81Kr in the Great Artesian Basin, Australia:: a new method for dating very old groundwater

The measurement of cosmogenic Kr-81 (t(1/2) = (2.29 +/- 0.11) x 10(5) yr) has been proposed for many years as a reliable tool for groundwater dating in the range from 10(5) to 10(6) yr. In this paper, we report on the first use of Kr-81 to determine the age of groundwater from four wells in the Great Artesian Basin in Australia. As the concentration of Kr-81 in old groundwater is only a few hundred atoms per liter, krypton was extracted from large (16 000 1) groundwater samples and was analyzed for the isotopic abundance of Kr-81 by accelerator mass spectrometry (AMS) with a cyclotron. Kr-81/Kr isotope ratios of (1.54 +/- 0.22) x 10(-13), (1.78 +/- 0.26) x 10(-13), (2.19 +/- 0.28) x 10(-13) and (2.63 +/- 0.32) x 10(-13) respectively, were measured for these samples. It is reasonable to assume that krypton dissolved in surface water in contact with the atmosphere has the known atmospheric Kr-81/Kr ratio of (5.20 +/- 0.40) x 10(-13). The observed reduction of isotope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge. This results in respective groundwater ages of: (4.02 +/- 0.51) x 10(5) yr, (3.54 +/- 0.50) x 10(5) yr, (2.87 +/- 0.38) x 10(5) yr and (2.25 +/- 0.42) x 10(5) yr. The main emphasis of this paper lies on the description of the analytic procedure to extract a reliable Kr-81 signal from large groundwater samples. Although the uncertainties are still relatively large (primarily due to counting statistics caused by the low cyclotron AMS efficiency), the new technique enabled for the first time a definite determination of residence times for old groundwater with Kr-81. It thus confirms the hope that this radionuclide may become a very valuable tool for groundwater dating. (C) 2000 Elsevier Science B.V. All rights reserved.