Temporal variations in the fractionation of the rare earth elements in a boreal river; the role of colloidal particles.

Rare earth element (REE) data from weekly sampling of the filtered (< 0.45 mu m) and suspended particulate phase during 18 months in the Kalix River, Northern Sweden, are presented together with data on colloidal particles an d the solution fraction(< 3 kDa). The filtered REE concentration show large seasonal and temporal variations in the river. Lanthanum varied between ap proximately 300 and 2100 pM. High REE concentration in the filter-passing f raction is related to increased water discharge and there is a strong corre lation between the REE concentration, organic carbon, Al and Fe. Physical e rosion of detrital particles plays a minor role for the yearly transport of particulate REE in this boreal river system. The suspended particulate fra ction, which is dominated by non-detrital fractions, accounted for only 35% of the yearly total transport of La in the river. Approximately 10% of the REE were transported in detrital particles during winter. At spring-flood in May, about 30% of the LREE and up to 60% of the HREE where hosted in det rital particles. Ultrafiltration of river water during spring-flood shows t hat colloidal particles dominate the transport of filter-passing REE. Less than 5% of the filtered REE are found in the fraction smaller than 3 kDa. T he colloidal fraction shows a flat to slightly LREE enriched pattern wherea s the solution fraction (< 3 kDa) show an HREE enriched pattern, compared w ith till in the catchment. Suspended particles show a LREE enriched pattern . Data indicate that the REE are associated with two phases in the colloida l (and particulate) fraction, an organic-rich phase (with associated Al-Fe) and an Fe-rich (Fe-oxyhydroxide) inorganic phase. The Ce-anomaly in the su spended particulate fraction in the river shows systematic variations, and can be used to interpret fractionation processes of the REE during weatheri ng and transport. There was no anomaly at maximum spring-flood but during t he ice-covered period the anomaly became more and more negative. The tempor al and seasonal variations of the Ce-anomaly in the suspended particulate p hase reflect transport of REE-C-Al-Fe-enriched colloids from the upper sect ion of the till (and/or from mires) to the river at storm events. (C) 2000 Elsevier Science B.V. All rights reserved.