Highly Contrasted Rare Earth Element Patterns in Shallow Groundwaters from Very Small Catchments

Abstract

Concentrations of Rare Earth Elements (REE) in large rivers (Amazon, Mississipi, Indus...) are often used as key constraints to determine chemical and physical erosion rates and fluxes on the global scale (Goldstein and Jacobsen, 1987; Sholkovitz, 1995; Elderfield et al., 1990; Gaillardet et al., 1997). Few works, however, have been dedicated to study fundamental processes which control REE hydrochemistry upstream, at the water-rock interface. Using data from groundwaters, Smedley (1991) suggested that REE in deep waters closely reflect the lithology of their host rocks. More recently, Johannesson et al. (1997) proposed that complexation processes play dominant role on the distribution pattern of REE in waters. Both authors acknowledged the usefulness of REE to investigate water-rock interactions. The present study concerns the spatial and temporal variability of REE concentrations in shallow groundwaters associated with three very small catchments located in western France (Brittany and Normandy). All the catchments benifit from the same temperate oceanic climatic conditions, and have low permeability soils. Basemant rocks have granitic or shaly compositions. The data are used to further elucidate the nature of fractionation processes that control the REE chemistry of shallow, continental waters.