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Many practical problems involve a phase change as a compound (eg. volatile organic compounds such as dichloromethane, benzene, toluene, etc.) migrate through a multiphased system. A common example is diffusion migration from a dissolved phase (eg. in contaminated water) into the gaseous phase (eg. in air in an unsaturated secondary leachate collection system). Under these conditions it is well known (eg. see Schwartzenbach et. al., 1993) that there is usually a concentration ‘jump’ at the interface between the contaminated water and the air, and that equilibrium is reached at the interface such that:

 

ca/w = KH’ cw/a

 

where.

ca/w = concentration in air at the interface,

cw/a = concentration in water at the interface,

KH’ = dimensionless Henry’s Law Constant, and is related to the Henry’s Law Constant KH by the relationship

 

KH’ = KH / (R T)

 

(R is the gas constant and T is the absolute temperature).

 

More generally, there is potential for phase change at interfaces other than air-water. For example, the migration of an organic compound from a dissolved phase in leachate through a “solid” geomembrane, may involve a phase change defined by [Rowe et al, 2004]:

 

cg = Sgf cf

 

where,

Sgf is the dimensionless ratio of the concentration at the geomembrane and water interface, typical values have been reported by Rowe et al, 2004. Thus, in general the concentration ratio at an interface where there is a phase change can be written as:

 

cn/w = ∆ cw/n

 

where,

cn/w is the gas or solid concentration (mol. m-3) in the n phase (i.e. gas or solid), cw/n is the concentration (mol. m-3) dissolved in the solvent of interest (eg. water), and ∆ is the dimensionless phase parameter (mol. m-3 mol.-1 m3).