ABSTRACT
The increasing severity and geographic extent of subsurface contamination problems has resulted in growing demands for predictive models of transport through both the vadose and saturated zones. Consequently, the list of available approaches and computer codes for modeling transport has grown steadily (e.g., van der Heijde and Elnawawy, 1993; Pennell et al., 1990). The practitioner must make an informed decision of which code to use based on (l) the specific nature of the problem, (2) the capability of the algorithm and code, and (3) his or her experience. This chapter is intended to assist the practitioner in assessing the capability of available methods by providing a perspective on state-of-the-art of vadose zone modeling approaches. While we do not catalog and categorize available codes, as this was done recently by van der Heijde and Elnawawy (1993), we do review some of the key assumptions inherent to various modeling approaches to offer a framework for assessing their strengths and weaknesses. Such a framework would be less important if there existed one body of widely accepted theory that provided a foundation upon which to base transport modeling and prediction. On the contrary, considerable debate remains in the scientific literature as to the validity of various assumptions and theories underlying the modeling approaches for the unsaturated zone. This should not preclude 250prudent application of the models, but nevertheless strongly dictates the appropriate modes of application and interpretation.
