Abstract

Soil water saturation is a very important parameter in the vadose zone and bioreactor landfills. To measure water saturation over large measurement volumes in both systems, the partitioning gas tracer technique (PGTT) was suggested as a useful tool. 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 PGTT measurements. A single-region model was used to describe transport of gas-phase tracers. Laboratory-scale partitioning tracer tests were conducted in four different sand packings. The results from the mathematical modeling and laboratory experiments were used to suggest guidelines for minimizing measurement error in field-scale PGTTs for water saturation measurement.

We have further assessed the influence of a number of important parameters on PGTT performance in two-dimensional systems: tracer diffusion coefficients, interstitial gas velocity, spatial variations in temperature, and the degree of domain heterogeneity. The molecular diffusion coefficient of conservative and partitioning tracers has a significant influence of PGTT measurements. Gas velocity was examined as a separate operational parameter in the two-dimensional model simulations. Our simulation results suggest that much higher gas velocities might be used to measure S_w in field tests without significant loss of measurement accuracy.