7. RECOMMENDATIONS FOR FUTURE STUDY

The developed model, VG model, attempts to further the work of vadose zone modeling through emphasis on the variable nature of subsurface system. Nevertheless, because this model incorporates one dimension (the vadose zone leaching) sub-model and two dimension (the saturated zone mixing and groundwater flow) sub-models, the following recommendations are given for future expansion of the model: 

1. The first part (the vadose zone leaching) of this model should be upgraded to a two-dimensional aspect. Obviously a two-dimensional model is limited for describing a three-dimensional system, but is more adequate than a one-dimensional approach. It can be demonstrated that, when there is a large horizontal contaminant concentration gradient, horizontal diffusion can be much greater than vertical diffusion. Steep horizontal concentration gradients are well known to be symptomatic of localized soil concentration by volatile organic chemicals (VOC). A two-dimensional model is the minimum required for considering density-driven gas flow, which is the predominant VOC transport mechanism.  
2. The first part (the vadose zone leaching) of the VG model should be expanded to include the governing equation term for the capillary fringe. In this dissertation, the capillary fringe of the sand soil column was considered small enough to be neglected both in soil column tests and in the model calculations. Much experimental data for vadose zone contaminant concentration have mismatched the bottom portion of the column, which may indicate the capillary zone. The mismatch zone may mean that the capillary fringe holds more water and liquid contaminant concentration. This affect will increase in clayey and finer soil columns. 
3. The peak point of groundwater loading concentration in the second part (saturated zone mixing) of the VG model should be continued to be further examined. In this dissertation, the peak point of groundwater loading concentration is matched in homogeneous and heterogeneous studies. The concentrations of peak point, however, were a little bit higher than the initial concentration of sodium chloride and nitrate. These peak points may involve consideration of a saline water density differentiation effect. 
4. The last part (the vertical two-dimensional steady state groundwater flow) of this model should be expanded to include a vertical two-dimensional solute transport aspect. The contaminant movement considered only the process of advection by which contaminants moves at the same speed as the average linear velocity of groundwater. A more effective complete description of the transport of a solute requires consideration of dispersion and chemical reaction processes such as used in the first part of this model. The development of a vertical two-dimensional heterogeneous solute transport model is another challenging aspect for groundwater flow.