U and Th Series Disequilibrium Studies in Ground and River Waters of the Upper Rhine Valley

Jost Eikenberg Paul Scherrer Institut, CH-5232 Villigen (PSI), SwitzerlandJOST.EIKENBERG@PSI.CH

Max Ruethi Paul Scherrer Institut, CH-5232 Villigen (PSI), Switzerland

Sixto Bajo Paul Scherrer Institut, CH-5232 Villigen (PSI), Switzerland

Aude Tricca CNRS, 1, Rue Blessig, F-67084 Strasbourg (Cedex), France

Peter Stille CNRS, 1, Rue Blessig, F-67084 Strasbourg (Cedex), France

Experimental: radiochemical methods and ultra low level a-spectrometric analysis

Ground and river waters in the Upper Rhine Valley (between Mulhouse and Strasbourg, Alsace, France) are analyzed (mainly by means of a-spectrometry) for the geochemically important radionuclides of the U and Th decay series in aquifers, i.e. 238U, 234U, 228Th, 228Ra, 226Ra, 224Ra, 222Rn, 210Pb and 210Po. Very effective and fast radiochemical separation procedures with almost complete (i.e. 100%) chemical recoveries are applied as follows: Uranium: chromatographic purification of 1-2 liter aliquots using anion exchange resin followed by electrodeposition in NaHSO4-medium for preparation of almost weightless sample discs. Radium: direct adsorption on MnO2-coated polyamide discs at a pH of 8. The analyses are carried out under N2 inert gas atmosphere to prevent infiltration of CO2 with possible precipitation of carbonates and other solids involving hydrolysis reactive species. 226Ra and unsupported 224Raex are determined directly after sampling using a-spectrometry, 228Ra is measured 3 weeks later on the same disc after reaching almost secular equilibrium conditions (complete decay of 224Ra and daughters as well as constant activities of 226Ra and daughters). 228Th is determined indirectly via 224Ra (and its decay products) from the same water aliquot via the adsorption method for Ra, however, after a waiting time of at least three weeks. This has the advantage that ingrowth of 228Th from decay of its precursor 228Ra can be neglected. Radon: g-spectrometric determinations of the 222Rn daughters 214Bi/214Pb obtained from 6 one liter (gas tight samples ) aliquots in an semiconductive intrinsic Ge counter. Reproducibility is checked with liquid scintillation counting using the emanation method. Polonium: co-precipitation with FeCl3-carrier under alkaline conditions using NH4OH followed by dissolution in 0.5 M HCl and spontaneous self-deposition on silver disc. Lead: repetition of the Po radiochemistry with the supernatant four months after the radioanalysis of Po with a different Po-spike to precisely obtain the chemical yield. The nuclide, half live, isotope yield spike, counting method and detection limits are presented in Table 1.

Results and Discussion

234U/238U and 87Sr/86Sr isotope ratios as well as U and Sr concentrations are used to characterize the different rivers in terms of mixing. In the flat regions of the Rhine valley, where the hydrologic head gradients are low and the saturated zone starts close to the surface, mixing of river water occurs on very small scales and is influenced by strong interaction with the surrounding groundwater of different composition. This indicates an aquifer system which is ecologically very sensitive to anthropogenic contaminants released either to the ground- or the river water systems. The U concentrations are highly variable and range from 0.1-10 mBq/L, as are the 234U/238U activity ratios with values close to unity (mainly groundwaters) to ratios of up to 2 in river waters from metagranitic and other acid metamorphic rocks, speaking for more selective leaching of more loosely bound 234U in mineral surfaces of the granitoide source rocks, whereas U in the groundwater is probably better controlled by dissolution/precipitation equilibria. There are positive correlations of U with HCO3 and bulk Sr, which leads to the conclusion that the carbonate sediments in the south of the test area are the main sources for U and (non-radiogenic) Sr. The U daughter radionuclides indicate a tremendous state of disequilibrium in the U- (and Th) decay series. 226Ra is highly variable in all the aquifers due to the different adsorption capacities of the surrounding rocks, 228Th is almost below the analytical detection limit, confirming the extremely low solubility products of Th-containing phases, whereas the Ra isotopes (228Ra and 224Raex) produced by decay of 232Th can still be found as aqueous species due to recoil and dissolution from surface bound Th. Although there are immense excesses of 222Rn compared to the mother nuclide 226Ra (101 vs. 10-3 Bq/L) in the groundwaters (due to prevailed Rn diffusion in a semi-closed system), the excesses of pure 222Rn supported 210Pb/210Po are much less pronounced, indicating relatively short residence times of these nuclides in the groundwaters.

Table 1: Compilation of the nuclides investigated, isotope yield tracers applied and detection limits obtained

radionuclide decay half live chemical yield counting method detection limit detection limit

spike isotope [Bq/liter] [g/liter]

238U, 234U 4.5·109, 2.5·105 y 232U *-spectrometry 5·10-5, 5·10-5 4×10-9, 2×10-13

228Th 1.9·100 y (224Ra) *-spectrometry 2·10-4 7×10-18

228Ra 5.8·100 y 133Ba proportional counting 2·10-3 2×10-16

226Ra, 224Ra 1.6·103 y, 3.7·100 d 133Ba *-spectrometry 5·10-5, 2·10-4 1×10-15, 3×10-19

222Rn 3.8·100 d (no separation) *-spectrometry, LSC 5·10-2 9×10-18

210Pb 2.2·101 y 209Po *-spectrometry 2·10-4 7×10-17

210Po 1.4·102 d 208Po *-spectrometry 1·10-4 6×10-19