Study Of The Distribution And Genesis Of Uranium In The Main Channel And Estuary Of Yellow River And The Southern Coast Of Laizhou Bay

U-series nuclides have been used extensively as tracers and chronometers of many oceanographic processes. Thus, the distribution of U in the oceans and its source and sink terms are of particular interest. The uranium concentration of river runoff, the dominant transport pathways which supply significant amounts of uranium to the ocean, is important to the evaluation of uranium budget and mass balance in the global ocean. The salinities and concentrations of suspended sediments in the estuarine mixing zone vary with time and space, which alter the element partitioning between solid phase and solution via adsorption and desorption and change the riverine input of uranium significantly. The intertidal salt marshes are also considered to be strong sinks of uranium at all salinities which will exert influences on the global uranium budget in the ocean.Yellow River is famous for its high suspended sediments. It has been found that the dissolved uranium in the Yellow River is not only higher than the average concentration of the world rivers but also exceed that of the average seawater. Therefore, it is of particularly interest in studying the concentrations, sources and behavior of uranium in the Yellow River and its estuary. The broad intertidal salt marshs along the southern coast of Laizhou Bay, which is adjacent to the Yellow River estuary, can serve as excellent sites to study the removal and evolution of uranium in the salt marsh sediments. In this study, the waters and sediments of the Yellow River and its estuary as well as the underground brine along the southern coast of Laizhou Bay were sampled. The uranium isotopes and other relevant elements of the samples were measured. Based on the data obtained from the field investigation and the laboratory simulation, the main results can be summarized below. 1. The concentrations of dissolved uranium in the main channel of Yellow River range between 2.04 -7.83μg/l, averaging of 4.38μg/l, which is not only much higher than the global average river water uranium concentration, but also higher than the average concentration for seawater of salinity 35. The uranium concentrations generally increase downstream. The 234U/238U activity ratios of the Yellow River water vary from 1.36- 1.67 with a majority of values of 1.4. The relatively high 234U/238U activity ratios indicate the contribution of silicate weathering as well as the groundwater input.2. The Chinese Loess Plateau covers most of the middle reaches of the Yellow River drainage basin. The arid-semiarid climate make the uranium accumulated in grain surface and weathering crusts of loess deposits and the severe physical erosion results in these loess grain and weathering crusts entering into the Yellow River and the uranium can be released or dissolved easily from these materials which will enhance the concentrations of uranium in the Yellow River. On the other hand, the uranium can be leached to the surface runoff and ground water and then enter into the Yellow River ultimately as dissolved uranium. Based on the data of equilibration experiments and leaching experiments as well as the water and sediments discharge of Yellow River, the sources of dissolved uranium in the middle reaches of Yellow River is estimated by a box model. Which give a result that the suspended sediments contribute two thirds while the ground water and runoff contribute one third of dissolved uranium in the middle reaches of Yellow River.3. The dissolved U behaves non-conservatively at salinities <20 in the Yellow River estuary. There is addition of U dissolved from suspended sediments and/or diffused from interstitial water of bottom sediment as about 3.5×104 kg/yr to the estuarine waters. In addition to the dissolved uranium inputs by the river, the inventory of uranium input from the estuary of Yellow River to the ocean should be 9.8×104 kg/ yr, which account for about 1% of the global riverine uranium flux. 4. Both field investigation and laboratory experiments indicate that in the Yellow River plume mixing zone, the variation in dissolved U with salinity assumes a similar pattern as that of PO43-. The diffusion of interstitial water from bottom sediment influences more the enrichment of PO43-, whereas desorption from the suspended sediments seems more responsible for the elevated dissolved U concentrations.5. The dissolved uranium concentration of underground brine along the southern coast of Laizhou Bay are higher than the concentrated seawater of Laizhou Bay under the same concentration factor and the discrepancy increase with the concentration factor, indicating sources of dissolved uranium other than the concentrated seawater. The wide intertidal salt marshes serve the condition of the removal of uranium from the seawater. The uranium deposited with the flocculation of colloid and/or coprecipitated along with the iron/ manganese oxides at the surface sediments may be leached by the groundwater during the geological evolution after being buried, or be released to the underground brine owing to the reduction of iron/ manganese oxides along with the degradation of organic material. The relatively high concentrations of HCO3- and PO43- in the underground brine can stabilize the high level of dissolved uranium by forming uranyl complex.