Tracers are used widely to determine the direction and velocity of ground‐water movement. Failures of tracer tests are most commonly a result of incorrect choice of tracers, insufficient concentrations of tracers, and a lack of an understanding of the hydrogeologic system being tested. Some of the most useful general tracers are bromide chloride, rhodamine WT, and various fluorocarbons. For certain purposes, dyed clubmoss and baker's yeast have proved valuable. Many radionuclides including 3H, 82Br, and 198Au are almost ideal for numerous purposes, but radiation hazards associated with their use together with local, State, and Federal regulations have discouraged widespread field applications in recent years within the United States.

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Chlorine 36 has many advantages as a dating tool for very old groundwater. These advantages include a suitable half-life (3.01 x l0 s years), simple geochemistry, conservative behavior in groundwater, and a general absence of subsurface sources at levels comparable to the atmospheric input. Recent advances in tandem accelerator mass spectrometry have permitted the analysis of 36C1 at the low abundance expected following residence in the subsurface for 106 years or more. In order to test the suitability of 36C1 for dating very old groundwater, the 36C1/C1 ratios of 26 groundwater samples from the Great Artesian Basin of Australia have been measured. Groundwater ages calculated from the 36C1 data compare favorably with ages computed independently from hydrodynamic simulations. INTRODUCTIONThe age of groundwater can be defined as the length of time the water has been isolated from the atmosphere. The concept of groundwater age is inherently somewhat ambiguous because, due to the effects of diffusion and hydrodynamic dispersion, no two water molecules in a given sample of water can be precisely the same age. Nevertheless, even though all samples are affected to some degree by mixing [Davis and Bentley, 1982], the concept of an "average" groundwater age is still a useful one. In this paper we compare two independent paethods of estimating this average groundwater age: (1) a new method of radiometric calculation dating, 36C1 tracing, and (2) the use of Darcy's law and the continuity equation.Up to the present time, the most successful radiometric method for the measurement of groundwater ages has been •4C dating. Carbon 14 dating of groundwater was first described by Munnich in 1957 and has since seen wide application. However, the inherent limitations of •'•C prevent its application in some circumstances where dating of groundwater is desired. One of these limitations is the age range over which dating is possible. With traditional direct-counting methods of measuring •4C, the maximum water age at which •4C can still be detected is less than 50,000 years. The advent of tandem accelerator mass spectrometry (TAMS) analysis and isotopic enrichment processes may advance this maximum age toward 80,000 years. Although this age limit is satisfactory for many shallow aquifers, deep regional flow systems commonly contain much older water. The need for a radio-nuclide with a longer half-life is particularly acute in hydrogeologic investigations of potential nuclear waste repositories in the deep subsurface. Such sites are intentionally located in low permeability formations containing very old water. Another disadvantage of •4C is the chemical reactivity of itsprincipal chemical form, the bicarbonate ion. The bicarbonate ion interacts with the aquifer matrix by precipitation or solution of carbonate minerals and exchange with carbonates, and is also produced biologically. These interactions complicate both determination of the initial •4C activity and the estimation of transport through the aquifer. A radioisotope with high sol...

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