The Importance of colloids for the behavior of uranium isotopes in the low-salinity zone of a stable estuary

Particle-mediated removal processes of U isotopes were investigated during spring flood discharge in the low-salinity zone (LSZ, up to 3 practical sal inity units [psu]) of a stable estuary. A shipboard ultrafiltration cross-d ow filtration (CFF) technique was used to separate particles (>0.2 mum) and colloids (between 3000 daltons (3 kD) and 0.2 mum) from ultrafiltered wate r (<3 kD) containing "dissolved" species. Sediment traps were used to colle ct sinking material. Concentration of Fe and organic C, which are indicator s of the major U carrier phases, were used to interpret the behavior of U-2 34-U-238 during estuarine mixing. Colloids dominated the river water transport of U, carrying <approximate to >90% of the U. On entering the estuary, colloids accounted for the dominant fraction of U to about a salinity of 1 psu, but only a minor fraction (<5% ) at 3 psu. A substantial fraction of the total U is removed at <1 psu by F e-organic rich colloids that aggregate and sink during initial estuarine mi xing in the Kalix River estuary. In contrast, at salinities >1 psu, there i s a general correlation between U and salinity in all filtered fractions. T he U-234/U-238 ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in U-234, compared with secular equilibrium (delta U-234 = 266-567). This clea rly shows that all size fractions are dominated by nondetrital U. Considera tion of U isotope systematics across the estuary reveals that substantial U exchange must occur involving larger particles at least to 1 psu and invol ving colloids at least to approximate to1.5 psu. Further exchange at higher salinities may also occur, as the proportion of U on colloids decreases wi th increasing salinity. This may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing i n the estuary. The results show that although U is a soluble element that shows generally conservative mixing in estuaries, removal occurs in the very low salinity z one, and this zone represents a significant sink of U. Variation in composi tion and concentration of colloidal particles between different estuaries m ight thus: be an important factor for determining the varying behavior of U between estuaries. Copyright (C) 2001 Elsevier Science Ltd.