Lithium isotopes in the Loire River Basin (France): Hydrogeochemical characterizations at two complementary scales

Abstract

We illustrate two different, but complementary, applications of lithium (Li) isotope tracers for river-basin characterization at two different scales within the Loire River basin (LRB) in France. The first example deals with the behaviour of Li and the fractionation of its isotopes during river weathering at the basin scale of the LRB (117,800 km2). The wide δ7Li range (+5.0 to +13.3‰) in Loire basin streams, spatialized between the headwaters and the lowlands, is consistent with distinct weathering conditions and the distribution of Li from bedrock in the basin between the rivers and secondary mineral phases during water/rock interaction. Additionally, suspended sediments in the LRB streams are significantly 6Li enriched (δ7Li from −8.7 to −7.6‰) compared to average river waters that range from +5.0 to +13.3‰. The second example focuses on the smaller scale Egoutier watershed (13 km2), part of the LRB, which shows that Li isotopes can be useful for distinguishing between natural input and anthropogenic pollution, like effluents from a water treatment plant connected to a hospital. Overall, we confirm that Li isotopes cannot be used as lithological tracers for river waters. However, we did find that Li isotopes can be good tracers of weathering conditions and of anthropogenic sources in an urbanized watershed.