Drilling Within the Critical Zone

ABSTRACT The critical zone (CZ), extending from the vegetation canopy to the base of weathered material or depth of groundwater, hosts coupled hydrologic, geochemical, and biological interactions that regulate soil health, water resources, and ecosystem sustainability. The subsurface CZ can extend tens to hundreds of metres below the surface and is largely inaccessible, except in happenstance exposures from quarries or road cuts through mountain hillsides. Drilling and subsequent borehole sampling, monitoring, and imaging reveal the importance of the deep subsurface for CZ evolution and function. However, drilling can be labour‐intensive, requires expensive, specialised equipment, and can only be done where the equipment can be deployed, limiting the number and placement of boreholes. Despite these challenges, drilling provides invaluable insights into deep CZ processes. To empower the next generation of CZ scientists to employ drilling and downhole techniques, this review synthesises emerging research objectives and methods commonly used during CZ drilling campaigns over the last 30 years. We focus on three CZ research themes: (1) physical and chemical weathering, (2) water storage and partitioning, and (3) solute, microbial, and gas dynamics. For each theme, we evaluate drilling techniques, sampling strategies, downhole logging approaches, long‐term monitoring, and analytical methods that collectively enable diverse hypothesis‐testing. We conclude by providing a vision for the future of drilling within the CZ, with a focus on novel drilling techniques aimed at recovering saprolitic material as well as borehole designs that can monitor and sample the vadose zone. Additionally, we emphasise that near‐surface geophysics and data‐model integration efforts are needed to expand borehole observations to the landscape scales necessary to advance CZ science and inform ecosystem and water resource management.