the observed BTCs (Fig. 4-4). At the slower flow rates, the pulse

resided in the column long enough to allow diffusion to bring the

mobile and immobile regions closer to physical equilibrium, thereby

masking the presence of an immobile-water region. Thus, at slow flux,

the conceptual assumption of the CD model, which considers all water to

be mobile, is falsely satisfied.

MIM model analysis

 The dimensionless parameters estimated by the MIM model for the

four displacement experiments through Column I are presented in Table

4-4. The retardation factor for all of the trials was held constant at

the measured value of 1.05 during each curve-fitting process. The

Peclet number, B, and w were allowed to vary.

 Although # is generally considered a constant for any given soil

sample, it showed a slight increase as the flux decreased. This

behavior is attributed to the inability of the model to distinguish

easily between the mobile and immobile regions when the flux is slow

enough to allow considerable diffusion between the two regions. This

implies that the best estimate of f is when the flux is infinitely

fast. Since the trial with the fastest flux exhibited the highest

degree of physical nonequilibrium (CD-model analysis), its MIM-model

analysis should yield the best estimate of f. Therefore, the BTC were

refitted to the MIM model holding # constant at 0.53 (Table 4-5).

In this study, f = 0.53 will be used as the value to approximate 0,

since R is very close to 1.0. The measured and MIM-estimated BTCs for

the four experiments from Column I are smooth, asymmetrical and show