Uranium colloidal transport and origin of the 234U– 238U fractionation in surface waters: new insights from Mount Cameroon

Abstract

In order to evaluate the spatial variability of the 234U– 238U fractionation and its origin in surface waters, ( 234U– 238U) activity ratios (a.r.) and trace element concentrations were measured in 34 streams and springs draining Mount Cameroon volcano. In addition, tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes. U concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element, even in diluted and organic-poor waters. Such influence, which may be related to the chemical and physical nature of colloids, does induce fractionations between U and Sr along with filter size. ( 234U– 238U) a.r. variations among the different ultrafiltered fractions of a sample remain small or even negligible. Only two samples exhibit a significant isotopic difference between colloid-associated and solute U. Colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters. At the scale of the volcano, the ( 234U– 238U) a.r. in surface waters is large, ranging from 1.038 to 1.397. Small 234U– 238U fractionations, between 1.038 and about 1.10, are attributed to surface and meteoric weathering of the bedrock. By contrast, the highest 234U– 238U fractionations (up to 1.397) are associated with groundwater contributions. Thus, variations in the ( 234U– 238U) a.r. could correspond to the water source and not to the surface weathering processes. Consequently, 234U– 238U disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters.