Key impact of soil (Fe/Organic C) ratio on REE shallow groundwater patterns. In : Goldschmidt conference 2011, 14-19 aoüt 2011, Prague, République Tchèque

A number of studies have been carried out over the last two decades on the chemistry of rare earth elements (REE) in stream waters and groundwaters. They occur usually as simple REE(III) species, with the exception of Ce. Indeed, among the REE, Ce is the only element that can be oxidized to the (+IV) state, modifying its behaviour. REE are potentially used as tracers of rock/water interaction processes, groundwater flow and mixing, soil genesis, and notably, as proxies for evaluating paleoceanographic and paleoclimatic change by using Ce anomaly development. Previous investigations of the REE geochemistry in groundwaters of the Kervidy/Coët-Dan catchment (Brittany, France) showed a systematic, topography related variation of REE signatures, notably a large negative Ce anomaly in the upper part which decreases from top to bottom of the transect. These studies also showed that REE are carried by organic colloids in these waters. A recent study showed that the strong spatial variation of negative Ce anomaly would be due to the input into the aquifer of REE-rich, Ce anomaly-free, organic colloids located in the uppermost, organic-rich soil horizons. Consequently, one major question arises: why organic colloids show a large negative Ce anomaly at the top and Ce anomaly-free at the bottom? In this context, soil/water interactions performed through soil column experiments, were carried out with several soil samples recovered from top to bottom of two toposequences to understand the origin of the variability of REE patterns in the Kervidy/Coët-Dan catchment. The major carriers phases of the REE pools were determined. Moreover, batch experiments were carried out to investigate the role of organic molecules in the development of Ce anomaly in solution. The results showed: (i) a strong Ce sequestration in oxidized phases such as secondary minerals formed at the frond of altered shale, (ii) a close relationship between soil Fe/Organic C ratio and Ce anomaly in soil solution. Moreover, this study showed (iii) the strong influence of organic molecules in the lack of Ce anomaly in waters by Fe oxyhydroxide dissolution. Furthermore, the REE signature in the Kervidy/Coët-Dan groundwaters appears to be provided in major part by the upper horizon of soil.