PARTICLE-TRANSPORT OF U-234-U-238 IN THE KALIX RIVER AND IN THE BALTIC SEA

The role of particles for U isotope transport was investigated in the Kalix River watershed, a particle-poor, Fe/Mn-rich river in northern S weden, and in the Baltic Sea estuary. Particles >0.45 mu m are strongl y enriched in U and contain 20-50% of the total riverine uranium budge t and <1% of the total U in brackish waters (3-7 PSU). The particles h ave high delta(234)U which is close to that of dissolved U in the asso ciated water, indicating that U on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved U. Part icles at the river mouth are dominated by nondetrital Fe-Mn oxyhydroxi des. Uranium and Fe are strongly correlated, clearly demonstrating tha t secondary Fe-oxyhydroxide is the major carrier of U in river water. There is no evidence for significant association of U with Mn-oxyhydro xide. Apparent U distribution coefficients (<(K)over cap (Fe)(d)>) wer e calculated for U between the authigenic Fe on particles and the solu tion. These values appear to be relatively constant throughout the yea r. This suggests an equilibrium between Fe in solution and authigenic Fe-oxyhydroxides on detrital particles. High values of <(K)over cap (F e)(d)> calculated for one summer as well as high U concentrations in b rackish waters can be explained by U scavenging by biogenic phases wit h low authigenic Fe content. Copyright (C) 1998 Elsevier Science Ltd.