Volatilization and vapor phase transport of organic contaminants in the subsurface.

The contamination of groundwater by volatile organic compounds is considered to be a threat to potable water supply. The mechanisms that control this pollution episode are under investigation to determine the most important factors governing contaminant motion. Improved groundwater protection and effective aquifer remediation are the long-range goals of these studies. Vapor phase partitioning and the transport of volatile organic compounds in the soil-gas phase are the focus of this study. A mathematical model that describes the motion of a non-aqueous phase and one other phase through the subsurface is presented. The organic phase and either the water or gas phase is allowed to move in response to pressure and gravity forces. Volatile components of the non-aqueous phase may partition to the gas and water phases. These components may be transported by advection in the mobile phases and by dispersive mechanisms in all three phases. The mathematical formulation separates the set of partial differential equations that describe phase flow from the set that describe component transport in the phases and proposes an iterative scheme between the equation sets. The numerical model based on this mathematical formulation is shown to be more flexible in its description of the problem and more efficient in the simulation of multiphase flow and transport than previous models. The numerical model is used to examine the gas phase transport of organic contaminants in the subsurface. Simulations show that the displacement of the soil-gas phase by an infiltrating organic liquid should be considered when modeling the transport of organic contaminants. The importance of the coordinate system used for a simulation is emphasized in several tests. Cartesian coordinates are often used to simulate the movement away from an infiltration event which is more closely described by an axisymmetric domain. This inappropriate coordinate system over-predicts the movement of the infiltrating organic liquid and the associated transport of organic contaminants. The simulation results indicate that the mobile nature of the soil-gas phase and the volatile nature of organic contaminants combine to make gas phase transport in the vadose zone an important phenomenon.PhDEnvironmental EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103479/1/9319618.pdfDescription of 9319618.pdf : Restricted to UM users only