Desiccation of unsaturated porous media; intermediate-scale experiments and numerical simulation

Soil desiccation (drying), involving water evaporation induced by air injection and extraction, is a potentially robust vadose zone remediation process to limit migration of inorganic or radionuclide contaminants through the vadose zone. Desiccation also has the potential to improve gas-phase-based treatments by reducing water saturation and therefore increasing sediment gas-phase permeability. Before this technology can be deployed in the field, concerns related to energy limitations, osmotic effects, and potential contaminant remobilization after rewetting must be addressed. A series of detailed, intermediate-scale laboratory experiments, using unsaturated homogeneous and heterogeneous systems, was conducted to improve our understanding of energy balance issues related to soil desiccation. The experiments were subsequently simulated with the multifluid flow simulator STOMP, using independently obtained hydraulic and thermal porous medium properties. In all experiments, the injection of dry air proved to be an effective means for removing essentially all moisture from the test media. Observed evaporative cooling generally decreased with increasing distance from the gas inlet chamber. The fine-grained sand embedded in the medium-grained sand of the heterogeneous system showed two local temperature minima associated with the cooling. The first one occurred because of evaporation in the adjacent medium-grained sand, whereas the second minimum was attributed to evaporative cooling in the fine-grained sand itself. Results of the laboratory tests were simulated accurately only if the thermal properties of the flow cell walls and insulation material were taken into account, indicating that the appropriate physics were incorporated into the simulator.