A new explanation for extreme ²³⁴U ²³⁸U disequilibria in a dolomitic aquifer

High ²³⁴U ²³⁸U activity ratios are found in the shallow groundwater of the phreatic Transvaal Dolomite Aquifer. The aquifer is uranium poor, while the waters are oxygen rich and young. Tritium and ¹⁴C are used to show that the disequilibrium increases the longer the water resides in contact with the dolomite host. The ²²⁸Ra and ²²²Rn isotopes were used as supporting evidence that ion exchange between the ²³⁸U series nuclides in the water and the carbonate wall rock is necessary in assisting in the fractionation process. The Wolkberg cave speleothems preserve a record of the uranium isotopic fractionations that evolved as water flowed through the aquifer. Extremely variable and elevated ²³⁴U ²³⁸U ratios (of 2-12) are characteristic. Individual caverns may exhibit very large ²³⁴U ²³⁸U variability in their drip water and in derivative carbonate precipitates (speleothems). Deeper chambers, where water has spent a longer time in contact with the aquifer, tend to exhibit a greater degree of uranium isotopic fractionation in their speleothems. The recorded fractionation process has been an ongoing one from at least the middle-late Pleistocene to the present. The speleothem study supports the contention that ion exchange processes that produce 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 uranium-poor environment).