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

The measurement of cosmogenic Kr-81 (t(1/2) = (2.29 +/- 0.11) x 10(5) yr) h as been proposed for many years as a reliable tool for groundwater dating i n the range from 10(5) to 10(6) yr. In this paper, we report on the first u se of Kr-81 to determine the age of groundwater from four wells in the Grea t Artesian Basin in Australia. As the concentration of Kr-81 in old groundw ater is only a few hundred atoms per liter, krypton was extracted from larg e (16 000 1) groundwater samples and was analyzed for the isotopic abundanc e of Kr-81 by accelerator mass spectrometry (AMS) with a cyclotron. Kr-81/K r 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 m easured for these samples. It is reasonable to assume that krypton dissolve d 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 iso tope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge. This results in respective groundwate r 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 thi s paper lies on the description of the analytic procedure to extract a reli able Kr-81 signal from large groundwater samples. Although the uncertaintie s are still relatively large (primarily due to counting statistics caused b y the low cyclotron AMS efficiency), the new technique enabled for the firs t 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 ri ghts reserved.