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A theoretical investigation of factors affecting the gas phase transport of evaporating organic liquids in the unsaturated zone is presented. Estimates of density-dfiven advective gas flow using a simple analytic expression indicate that significant advective gas flow will result from the evaporation of volatile liquids in soils having a high permeability. Numerical simulations using a two-dimensional cylindrical geometry and including the effects of phase partitioning between the solid, gas, water, and organic liquid phases show that mass transfer due to density-dfiven flow may dominate the gas phase transport of some organic chemical vapors in the unsaturated zone. 2159

Currently a common method of aquifer cleanup is to extract the polluted ground water and, after reducing the concentration of contaminants in the water below a certain level, the treated water is either injected back into the aquifer, or if it is environmentally and economically feasible, released to a surface‐water body. The proper design of such an operation is very important, both economically and environmentally. In this paper a method is developed which can assist in the determination of the optimum number of pumping wells, their rates of discharge and locations, such that further degradation of the aquifer is avoided. The complex potential theory has been used to derive the equations for the streamlines separating the capture zone of one, two, or more pumping wells from the rest of the aquifer, A series of capture‐zone type curves are presented which can be used as tools for the design of aquifer cleanup projects. The use of these type curves is shown by an hypothetical field case example.

A multidimensional integral finite difference numerical simulator is developed for modeling the steam displacement of nonaqueous phase liquid (NAPL) contaminants in shallow subsurface systems. This code, named STMVOC, considers three flowing phases, gas, aqueous, and NAPL; and three mass components, air, water, and an organic chemical. Interphase mass transfer of the components between any of the phases is calculated by assuming local chemical equilibrium between the phases, and adsorption of the chemical to the soil is included. Heat transfer occurs due to conduction and multiphase convection and includes latent heat effects. A general equation of state is implemented in the code for calculating the thermophysical properties of the NAPL/chemical. This equation of state is primarily based on corresponding states methods of property estimation using a chemical's critical constants. The necessary constants are readily available for several hundred hazardous organic liquid chemicals. In part 2 (Falta et al., this issue), the code is used to simulate two one‐dimensional laboratory steam injection experiments and to examine the effect of NAPL properties on the steam displacement process.

Analytical solutions are developed for modeling steady state gas flow to a single vacuum extraction well in the unsaturated zone. The solutions are applicable to isotropic as well as anisotropic homogeneous subsurface media in which the ground surface is open to the atmosphere. Analytic expressions are given for the gas pressure field, and for the stream function distribution. Streamline travel times are computed using a simple numerical technique employing the analytical solutions. The pressure solution may be used to analyze in situ gas pumping tests to determine average horizontal and vertical gas permeabilities. The stream function solution and streamline travel times may be used to optimize the design of vapor extraction systems for volatile organic chemical removal from the unsaturated zone. Application of the solutions to several examples illustrates the strong effect of screen location and anisotropy on the induced gas flow field, and dimensionless type curves are given for a variety of gas well geometries.