Isotopic ratios (Pb, Sr, Nd) trace and rare earth elements (REE) are used to identify anthropogenic pollution in river- and groundwaters, to determine the transport mechanisms of heavy metals in aquifers and river waters and to decipher exchanges between the different water masses.
We measured rare earth element concentrations in ground- and river waters from the Upper Rhine Valley which can be divided in two groups: on one hand the carbonated water samples from the Alsatian plain and on the other hand the light acidic waters from the dominantly granitic alsatian mountains (the Vosges). Both of them, particularly the carbonated waters, which have a pH ranging between 7 and 8.5, are strongly depleted in REE. The REE concentrations vary between 0.1 ppt and 100 ppt and are far below the detection-limit of the ICP-MS (Inductively Coupled Plasma - Mass Spectrometry) presently used in our study (10 ppt). We therefore developed a specific enrichment method involving liquid-liquid extraction with HDEHP as organic solvent. This method allowed the enrichment of the REE by a factor of 100 requiring a relativly reduced initial sample volume (1 liter). Fractionation between heavy and light REE are controlled by the extraction of an adequate standard solution.
Based on Sr isotopic data, three different sources can be identified in the alsatian hydrosystem: a carbonated one with a 87Sr/86Sr=0.7085, a silicated one with a 87Sr/86Sr=0.7102, and the one corresponding to the alsatian brines with a 87Sr/86Sr =0.7094, the isotopic compositions of the rivers resulting of a mixing between the three poles.
The filtration experiments indicate that most of the REE in the suspended load are associated with the colloidal phases ranging in size between 0.22 µm and 0.45 µm. Nevertheless, following the international convention we considered the fraction <0.45 µm as the dissolved load. The waters exhibit two different patterns: the ground and the Rhine water are characterized by a positive europium anomaly probably as a sign of exchange with hydrothermal waters and a strong heavy rare earth element (HREE) enrichment towards the light ones. This enrichment might be due to the the higher stability of the carbonate and hydroxide HREE complexes relative to the LREE complexes. In contrast to the carbonated waters, the light acidic ones
(pH comprised between 5 and 6.5) exhibit more typical river water REE distributions with an enrichment in the intermediate REE (IREE) and a HREE depletion as a result of the bedrock leaching. The negative cerium anomaly in all the waters points to its oxidative scavenging, this being more accentuated in the acidic waters.
The suspended load has been submitted to several leaching experiments to determine the repartition of the REE in the different phases. The LREE, with exception of Cerium, are easily and almost totally removed from the solid (80-90% of the whole LREE) by leaching with HCl 1N while the HREE remain to some extend in the detrital phase (30% of the whole HREE). The correlation of the REE with Mn and Fe and their easy dissolving during leaching hint to REE scavenging by oxides. But we can not rule out the influence of clay minerals as indicated by correlation of the REE with Al.