Release of heavy metals in lake sediments under hydrodynamic flux

Abstract: To study the release flux of heavy metals in sediment of the lake under different hydrodynamic conditions, an interior experiment was designed and carried out. The experiment aimed to simulate the migration of sediment in the lake and to investigate the release regularity of heavy metals in sediment under different hydrodynamic conditions. The sediment in Xingzi Reach of Poyang Lake was selected as the experiment material and a circulating facility was designed. The circulating facility was composed of an inlet tank (0.5 m3), an experimental tank (2 m×1 m×0.5 m), an outlet tank (0.5 m3), a storage tank (0.5 m3) and a pump. First, sediment had been tiled at the bottom of the experimental tank with the thickness of 8 cm for half an hour. Then water of 30cm deep was filled in three tanks. The experiment was carried out by pumping water into the inlet tank from the storage tank, going through the experimental tank, the outlet tank and finally back to the storage tank. The average flow velocity was controlled at 0 to 5, 15, 25, 35, 45, and 65 cm/s as the highest speed by the gate. The test of each speed maintained 1 hour to ensure the water was circulated through the facility at least once. In this case, the test not only simulated different migration state under different hydrodynamic conditions, which were "stationary state", "slight migration state" and "intense migration state", but also detected the total concentration of heavy metals in water under different flow velocity conditions at the same time. In order to reduced the experimental error, the sample was taken 3 times to calculate an average concentration. Through analyzing varied heavy metal concentration under different hydrodynamic conditions, some conclusions were drawn as below: When flow velocity was less than 25cm/s, heavy metals were released only in interstitial water for not much sediment was suspended and heavy metal concentration was not increased greatly with slight interference; With increasing flow velocity, sediment shifted from "stationary state" to "slight migration state" and the concentration of Cu, Zn, Cd and Pb rose evidently for that much sludge moved up, smaller particle mud of heavy metals went up to overlying water, and large quantity of heavy metals released in interstitial water at the same time; When the flow velocity reached 60-70 cm/s, "intense migration state" was achieved since the movement of mud at the bottom was relatively severe and silt by piece. The concentration of Cu, Zn, Cd and Pb in water and its release flux in sediment speeded up. When flow velocity was 65 cm/s, the density of Cu, Zn, Cd and Pb was 127.16, 220.5, 0.28, 11.07 μg/L respectively. The release flux was calculated by analyzing the concentration changes of Cu, Zn, Cd and Pb in the water under different flow velocity conditions. The relationship between release flux of heavy metals in sediment of the lake (y) and speed (x) was established by linear fitting. The result shows that release flux of Cu, Zn, Cd and Pb in sediment of the lake has an exponential relationship with speed. In addition, to test the applicability of the established related equation, this paper applied the equation to field experiment of Poyang Lake. First, the concentration of heavy metals in Poyang Lake under different hydrodynamic conditions was detected and the velocity was recorded. And then the release flux of heavy metals in Poyang Lake was calculated by referring to the relationship which was established from the interior test. The concentration of heavy metals in Poyang Lake was calculated (that is the calculated concentration) according to the release flux. Finally, the calculated concentration was compared with measured concentration. The result shows that the values between calculated concentration and measured concentration are very close with the error range of 5%-20%, which indicated that the relationship between release flux of heavy metals and speed set in this paper was basically reasonable and could adapt to conditions similar with Poyang lake and the sediment particle size of 50 to 200 μm, of the experimental facility was feasible and operable. The results can provide scientific basis on control and governance of heavy metal pollution.