Thermal ionization mass spectrometry measurements of Ra-226 and U isotopes in soils, surface and groundwaters: Analytical aspects and pore water-matrix interactions revisited

Recent developments in thermal ionization mass spectrometry and in the chemical separation of Ra and Ba through cation exchange chromatography now allow the analysis of 226Ra and U isotopes (234U, 238U) in smaller samples than those required using conventional counting techniques. Routine measurements are now obtained with picogramme (U) to subpicogramme (Ra) quantities of these elements (i.e. in water samples of a few millilitres for U isotopes, and of ≤200 mL for 226Ra). Details are given on the analytical procedures used for 226Ra measurements in low mineralized waters. These technical improvements allow the revisiting of porewater-matrix interactions in soils and aquifers. Two case studies are examined. The first concerns the well known Cretaceous chalk aquifer in the Paris Basin, where matrices and pore waters from both the shallow unsaturated and deeper saturated (confined and unconfined) zones were analysed. The second site is a small catchment basin of the Precambrian Shield in eastern Canada, where soils, surface and ground waters were also sampled. At all sites, 226Ra/228U activity ratios in waters are much lower than the secular equilibrium value, indicating that the two elements have distinct entry functions during early mineralization processes. Similarly, Ra/Ba and Ra/Ca ratios depart significantly from the conservative behaviour expected if co-dissolution and co-precipitation were the only mechanisms involved. However, during the early infiltration stage in the unsaturated zone and in the upper unconfined part of the chalk aquifer, dissolved Ra contents correlate (r=0.9) with those of Sr, indicating a prominent input through dissolution processes, whereas 226Ra values for water samples from the crystalline basement of eastern Canada are correlated with conductivity data, rather suggesting Ra uptake through rock etching processes. In both cases, significant uptake of Ra occurs during early mineralization processes. Later, Ra dissolution occurs either through recoil effects from matrix lattice, and/or through decay of 230Th produced by porewater U and adsorbed at grain surfaces, as shown by former studies