The convection-dispersion equation (CDE) is often used to describe solute transport in geologic systems under saturated and unsaturated conditions. Dispersivity, one of the parameters of the CDE, is a measure of the dispersive properties of a geologic system. Traditionally, it has been considered as a characteristic single-valued parameter for an entire medium (Pickens and Grisak, 1981b). However, a number of studies have shown that a constant dispersivity is not always adaquate, and a dispersivity that is dependent on the mean travel distance and/or scale of the geologic system is often needed (Fried, 1972; Sudicky and Cherry, 1979; Pickens and Grisak, 1981a; Gelhar, Welty, and Rehfeldt, 1992; Khan and Jury, 1990).