Foam-Delivery of Remedial Amendments Into the Vadose Zone: Foam Transport.

Metal and radionuclide contaminants reside in the vadose zone at the U.S. Department of Energy’s Hanford Site.  Remediation technologies that immobilize these contaminants in situ are being explored. One of the challenges is associated with delivery of reagents to the subsurface.  Reagents introduced in liquid solutions tend to move vertically because of gravity and enhance plume movement to a larger depth, which poses an even greater threat on the aquifer. Foam is being evaluated as a delivery fluid because it consists of a gas phase dispersed within a liquid phase. It can be stabilized by surfactant adsorbed at the gas-liquid interface. Foam delivery can potentially distribute remedial amendments to the deep vadose zone for site remediation with minimal spreading of contaminants and achieve relatively more uniform distribution. However, research on foam transport in the vadose zone has been limited. Because the foam flow is non-Newtonian, foam mobility is dependent on surfactant concentration, foam quality, and foam flux. Hence, classic theory of flow cannot simply be extended to describe foam flow. Based on the piston-type flow assumption, we developed an analytical solution for transient movement of foam in a one-dimensional semi-infinite medium. Different from the existing fractional flow theory, foam is considered as a compressible phase. An empirical relationship between foam mobility and foam quality and injection rate was established for the non-Newtonian foam flow. Column experiments were carried out to investigate the movement of foam with different foam quality and injection rates. The analytical solution can correctly describe the foam front location, pressure variation with distance and time, and aqueous saturation obtained from the experiments. The analytical solution is useful for scoping investigation of foam transport under relatively simple conditions.