Vadose Zone

Researchers studying the vadose zone in agricultural settings have traditionally made surprisingly infrequent use of isotopic methods (except for nitrogen isotopes and a few other approaches related to nutrients). This is beginning to change, as evidenced by studies such as Komor and Emerson [1994], employing O and H as tracers for evaporation processes in a study focusing on the redistribution of pesticides. However, the most interesting advances are in an area of vadose-zone hydrology that is still a frontier: processes in arid-region vadose zones.

Allison et al. [1994] and Phillips [1994] have recently published reviews of the application of isotopic and environmental tracers to hydraulic processes in arid and semiarid vadose zones. Both emphasized the relative advantages of tracer methods in desert settings where inherent uncertainties pose major difficulties for the more traditional water-balance and soil-physics approaches. The most widely used tracers include the pulses ofH and Cl that were deposited as a result of thermonuclear weapons testing,O and H in the water molecule as tracers for evaporation and redistribution processes, and chloride concentration as a measure of the soil water balance. The chloride mass balance approach, which relies on the progressive changes of chloride concentration from the atmosphere to the soil, and with depth in the soil, as a measure of the evapotranspirative loss from the system is particularly attractive because of its simplicity and low expense [ Allison, et al., 1994]. The measurement of a moderate number of deep chloride profiles through desert soils has lead to the realization that in many cases the vadose zones store many thousands of years worth of atmospheric deposition. Stone [1992] has pointed out that, if downward displacement is reasonably uniform and one-dimensional, the vadose zones may contain a sequential record of paleohydrological conditions at the soil surface. In profiles from southern Australia containing up to 20,000 years worth of chloride accumulation, Stone [1992] found that evapotranspirative removal of soil water water was greater during the last glacial maximum. In contrast, in western Texas, Scanlon [1991], in a study which found chloride inventories of a similar magnitude, determined that evapotranspiration was less effective in extracting soil water during the last glacial period. Phillips [1994] surveyed chloride profiles across the entire southwestern United States and came to the same conclusion. However, independent evidence indicates that paleoclimatic conditions were very different in the desert areas of the United States and Australia during the glacial period, so the differing conclusions probably reflect geographical variability. Cook et al. [1992] have performed a thorough analysis of the potential effects of diffusion and mechanical mixing (dispersion) on vadose zone profiles.

Several recent studies have illustrated the usefulness of tracer techniques in applied research. Walker et al. [1991] have shown how chloride profiles can be used to quantify changes in recharge following land use changes. This is a problem of great practical sigificance in southern Australia where increases in recharge following clearing of scrub to improve grazing are beginning to flush saline water from the vadose zone into aquifers. Edmunds and Gaye [1994] have demonstrated how chloride profile surveys can be used to estimate the spatial distribution of diffuse recharge in the Sahel. Phillips et al. [1990] have proposed to use Cl to monitor migration of radionuclides from nuclear weapons test sites and showed that Cl vented from the Gnome Site nuclear explosion had moved less than two meters down through the soil profile.

One of the major motivations for studying water in desert vadose zones is the increasing use of this setting for hazardous and radioactive waste disposal. Isotopic and environmental tracers are beginning to be applied to these site characterization problems. Scanlon et al. [1990] and Scanlon [1992] have reported on the use of Cl and H as tracers for water movement at a proposed radioactive waste facility in west Texas. Stephens and Coons [1994] demonstrated the usefulness of the chloride mass balance method for assessing the performance of an abandoned landfill in New Mexico. In an application of great potential importance, Fabryka-Martin et al. [1993] have shown that bomb-fallout Cl may prove to be a critical tracer for unsaturated flow processes through the fractured tuff at the proposed Yucca Mountain high-level nuclear waste repository.