The Vadose Zone Deserves More Attention

The vadose zone, the variably saturated layer between the surface and groundwater table, plays a significant yet often overlooked role in subsurface contaminant fate and transport. Contaminants entering and exiting the subsurface must pass through this zone, but characterization methods are comparatively less developed than those applied to the saturated zone. Despite its importance, the vadose zone's complexity and transient nature present challenges to site characterization that have historically limited our ability to fully understand it.

Conventional vadose zone field investigations typically rely on well-established technologies, such as coring, seepage lysimeters, and geophysical methods like electrical conductivity and time domain reflectometry. While these methods continue to be used and refined, the rise of real-time and data-intensive tools holds significant promise in transforming vadose zone characterization, providing more detailed information than ever before.

New challenges have also emerged. The growing concern over per- and polyfluoroalkyl substances (PFAS), a diverse group of synthetic chemicals, has highlighted the need for improving our understanding of vadose zone retention processes. Some PFAS have surfactant properties and are known to interact strongly with air-water interfaces which can contribute to greater accumulation in the vadose zone. Investigating the subsurface transport behavior of these and other chemicals is critical to the development of robust characterization and remediation strategies.

Collectively, these papers underscore the important roles of the vadose zone in contaminant fate and transport, as well as the need for continued research to uncover the processes yet to be discovered in this dynamic environment.

We extend our sincere thanks to the authors, coauthors, and peer reviewers who contributed to this Special Issue. We hope it will serve as a valuable resource, inspiring further research and fostering a deeper appreciation of the vadose zone's roles in our practice. There is still much to learn about this critical, yet often overlooked, component of the subsurface environment.