through and a higher peak concentration than the experimentally

observed BTCs. The differences between the two estimated and observed

BTCs could be due to natural variability between the two soil samples.

Although the two columns contained nearly identical volumes of gravel,

the particle density of the gravel in Column II was higher. Since the

more dense gravel had less porosity (Table 3-3), less of the total

porosity of Column II was associated with the gravel and a greater

portion is associated with the remaining fine-earth fraction. However,

considering that all model parameters were independently estimated, the

MIM-model-generated estimates closely described the observed

asymmetrical BTCs.


 Conclusions


 The data from this study show that this soil, which contains a

strongly aggregated fine fraction and porous gravel, produced

asymmetrical BTCs for tritiated water. The degree of asymmetry

increased with increasing flow rates. The classical CD model was found

to be inadequate in describing water movement in this soil, due to the

inability of the model to account for diffusive mass transfer of water

into stagnant or immobile-water regions. The MIM model adequately

described water movement at all flow rates, and estimated that about

50% of the total-water content was in immobile regions. The high

immobile-water content and the relatively large dispersivity indicated

that, under natural field conditions consisting of short but intense

tropical rain storms, water transport in the larger soil pores could