| In order to avoid the situation that there is no water flow and to improve the proportional relationship between water and sediment transport by flushing the reservoirs and reducing the sedimentation in the lower reaches of the Yellow river, the Water-Sediment Regulation Scheme (WSRS) has been carried out in the river. Meanwhile, this management has changed the transport rules of water and sediment, and bring significant effects on the terrigenous matter transport.The main results in the thesis were drawn as follows:Dissolved uranium concentration in the main channel of the Yellow River ranged from 0.84 ± 0.08 μg/L to 5.58 ± 0.38 μg/L, and 234U/238U activity ratio was between 1.32 ± 0.18 and 2.31 ± 0.21. Dissolved uranium concentration exhibited an increasing trend from the upper reaches to the lower reaches along the main channel of the Yellow River. Except the high value (2.31 ± 0.21) near the source, the 234U/238U activity ratios (1.3～1.7) kept in a relatively stable level in the Yellow River.Dissolved uranium concentrations at station Lijin ranged from 2.72±0.18 μg/l to 7.57±0.66 μg/l, averaged 5.49±0.28 μg/l, with a seasonal variation of lower concentrations in Summer and Fall compared with Winter and Spring. Monthly fluxes of water, sediment and dissolved uranium changed notably, and concentrated in the periods of the Water-Sediment Regulation Scheme and rainfall flood. The fluxes of dissolved uranium at station Lijin were 1.04×105 kg/y,1.31×105 kg/y and 6.41 × 104 kg/y in 2010,2013 and 2014, respectively, which accounted for 0.64%-1.31% of the dissolved uranium flux of the whole world rivers.A nonconservative behavior of dissolved U with an addition of U to the Yellow River estuarine waters has been observed at salinity<15, which might be the result of desorption from suspended sediment. The addition of dissolved U to the estuarine waters was estimated about 3.13 × 104 kg/y, which accounted for almost 24% of dissolved U flux at station Lijin. Dissolved U concentration exhibited a reasonably conservative distribution in the Yangtze River, which would imply a simple two end-member mixing.Reservoirs and the WSRS have great effects on the transports of uranium isotopes in the Yellow River. During the first stage of the WSRS, flood water released from the Xiaolangdi Reservoir soured the lower river bed, and dissolved uranium in the pore water of the bottom sediment was released to river water. During the second stage, a significant increase of dissolved uranium concentration in the lower reaches of the Yellow River was related to a large amount of fine grain size suspended sediment expelled from the Xiaolangdi Reservoir. Because of the anoxic environment in the bottom of the Xiaolangdi Reservoir, the tetravalent state of uranium could be oxidated and then released into river water by exposing to oxic environment during the second stage. Based on the budget of dissolved uranium in the section of the Yellow River between stations Xiaolangdi and Lijin, excluding the input of dissolved uranium from the upstream at station Xiaolangdi, released from the suspended sediment and diffusion of porewater from the bottom sediment were the main sources. Dissolved uranium output from the Yellow River via water consumption (water diversion and supply to groundwater) was the main loss in river water of this section between stations Xiaolangdi and Lijin.During the WSRS, the fluxes of dissolved uranium from the Yellow River to sea were mainly controlled by water discharge. The flux of dissolved uranium from the Yellow River to the sea during the WSRS were 2.65×107 g,2.40×107 and 0.97× 107 g in 2010,2012 and 2014, respectively, which accounted for over 25%(2010) and 15%(2014) of annual uranium flux. The WSRS was an important period of dissolved uranium transported from the Yellow River to the sea. |
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