Chemical Geology (Isotope Geoscience Section), 58 (1985) 83-88 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 83 INTERPRETATION OF 234U/238U ACTIVITY RATIOS IN GROUNDWATERS B.L. DICKSON and M.R. DAVIDSON CSIRO Division of Mineral Physics, North Ryde, N.S. W. 2113 (Australia) CSIRO Division of Mineral Physics, Sutherland, N.S. W. 2232 (Australia) (Received January 31, 1985; accepted for publication July 22, 1985) Abstract Dickson, B.L. and Davidson, M.R., 1985. Interpretation of *34U/23*U activity ratios in groundwaters. Chem Geol. (Isot. Geosci. Sect.), 58: 83-88. The quasi-steady variation in 234U/23*U activity ratio has been determined along an idealised homoge- neous one-dimensional aquifer in which the transfer of radionuclides from the aquifer material to solution occurs by both or-recoil and ion exchange. The results highlight the importance of considering these two effects in predicting the variation of 234U/33*U activity ratios in a flowing system. In particular, it was shown that such ratios in old waters should achieve the moderate values indicated by measurements, 1. Introduction Isotopic disequilibrium between the two U isotopes 234U and 238U occurs in most ground- waters with 234U/23BU activity ratios (A.R.) commonly in the range 0.5-6.0 (Osmond and Cowart, 1982). Both isotopes occur in the 23gU decay series (Fig. 1) and excess 234U in solution results from either the direct recoil of 234Th on the a-decay of 238U (Kigoshi, 1971; Kronfeld, 1974) or by preferential leaching of 234U from radiation-damaged areas (Rosholt et al., 1963; Fleischer, 1982). Ura- nium concentration and A.R.‘s within a con- fined aquifer typically show a pattern of in- creasing concentration and near-equilibrium A.R. (- l.O), where oxidising waters are en- tering the aquifer, and of falling concentra- tion and rising A.R., where the waters enter reducing zones (Andrews and Kay, 1982; Osmond and Cowart, 1982). Further down- flow of the reducing zone, the U concentra- tion remains relatively constant whereas the A.R. decreases. By relating the decrease in A.R. to the rate of decay of 23aU (Kronfeld and Adams, 1974; Barr et al., 1978; Osmond and Cowart, 1982) attempts have been made to use the variation of A.R. in the reduced area of an aquifer as a method of determining groundwater age. However, the results have been generally dis- appointing with most of the studied aquifers showing a decrease in 234U relative to 238U at rates higher than those associated with radioactive decay. The apparently rapid de- crease of A.R. and the failure of very old waters to have very high A.R.‘s have been variously ascribed to mixing of fissure and pore waters with different A.R.‘s (Andrews and Kay, 1982), to cementation of surfaces 0168-9622/85/$03.30 o 1985 Elsevier Science Publishers B.V.