SEASONAL URANIUM DISTRIBUTIONS IN THE COASTAL WATERS OFF THE AMAZON AND MISSISSIPPI RIVERS

The chemical reactivity of uranium was investigated across estuarine g radients from two of the world's largest river systems: the Amazon and Mississippi. Concentrations of dissolved (<0.45 mu n) uranium (U) wer e measured in surface waters of the Amazon shelf during rising (March 1990), flood (June 1990) and low (November 1991) discharge regimes. Th e dissolved U content was also examined in surface waters collected ac ross estuarine gradients of the Mississippi outflow region during Apri l 1992, August 1993, and November (1993). All water samples were analy zed for U by isotope dilution inductively coupled plasma mass spectrom etry (ICP-MS). In Amazon shelf surface waters uranium increased noncon servatively from about 0.01 mu g l(-1) at the river's mouth to over 3 mu g l(-1) at the distal site, irrespective of river discharge stage. Observed large-scale U removal at salinities generally less than 15 im plies a) that riverine dissolved U was extensively adsorbed by freshly -precipitated hydrous metal oxides (e.g., FeOOH, MnO2) as a result of flocculation and aggregation, and b) that energetic resuspension and r eworking of shelf sediments and fluid muds on the Amazon shelf release d a chemically reactive particle/colloid to the water column which can further scavenge dissolved U across much of the estuarine gradient. I n contrast, the estuarine chemistry of U is inconclusive within surfac e waters of the Mississippi shelf-break region. U behavior is most lik ely controlled less by traditional sorption and/or desorption reaction s involving metal oxides or colloids than by the river's variable disc harge regime (e.g., water parcel residence time during estuarine mixin g, nature of particulates, sediment storage and resuspension in the co nfined lower river), and plume dispersal. Mixing of the thin freshwate r lens into ambient seawater is largely defined by wind-driven rather than physical processes. As a consequence, in the Mississippi outflow region uranium predominantly displays conservative behavior; removal i s evident only during anomalous river discharge regimes. 'Products-app roach' mixing experiments conducted during the Flood of 1993 suggest t he importance of small particles and/or colloids in defining a deplete d U versus salinity distribution.