Tritium age dating
Although a shallow upper zone of arid-region soils is typically hydraulically active, response time in deep desert vadose zones is on the scale of 10 years (Phillips, 1994; Murphy et al., 1996). Even the basics of water flow in arid-region vadose zones are still incompletely understood. Instead, attention has focused on the application of Probably the most exciting development in this setting has been the adaptation of solid source mass spectrometric methods originally developed for "hard rock" geochemistry to the investigation of heavy isotope ratios in deep ground water. At great depths the hydraulic properties are generally very poorly known and deep flow systems may as much reflect processes under ancient tectonic and climatic regimes as they do the influence of current conditions. They are studied mainly for the information they give about the ground water flow regime rather than the nature of the chemical activity in the ground water system.
Although in some cases the systematics of the tracer behavior have been worked out during investigations of ground water systems, more commonly the systematics have been previously well understood from independent investigations and the focus has mainly been on what the tracers can reveal about ground water flow and transport. This has resulted in an increased emphasis on environmental tracer methods, partly because tracers are directly relevant to predicting the movement of dissolved contaminants and partly because the time scales for flow in arid vadose zones are often so slow that information from short-term physical monitoring may be difficult to extrapolate to the longer scale appropriate for solute transport. Figure 1 Cumulative water volume as a function of cumulative chloride mass (both per unit area) for three boreholes in the Pasco Basin, Washington. One frequently used tracer in this situation is also one of the simplest—chloride. The recharge rates are calculated from the slope of the line and the total chloride accumulation times (in parentheses) from the chloride inventory. Chloride inventories with depth are commonly used to estimate net infiltration rates (see Figure 1), and increases in concentration are used to estimate evapotranspiration (Allison et al., 1994). Based on the geological history of the site, the actual accumulation time is known to be between 13,000 and 15,000 years. Ancillary tracers include tritium (Scanlon, 1992), Cl (Tyler et al., 1996), and the stable isotopes of hydrogen and oxygen (Liu et al., 1995).
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