Contaminant-transport analyses, risk assessments, and site remediations are all constrained by the complexity of the subsurface environment and by our insufficient knowledge of that environment. Most current subsurface characterization methods provide measurements for very small spatial domains, such that they are essentially point values. While such methods can provide accurate and precise data for small scales, their use for characterizing larger domains is generally constrained by sample-size limitations. Thus, methods that provide measurements at larger scales are being developed to complement the point-sampling methods. One such group of methods is based on the use of tracer tests. This review will cover “partitioning” tracer tests, which can be used to measure immiscible-liquid saturation of organic contaminants, soil water content, and fluid–fluid interfacial areas in subsurface systems. The conceptual basis and implementation of these methods will be briefly reviewed, with a focus on vadose zone applications.

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