A NEW EXPLANATION FOR EXTREME U-234 U-238 DISEQUILIBRIA IN A DOLOMITIC AQUIFER

High U-234/U-238 activity ratios are found in the shallow groundwater of the phreatic Transvaal Dolomite Aquifer. The aquifer is uranium poo r, while the waters are oxygen rich and young. Tritium and C-14 are us ed to show that the disequilibrium increases the longer the water resi des in contact with the dolomite host. The Ra-228 and Rn-222 isotopes were used as supporting evidence that ion exchange between the U-238 s eries nuclides in the water and the carbonate wall rock is necessary i n assisting in the fractionation process. The Wolkberg cave speleothem s preserve a record of the uranium isotopic fractionations that evolve d as water flowed through the aquifer. Extremely variable and elevated U-234/U-238 ratios (of 2-12) are characteristic. Individual caverns m ay exhibit very large U-234/U-238 variability in their drip water and in derivative carbonate precipitates (speleothems). Deeper chambers, w here water has spent a longer time in contact with the aquifer, tend t o exhibit a greater degree of uranium isotopic fractionation in their speleothems. The recorded fractionation process has been an ongoing on e from at least the middle-late Pleistocene to the present. The speleo them study supports the contention that ion exchange processes that pr oduce sufficient enrichment of uranium on carbonate can provide ideal conditions for generating uranium isotopic fractionation in the water of carbonate aquifers (in the absence of redox changes and within a ur anium-poor environment).