Volatile contaminants exist in soils in three possible forms: dissolved in water, volatilized as vapor, and adsorbed by solid particles.The mass partitioning or sorption between them is an important factor that affects contaminant transport, which could be represented by retardation coefficient in the case of that equilibrium partitioning or sorption is attained.The most important mechanism affecting contaminant transport is nonequilibrium in sorption and desorption occurring in soil between solid particles and water phase, which should be described with a series of coefficients due to the innate particle-scale heterogeneity of soils.