Gas-Phase Tracer Column Experiments to Detect and Quantify Soil Moisture During Desiccation

Desiccation (soil drying) applied to the unsaturated zone is being considered at arid sites to reduce vertical contaminant migration towards the groundwater. A proposed technique to test desiccation progress is the gas-phase partitioning tracer test method. So far, the method has been used successfully using a variety of tracers in column studies to determine water content. We will discuss a series of one dimensional, 1-m long column studies were the technique has been applied before, during, and after desiccation. Sulfur hexafluoride was the nonpartitioning tracer while difluoromethane and trichlorofluoromethane served as the partitioning tracers. Gas samples were obtained at three different locations during the tracers tests. A dual energy gamma radiation system has been used to determine water saturation at the various stages of the experiment. The results show that the tracer method provides good estimates of the average water saturation between the inlet and the sampling location for several porous media and several initial water saturations.