Gas-phase Partitioning Tracer Tests to Quantify Water Content in Relatively Dry and Desiccated Porous Media

Soil desiccation is a potentially robust remediation process to slow migration of inorganic or radionuclide contaminants through the unsaturated zone. The application of gas-phase partitioning tracer tests has been proposed as a means to estimate initial water volumes and to monitor the progress of the desiccation process at field sites. Tracer tests have been conducted in porous medium columns with various water saturations using sulfur hexafluoride as the conservative tracer and tricholorofluoromethane and difluoromethane as the water-partitioning tracers. For porous media with minimal silt and/or organic matter fractions, tracer tests provided reasonable saturation estimates for water contents in dry materials. However, for sediments with considerable silt and/or organic matter fractions, tracer tests only provided satisfactory results when water contents were at least 0.03 - 0.05, depending on the porous medium. For dryer conditions, the apparent tracer retardation increases due to direct air - soil sorption, which is not included in traditional retardation coefficients derived from advection-dispersion equations accounting only for air - water partitioning and water - soil sorption. Based on these results, it is suggested that gas-phase partitioning tracer tests may be used to determine initial water contents in sediments, provided the initial water saturations are sufficiently large. However, tracer tests are not suitable for quantifying moisture content during and after the desiccation process when water contents are expected to be low.