Abstract:The movement of lake sediments under different hydrodynamic conditions was simulated, and vertical distribution characteristics of heavy metals in lake under different hydrodynamic conditions were explored with self-designed circulating flume and sediments sampled from Xingzi section of Poyang Lake. The flume device was composed of inlet tank (0.5 m 3 ), test area (2 m in length, 1 m in width, and 0.5 m in height), outlet tank (0.5 m 3 ), storage tank (1 m 3 ) and pump (maximum flow rate of 100 m 3 /h). Prior to testing, sediment was spread at the flume bottom, and lightly pressed and flattened to a thickness of 8 cm. After remaining it untouched for 0.5 h, water was added into the device. Water depth in the flume was controlled at 140 cm. Water was pumped from the storage tank into the inlet tank, and flowed through the test area, the outlet tank and eventually returned to the tank. During the test, the flow rate in the flume was gradually increased from 0 cm/s, and its flow rate was set through the control of the gate as 0 cm/s, 5 cm/s, 15 cm/s, 25 cm/s, 35 cm/s, 45 cm/s and 65 cm/s. To ensure that the water flowed through the flume at least once, the testing period was maintained 1 h under each flow rate, and then immediately proceeded to the next flow rate until it reached the maximum flow rate. During the test, the incipient motion characteristics of sediment, including critical motion state, slight motion state and plenty motion state, were initially simulated. The water depth in the flume was divided into four layers. The total heavy metal concentration and dissolved heavy metal concentration were measured at different flow rates and different water depths. In order to minimize deviation, heavy metal concentration was measured three times and the average value was taken under each group of water flow and water depth condition. When the sediment was in critical motion state at flow rate of less than 25 cm/s, a diluted suspension at the sediment surface was appeared with the increase of flow rate. The total heavy metal concentrations were similar at different water depths in this period. When the sediment was in slight motion state at flow rate of 25 cm/s to 50 cm/s, water became turbid, with part of the sediment washed up. Shear stress at sediment surface was obviously larger than that in critical motion state, and the total heavy metal concentrations were increased from water surface to bottom, although it was not obvious. When the sediment was in plenty motion state at flow rate of 60 cm/s to 70 cm/s, a large amount of sediment was washed up, and water became turbid in a short period. Large quantities of heavy metals in the sediment were released into water, and the total heavy metal concentration was increased obviously along the water depth direction. In addition, by analyzing the concentrations of dissolved Cu, Zn, Cd and Pb in each layer with curve fitting method, vertical distribution characteristics of heavy metals under different hydrodynamic conditions were revealed. The basic formula was established between heavy metal concentration and height above sediment surface. The results showed that when the hydrodynamic condition was strong, the concentrations of dissolved Cu, Zn, Pb were increased logarithmically from water bottom to water surface. Furthermore, to detect the applicability of the established formula, the testing results were applied to measurement of Poyang Lake, water samples collected from Poyang Lake Bridge section and Xingzi section at 0.3 m, 0.8 m, 1.3 m and 1.8 m above sediment were used to measure the dissolved heavy metal concentration. Flow rate at the two sections were 39 cm/s, 75 cm/s and 44 cm/s, 68 cm/s, respectively, when the samples were collected. By substituting the measured data into the established formula, the obtained result showed that the relative error between calculation results and measured results was within ±15%. The formula was applicable at flow rate of 65 cm/s to 75 cm/s in lakes with sediment grain size of 50 μm to 200 μm. The established formula was basically reasonable. The device was feasible for the experiment with strong operability. The research results could provide references for revealing heavy metal release characteristics under different water conditions.
