The behaviour of the rare earth elements during mixing of river and sea waters

Rare earth element concentrations have been measured in organic-rich Luce river water and coastal sea water. Concentrations (e.g. approx. 350 to 1850 pmol/kg Nd in the Water of Luce and approx. 45 to 350 pmol/kg Nd in Luce Bay) are related to the presence of particles, with 30 to 60% of the REE associated with > 0.4 to 0.7 μm particles, and to riverine Fe concentrations. REE fractionation occurs in the river water: the sub-micrometre river water is heavy REE enriched whereas the coarser fraction has a more shale-like REE pattern. Laboratory experiments show that the REE in organic-rich river waters are chiefly associated with Fe-organic matter colloids which flocculate during estuarine mixing. Preferential removal of heavy REE (approx. 95%) relative to light REE (approx. 60%) occurs, but no Ce anomaly is developed. In contrast, no REE removal occurs during estuarine mixing with organic-poor river water. (author)