Upper soil horizons control the rare earth element patterns in shallow groundwater

- The dissolved soil-REE patterns were similar to those of shallow groundwaters.
- REE signature observed in shallow groundwaters is affected by soil-Fe oxyhydroxides.
- Fe/OC ratio of soil controls the development of the Ce anomaly in shallow groundwaters.
- Topography-related REE pattern is controlled by the upper soil horizons.

Several studies dedicated to the aquatic geochemistry of rare earth elements (REEs) have displayed a wide topography-related spatial variability in the REE signatures of shallow groundwater. The aim of this study was to understand the processes leading to this specific REE signature, notably with regard to the size of the Ce anomaly. Soils were sampled in order to encompass the expected topographic variability in the organic carbon (OC)/Fe(Mn) ratio. Leaching experiments were performed with the uppermost horizons of the soil. The REE patterns that developed in the soil leaching solution were similar to the REE patterns for the shallow groundwater collected in this catchment. The negative Ce anomaly evolves in a similar manner according to the topography. This spatial variation is strongly correlated with the soil OC/Fe ratio. For a low OC/Fe ratio, the negative Ce anomaly amplitude in the soil solution is large, whereas a high OC/Fe ratio generates a small or insignificant Ce anomaly. Reductive dissolution experiments using soil with low OC/Fe ratios demonstrated that the REE pattern for soil Fe oxyhydroxides exhibited a positive Ce anomaly and HREE enrichment, indicating a preferential association of these elements with Fe-oxyhydroxides. The rare earth element signature observed in the shallow groundwater is affected by Fe oxyhydroxides present in the upper soil horizons. In contrast, in soil with a high OC/Fe ratio, the REE pattern obtained under reducing conditions did not exhibit any Ce anomaly, suggesting that in the bottomland, the REE signature is affected by the OC content in the uppermost soil. This study highlights the impact of organic matter on the Fe pedofeatures, which control the development of a negative Ce anomaly in shallow groundwater.