USING A MULTIREGION MODEL TO STUDY THE EFFECTS OF ADVECTIVE AND DIFFUSIVE MASS-TRANSFER ON LOCAL PHYSICAL NONEQUILIBRIUM AND SOLUTE MOBILITY IN A STRUCTURED SOIL

Waste management problems for shallow land burial facilities in the hu mid eastern United States are usually complicated by slow but continuo us movement of wastes through the soil matrix and discrete but rapid p ulses of wastes through macropores and fractures. Multiple-pore-region models employed to describe flow and solute transport in the soils us ually consist of multiple mass transfer coefficients that cannot be me asured experimentally, and their effects on subsurface mass transport are poorly understood. The objective of this research was to study the individual and concurrent effects of interaggregate advection and dif fusion on mass transport in a structured soil. The interactions of the se two mass transfer processes and local solute concentration equilibr ium are examined for a heterogeneous soil. Pore region water retention , hydraulic conductivity, and dispersivities, obtained from independen t measurements and published calibration results, were used to test a novel three-pore-region, one-dimensional numerical model. Advective an d diffusive mass transfer coefficients were estimated using mass trans fer equations and fracture spacings published in the literature. The m ass transfer coefficients were then varied systematically, and the sen sitivity of local fluid pressure and solute concentration nonequilibri um to interregion mass transfer were analyzed. Our results indicated t hat time-dependent interaggregate advection and diffusion were importa nt processes controlling solute mobility in heterogeneous media. Under transient flow conditions, interaggregate advection may reduce the si gnificance of interaggregate diffusion that otherwise dominates intera ggregate mass transfer under steady state conditions. Nonetheless, the equilibrium of local solute concentrations was 20 times more sensitiv e to diffusive mass transfer than to advect;ve mass transfer, which su ggests that site characterization efforts should be directed more towa rd the former process. Unfortunately, characterization efforts of this type are not commonplace and if available are frequently ignored beca use they add a difficult reality to complex waste management problems. Since advective and diffusive mass transfer may be important processe s limiting the efficiency of cleanup activities such as pump and treat , it is perhaps time to include the characterization of these processe s and quantification of the timescale of physical nonequilibrium in si te remediation efforts.