Rock/water interaction timescales involving U and Th isotopes in a Permo-Triassic sandstone

The aim of this work is to establish the timescales for rock/water interactions, using uranium series radionuclides measured in ground waters and solids of the Chat Moss area of the Lower Mersey Basin Permo-Triassic sandstone acquifer. The measured characteristics of the fresh groundwaters of the area are consistent with the idea that the saline groundwaters are being flushed from the aquifer by the fresh groundwater. As mixing takes place, the Cl- concentrations rise, Ca2+ and Mg2+ are removed from the flushing water and are replaced by Na and K from the sandstone causing renewed calcite dissolution resulting in elevated HCO3- contents and pH. The sampled groundwaters have U concentrations in the range of 0.3 to 13.6 ppb and high 234U/238U activity ratios ranging from 1.6 to 13.8. The corresponding radium and radon contents are relatively low. The solid surfaces of the matrix material have been characterized indicating active exchange is taking place between the solid surfaces and fluids. A one-dimensional dispersion model has been used to estimate timescales for these exchange processes, and retardation factors for U and Th assuming dynamic equilibrium. Thus, for the data at a 50 m horizon a groundwater mean residence time of 250-750 y has been estimated consistent with Darcy flow estimates and with radiocarbon data. Similar calculations applied to the data from a deeper horizon of 200 m containing a mixture of younger fresh water with much older saline groundwaters has yielded a mean residence time of ∝ 1250 y. The model yields retardation factors for uranium and thorium in this aquifer of the order of 103 and > 105 respectively. Values of precipitation rate constants for uranium and thorium indicate that this process is much more significant for thorium than for uranium in this aquifer. (orig.)