Shallow aquifers are presently a particularly important part of the hydrogeological environment for groundwater hydrologists. This is because much of both the research and the ``commercial'' activity in the field is focused on contamination in shallow aquifers. Tracer methods that provide advances in characterizing transport in shallow aquifers are thus likely to play an important role in solving practical problems.
The advances in the preceeding three years have originated, not from new discoveries, but rather
from the refinement of two methods first proposed and
explored many years ago. Dating of groundwater by measuring both 
H and
its decay product, 
He, was first proposed by Tolstikhin and Kamensky
in 1969 and revived by Schlosser et al. [1988] and Poreda et al.
[1988]. The
approach has the distinct advantage that the measurement of both the parent
and daughter in a sequence of samples allows the shape of the input function
to be determined and the effects of diffusion and dispersion to be separated
from those of radioactive decay. The application of this approach to
estimating the spatial distribution of recharge has been described by Solomon
and Sudicky [1991] and has been tested by Solomon et al. [1992 and 1993].
The second method, rather than using the time variation of radioactive fallout from nuclear weapons testing, relies on the atmospheric buildup of products of the refrigeration industry: freons (chlorofluorcarbons). These relatively stable, inert, artificial compounds have been accumulating in the atmosphere since the late 1940's. Their inert, conservative nature makes them excellent tracers for the flow of groundwater and their simple pattern of increasing concentration (unlike the pulse of tritium fallout) permits a straightforward correlation of concentration with recharge age. The smooth increase of concentration with time also minimizes distortions of apparent ages due to dispersion. Impressive demonstrations of the ability of this approach to decipher the flow systems of shallow aquifers in Oklahoma and the Delmarva Peninsula have been published by Busenberg and Plummer [1992] and Dunkle et al. [1993]. Reilly et al. [1994] have used chloroflurocarbon recharge age determination as a means to calibrate a numerical flow and transport model and then used the model to simulate tritium transport in the aquifer. This analysis indicated surprisingly small amounts of dispersion during aquifer transport.
These demonstrations only scratch the surface of the potential applications
of this suite of techniques. In particular, combining freon dating with
H/
He measurements could produce data sets that would aid in
understanding groundwater dispersion in a fashion analogous to
heavily-instrumented artificial tracer tests (e.g., Borden [ Mackay et al., 1986], Cape
Cod [ LeBlanc et al., 1991]), but over longer flow time, greater flow
distances, and for less expense and effort! In addition to this contribution
to basic science, the techniques should be readily applicable to determining
flow geometries and rates in contaminated aquifers. This information is
critical to understanding the origins of contamination, assessing its future
migration, and planning for remediation.