**2.3 Vadose zone studies**

A number of studies of the vadose zone in arid environments have been conducted elsewhere primarily for water resources evaluation. In the last two decades of the twentieth century, however, emphasis shifted from water resources to waste disposal and the transport of salts and other contaminants. Arid areas are being proposed for low-level and high-level radioactive waste disposal. Most of the studies related to the vadose zone in arid settings were conducted in the western United States, in regions that are designated as waste facilities. Some of these sites include Hanford Washington, Sandia New Mexico, Ward Valley California, Eagle Flat Texas, Nevada test site and Yucca Mountain Nevada (Scanlon et al., 1997). The increasing interest in the desert environment for waste facilities, in general, and radioactive waste, in particular, raises the need to understand the importance of preferential flow in the subsurface. One could assume that a thick vadose zone combined with low precipitation promotes the safest possible environment for waste disposal. However, fast flow via fractures, cracks, and macropores had been suggested as a major mechanism leading to contaminant transport much faster than anticipated by models that predicted transport based on average soil properties. For hydrologic studies, dual-porosity models exist (i.e. HYDRUS) (Simunek, 2008), but they are difficult to parameterize for this kind of soils.

### **Salt accumulation**

Salinization is a significant issue to consider in arid environments is the salinization of both soils and groundwater. The low precipitation combined with high evapotranspiration and often-slow flow rates through the subsurface, result in higher concentrations of salts. Human-induced salinization has a long history. A major source of salts accumulating in the upper vadose zone is irrigation water, which is essential for sustaining agriculture in arid lands. More than one-third of the developed agricultural lands in arid and semiarid regions reflect some degree of salt accumulation. High salinity in agricultural lands imposes stress on the growing crops that can lead to decreased yield and in some cases complete crop failure. This problem emphasizes the need for careful management of desert land and water balance.

### **2.4 Water management issues**

Despite the difficulties for plants, animals, and humans to live in desert regions, they are increasingly being utilized because of pressure from world population growth. This problem is expressed in the expansion of agricultural activities onto desert lands as well as by the formation and rapid growth of urban and industrial centers. These trends not only result in a growing demand for usable water, but also for the increased disposal of vast amounts of wastewater and solid wastes (e.g., radioactive wastes, hazardous wastes, and municipal solid wastes). In several cases, international conflicts have developed due to water rights in arid regions. Large rivers crossing desert regions are often the only potential source for water that is essential for agriculture, industrial use, and drinking water. For example, the rights to use the water of large rivers in Africa (e.g., the Nile) and in the Middle East (e.g., the Euphrates and Jordan) remain one of the major issues that govern the relations and conflicts between the countries upstream, where most of the river water discharges, and the countries that use the river water downstream.
