ABSTRACT
In this chapter we will consider briefly the general geologic aspects of the vadose zone in the United States. As discussed in Chapter 1, the vadose zone is a long-neglected portion of the hydrologic system. This neglect has been particularly true of the geologic aspects of the vadose zone. As a consequence, publications summarizing the extent and nature of the vadose zone in different geologic settings are virtually nonexistent. Numerous depth-to-water maps, nevertheless, exist which will suggest, but not define in detail, the thickness of the vadose zone in many regions. Specifics of moisture content, composition of soil gas, chemistry of vadose water, mineralogy of solid particles, velocity of downward moving fluids, and standard hydrogeologic measures such as permeability, porosity, and dispersivity are rarely determined for the vadose zone except for the upper two or three meters which are of direct interest to agriculture.
