Water column samples from three vastly contrasting coastal regimes (Framvaren Fjord, Norway, the Amazon and Mississippi River outflow regions) were analyzed to study the estuarine transport and scavenging processes of 210Pb, 210Po, 234U and 238U. The Amazon and Mississippi River outflow regions are two of the world's largest river-ocean mixing systems that contrast each other sharply in many physiographical and geochemical features.

The Amazon is a tropical, high energy, shelf environment where estuarine physico-chemical processes are strongly influenced by sediment-water interactions. The Mississippi system on the other hand discharges much of its load close to the shelf-break where estuarine geochemical processes are largely separated from benthic processes. The distribution of uranium in these two systems reflects the vastly contrasting environments. In the Amazon, U is consistently non-conservative - showing removal at all river discharge stages. Removal of U from this water column onto particulates involves scavenging by colloidal-sized metal oxides, flocculation and subsequent aggregation up the particle size spectrum. In the Mississippi River outflow region, U is conservative at all normal river discharge stages. However, during flood/drought conditions uranium does exhibit non-conservative estuarine behavior and U removal is thought to be a function of the reactivity of the carrier phase.

Framvaren Fjord represents a unique environment in which to investigate estuarine biogeochemistry. At about a salinity of 21 and a depth of 20 meters (well within the photic zone), a very sharp O2/H2S boundary controls the vertical distribution of a suite of trace metals and radionuclides. The uptake and release of such elements by dense communities of anoxygenic phototrophic bacteria (e.g., Chlorobium and Chromatium ssp.) create very sharp concentration peaks at the redoxcline. Such bio-concentration at the O2/H2S boundary has been observed in the vertical profiles of 210Po, 210Pb, Fe, Mn, U, Ba and Sr. Mechanisms or processes to create such distributions must be biogenic and are most likely microbially mediated. Results from this study indicate that even elements once thought to be quite non-reactive in natural waters, such as uranium, can in fact be susceptible to both biotic and abiotic enrichment/depletion processes.