²³⁴U / ²³⁸U activity ratio disequilibrium technique for studying uranium mobility in the Opalinus Clay at Mont Terri, Switzerland

Mobility of naturally occurring ²³⁸U and ²³⁴U radionuclides was studied in a low permeability, reducing claystone formation (Opalinus Clay) near its contact with an overlying oxidising aquifer (Dogger Limestones) at Mont Terri, Switzerland. Our data point to a limited redistribution of U in some of the studied samples. Observed centimetre-scale U mobility is explained by slow diffusive transport of ²³⁴U in the pore waters of the Opalinus Clay driven by spatially variable in situ supply (by α -recoil) of ²³⁴U from the rock matrix. Metre-scale mobility is interpreted as a result of infiltration of meteoric water into the overlying aquifer which developed gradients of U concentration across the two rock formations. This triggered a slow in-diffusion of U with (²³⁴U / ²³⁸U) > 1 into the Opalinus Clay as attested by a clear-cut pattern of decreasing bulk rock (²³⁴U / ²³⁸U) inwards the Opalinus Clay, away from the Dogger Limestones.