds Creek Experimental Watershed and Critical Zone Observatory, Idaho. is calibrated at the borehole by linking bulk τ to geophysical indicators of weathering and is extended spatially using surface electrical resistivity tomography data. The resulting reveals laterally extensive, clay‐rich weathered zones that are not explained by meteoric infiltration alone and are consistent with interpretations of bottom‐up weathering driven by convection of geothermally influenced fluids. Geophysical Tau provides a quantitative framework for integrating geochemical and geophysical observations to interpret subsurface mass loss and weathering and is transferable to analogous volcanic settings worldwide.