The very long flow paths and generally low permeability of regional-scale flow
systems result in very long residence times and hence different approaches to
environmental and isotopic tracing. One nuclide with an appropriate half
life, 
Cl (301,000 years), has begun to be widely applied to groundwater
problems over the past decade due to the enhanced analytical capability provided by accelerator mass
spectrometry. Work by Torgersen et al. [1991] has
provided new information about the flow rates and paleohydrology of the Great
Artesian Basin in Australia. Dowgiallo et al. [1990] used 
Cl to study
another deep sedimentary basin aquifer system, in Poland. In a rather
different application, Kaufman et al. [1990] estimated residence times of
brines in a rift environment: the Magadi-Natron basin in East Africa.
Finally, numerous isotopic tracers have been applied to deciphering the
hydrological regime in deep, low permeability rocks being studied as possible
environments for the disposal of high-level radioactive waste (e.g., [ Pearson
et al., 1991; Andrews et al., 1989]).
Some of the most interesting approaches to the analysis of very large flow
systems have arisen out of the application of traditional ``hard rock''
isotopic methods to groundwater. Banner et al. [1989 and 1990] and Musgrove
and Banner [1993] have used 
Sr, 
U/
U, and other heavy
element isotope analyses more commonly associated with rock than water
analyses to trace the sources and rates of a deep flow system extending across
the midcontinental United States. Stueber et al. [1993] have applied similar
techniques to understanding the evolution and migration of formation waters in
the Illinois Basin. The full potential of these methods for groundwater
studies has not been adequately explored and we can expect new insights in the
coming years. This progress will be aided by the publication of approaches
for modeling rock-water interaction such as that of Johnson and DePaolo
[1994].
Experimental advances of the past few years have encouraged other, previously suggested, isotopic techniques. Although measurements of chlorine isotope ratios were first performed many years ago [ Hoering and Parker, 1961], insufficient analytical precision prevented discrimination among natural samples. Advances in sample processing and gas-source mass spectrometry [ Long et al., 1993] have made available a new tool for tracing the origins of chloride in groundwater. Eggenkamp [1994] has presented data showing intruiging trends in the stable chlorine isotope ratios of deep formation waters from the Paris Basin and North Sea. The processes responsible are not yet clear, but the covariance with other isotopic and geochemical tracers promises to add to our understanding of the evolution of formation water. Eggenkamp as well as Eastoe and Guilbert [1992] also give data showing distinctive stable chlorine isotope compositions of hydrothermal and fumerole waters. Finally, Fritz and Whitworth [1994] have produced experimental results showing that the isotopes of lithium are fractionated during hyperfiltration. This advance opens the way for using lithium isotopes as tracers of membrane processes in deep sedimentary basins.