Foam delivery of amendments for vadose zone remediation: propagation performance in unsaturated sediments.

Foam delivery of remedial amendments is potentially useful for vadose zone remediation because aqueous amendments are delivered while a low water content is maintained in the treated zones. The transport behavior of foam in the vadose zone has not been well studied. Benchtop-scale column tests were conducted to investigate foam transport, foaming gas and liquid propagation, pressure buildup and distribution, the influence of foam quality and sediment permeability on foam injection pressure, and liquid uptake and distribution during foam flow in unsaturated sediments. Results indicated that under unsaturated conditions, the foaming gas front traveled much faster than the foam front where bubble rupture was significant. A liquid front was formed ahead of the foam flow front. The accumulated liquid in this front was mainly composed of the water injected with the foam. The foam injection pressure was primarily attributed to the resistance to foam flow by the sediment but not to the movement of the water ahead of the foam front. The foam had a high effective viscosity compared with water. A near-linear pressure distribution within the foam-occupied column section was observed. At a fixed injection rate, higher foam quality resulted in lower injection pressure. Higher permeability sediments did not necessarily have a lower foam injection pressure because other factors, such as the foam injection rate, foam bubble diameter, and sediment pore size, also influenced the pressure. In the foam flushed region, the liquid uptake and water content distribution in the sediment was negligibly affected by the initial water content and the foam injection rate within the ranges tested in this study. The implications of these findings to field remediation application were considered.