To understand the U transport in rivers and estuaries, knowledge of geochemical interactions between particulate, colloidal and dissolved fractions is important. Andersson et al. (1995) studied the U-Th transport in fresh- and brackish water from the Baltic Sea area, and suggested that particles and colloids are important for the Th transport. The objectives of the present study include the role of particles (>0.45µm) for U transport, and the relationship in 234U/238U between particles and "dissolved" (<0.45µm, includes U in solution and colloids) fractions. Using TIMS, we have examined the U concentrations (C238U) and 234U/238U (reported as d234U) in particulate and "dissolved" fractions in water along the Kalix River, northern Sweden and in brackish water from the Baltic Sea. The role of colloids in the system is reported by Porcelli et al. (1996).
Variations in concentration of suspended particulate material (SPM, >0.45µm) influence the U transport in the Kalix River substantially. During high discharge in spring, the proportion of U in the suspended load decreases downstream from 51% in the headwater to 18% at the river mouth, where the annual average particulate U transport is about 12% of the total load. At the mouth of the Kalix River, there is a strong seasonal variation in the particulate U concentration, from 8 to 35ppm (ppm of the total particulate load) with a maximum before the spring discharge. The particulate fraction contains a large portion of authigenic Fe-oxyhydroxides and 18 to 50% of the particles consists of Fe. The particulate compositions show strong correlation between Fe/Al and U/Al, indicating that "dissolved" U is being scavenged by Fe-oxyhydroxides. These results demonstrate that particles are important for the U transport in river water, and that the particulate U is associated with an authigenic Fe phase.
In the Kalix River the "dissolved" C238U increases downstream from 55ng/kg in the headwater to 184ng/kg at the mouth, whereas the "dissolved" d234U is higher in the headwater (1005) than at the river mouth (896). The observation that the d234U is higher in the mountainous headwater, compared to the river mouth implies that preferential leaching of 234U from the bedrock or the soil dominates over congruent weathering, which should yield U closer to equilibrium. This demonstrates that also within a mountainous area, where physical weathering prevails, can a substantial amount of mobile 234U be found in the water.
All the measured particles are substantially enriched in 234U, demonstrating that the U in the suspended particles cannot be derived solely from detrital silicate material, which should be in radioactive equilibrium (d234U=0). At the mouth of the Kalix River the particulate fraction has d234U lower (798) than in the dissolved load (896). Contrary to this, the headwater sample has higher d234U in the particulate fraction (1198), compared to the "dissolved" fraction (1005). The U in the particulate fraction can be treated as a two-component mixture between silicate detrital material and a second non-detrital phase, enriched in 234U derived from "dissolved" U. By assuming crustal U/Al ratios in the detrital particles, we calculate the portion of U derived from the non-detrital component to account for ~90% of the particulate U. Also in the headwater, the major portion of the 234U in the particulate fraction must be derived from a "dissolved" component, although our model is not in full accordance with the data. Sorption of dissolved or colloidal U on authigenic Fe-oxyhydroxides is a plausible mechanism for forming a non-detrital U phase enriched in 234U. However, both the POC and TOC (Particulate/Total Organic Carbon) in the Kalix River is high (TOC=2-13mg/L). Organic material, originally rich in U, or U sorption on organic material in the river water might be an additional explanation for the high d234U in the particulate load.
The SPM concentration in the Baltic Sea is about 10 to 100 times lower, compared to the river water. The proportion of U in the particulate fractions comprises 0.1 to 0.5% of the total load. In the Gulf of Bothnia, (salinitiy=3.3) the d234U in the particulate fraction is 180 and in the "dissolved" 247. We estimate, using the same assumptions as in river water, that roughly equal amounts of detrital and non-detrital U is present in the particulate fraction in the Gulf of Bothnia. With increasing salinity, the SPM concentration decreases and the concentration of seawater derived "dissolved" U, which has lower d234U(~150) than river water, increases. The difference in d234U, between particulate and dissolved fractions will therefore be smaller. In the central Baltic Sea (salinity=7.2), the particulate (178) and "dissolved" (173) d234U show no resolvable difference within error.
Particles enriched in both C238U and d234U, compared to detrital material, are delivered by the Kalix River to the Baltic Sea. Sedimentation of these particles yields U enriched sediments. Analyses of Fe-Mn concretions in the Gulf of Bothnia show that the non-detrital fractions in these sediments have about twice the C238U and higher d234U compared to their deep sea counterparts (Ingri, unpublished).
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