Understanding the deep critical zone in the South Carolina Piedmont using borehole sampling and imaging

Quantifying subsurface weathering and water storage potential in mountain landscapes is important for understanding the deep critical zone and its central role in ecology, hydrology, and geomorphology. Surface-based geophysical techniques, such as seismic refraction and resistivity surveys, have emerged as powerful tools for quantifying subsurface weathering and water storage potential, especially where data from boreholes are available for calibration. However, borehole calibration data are rare. Here we present geochemical and geophysical data from four boreholes drilled to depths of 19, 45, 45 and 70 m in the South Carolina Piedmont in Spring 2022. Previous surface-based geophysical surveys at this site have suggested a strong connection between seismic velocity structure and subsurface weathering predicted from a model of the subsurface stress field (St Clair et al., 2015). Our new downhole geophysical measurements include optical and acoustic borehole imaging and logs of sonic velocity, spectral gamma, and nuclear magnetic resonance. We interpret these measurements in terms of porosity, fracture spacing, and material strength. Our new geochemical analysis of 240 bulk samples from cores yields concentrations of major, minor, and trace elements, which we interpret in terms of strain and mass loss that have occurred during weathering of the underlying gneissic bedrock. We use our measurements, together with previously published data from a 65-m borehole in the same bedrock (Holbrook et al., 2019), to determine (i) whether differences in bedrock composition can explain mismatches between the surface-based geophysics and the subsurface stress model; and (ii) whether physical and chemical weathering are tightly coupled due to feedbacks between the opening of fractures, the flow of meteoric water, and the breakdown of minerals by weathering.