Environmental tracers age dating young groundwater
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).
One reaction to the new circumstances was to call for increased emphasis on research that had relatively immediate societal applications.
Hydrology clearly falls into this category; indeed, one common criticism of hydrological research is that the direction of research is driven too much by perceived applications.
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.
That report focuses on the nature of scientific problems in hydrology and the means by which solutions to those problems might be efficiently promoted: "The choice of research problems is occasioned by its level of development within the hierarchy of the science, by the availability of new methods with which to solve it, and by the desire to understand a hydrologic phenomenon more deeply. Later the report states that "achieving this comprehensive understanding will require the kind of long-term disciplinary and interdisciplinary effort that can be sustained only by a vigorous scientific infrastructure" (p.
Even the basics of water flow in arid-region vadose zones are still incompletely understood.
Scientific disciplines that have a large theoretical component (e.g., the mathematical description of solute transport in ground water) tend to develop in a coherent way as individual researchers explore and build upon earlier theory, with only occasional correction from experimental results. Water movement in desert soil traced by hydrogen and oxygen isotopes, chloride, and chlorine-36, Southern Arizona.
New developments are driven in large part by intellectual assessment of immedi- ately preceding work.
(NRC, 1991) emphasizes that "environmental isotopes are a key tool in studying the subsurface component of the hydrologic cycle." Despite recently increased interest in applications of environmental tracers, no clear path of development over the past 5 to 10 years can be laid out.
This diffuse and unpredictable nature of development is a direct outcome of the opportunistic nature of the field.