Water Saturation Measurements by Gas Tracers in Unsaturated Porous Media—Effect of Mass Transfer Limitations

Abstract

Water saturation is a key parameter in studies of pollutant transport in the vadose zone and an important factor controlling the rate of waste degradation in municipal solid waste landfills. The partitioning interwell tracer test (PITT) has been suggested as a useful tool for measuring water saturation over large measurement volumes in both systems. However, heterogeneous water distributions may result in mass transfer limitations, which can affect the accuracy of the measurements. In this study, we examined the influence of water saturation, Henry's Law constant, injected tracer mass, tracer quantification limit, and local rates of mass transfer between air and water on PITT measurements. A single‐region model was used to describe transport of gas‐phase tracers in a one‐dimensional system and to investigate the effect of these factors on measurement error. To verify the conclusions drawn from this modeling exercise, laboratory‐scale partitioning tracer tests were conducted in four different sand packings. Finally, the results from the mathematical modeling and laboratory experiments were used to suggest guidelines for minimizing measurement error in field‐scale PITTs for water saturation measurement.