Characteristics And Controlling Factors Of Heavy Metals Transport By Surface Runoff From A Contaminated Red Soil

The heavy metal contamination is becoming more and more serious around red soil region in the south of China. Heavy metals may enter into surface water and food chains along with the process of rainfall, which brings great dangers to human health directly or indirectly. In this paper, a simulated rainfall experiment was conducted based on different rainfall intensities (55,90,120mm/h), slope gradients (5°,10°,15°), rice straw coverages (0%,40%,100%) and lime addition amounts (0%,0.25%,0.50%). The transportation characteristics and mechanisms of heavy metals in contaminated soil (non-artificial added) transporting with surface runoff were studied to provide a guidance and theoretical basis for preventing non-point source pollution of heavy metals, and realizing safe utilization of heavy metals contaminated soil. The main results are as follows:(1) Surface runoff decreased significantly with the decrease of rainfall intensity, the raise of rice straw coverage and the increase of lime addition amount, the decrease rate ranged from58.7%to77.6%,30.8%to50.1%and14.9%to18.3%, respectively. However, slope gradient had a smaller effect on the runoff. The sediment loss amount in surface runoff increased significantly with the increase of rainfall intensity and slope gradient, the increase rate ranged from812.5%to1623.4%and44.7%to73.0%. On the contrary, it decreased significantly under straw mulching and lime addition conditions, the decrease rate ranged from92.8%to97.4%and44.5%to59.2%respectively.(2) The initial concentration of dissolved Cd、Cu and Ni was high(within the first10min after runoff generation) under the conditions of90,120mm/h and steep slope (15°), and then it decreased rapidly with the runoff time. The concentration of dissolved Zn in runoff fluctuated with the extension of runoff time. Under55mm/h condition, the initial concentration was relatively low, however it began to rise with the extension of runoff time. Under gentle slope (5°) and middle slope (10°) condition, the concentration of dissolved Cu in runoff decreased rapidly with runoff time, while dissolved Cd、Ni and Zn fluctuated. Under high rice straw coverage (100%) condition, the concentration of four tested heavy metals declined fluctuatedly, but it changed relatively stablely under bare land (0%) and low coverage (40%) conditons. In different lime addition treatments, dissolved concentration of four tested heavy metals declined fluctuatedly with the extension of runoff time.(3)The accumulative loss amount of dissolved Cd、Cu、Ni and Zn increased with the increase of rainfall intensity and rice straw coverage, the increase rate ranged from52.6%to277.8%and40.0%to153.7%. With the increase of lime addition amount, the accumulative loss amount of dissolved Cd decreased significantly by69.4%～86.1%, However the accumulative loss of dissolved Cu increased significantly by14.3%57.1%. The accumulative loss amount of dissolved Ni and Zn decreased at first and then increased. No significant effect on dissolved cumulative loss amount of four tested heavy metals was found under the conditions of different slope gradient.(4)The initial concentration of total Cd、Gu、Ni and Zn was high (within the first10min after runoff generation) under the conditions of120mm/h and steep slope (15°), then it decreased rapidly along with the extension of runoff time. And20-30min after runoff generation, the concentration rose under steep slope (15°)condition, but it failed under120mm/h condition. Under90mm/h and gentle slope (5°) and middle slope (10°) conditions, the initial concentration was relatively low, but it gradually reached the peak during the time between10and20min after runoff generation, then it tended to be stable, but it decreased rapidly in later period of runoff (40min after runoff generation). Under55mm/h and straw mulching (40%,100%) conditions, the concentration of total Cd、Cu、Ni and Zn changed smoothly in the whole process of rainfall, while it declined rapidly within the20min after runoff generation and then tended to decrease steadily in different lime addition treatments.(5) The accumulative loss amount of total Cd、Cu、 Ni and Zn increased with the increase of rainfall intensity and slope gradient significantly by454.8%～1225.0%and41.2%～68.7%respectively, and it increased with the decrease of rice straw coverage significantly by40.0%～1735.9%. But no significant effect was found in lime addition treatments. The accumulative loss amount of four tested heavy metals in runoff is:Zn> Ni> Cu> Cd, and they were more likely to lose under bare land,120mm/h and steep slope conditions. Moreover, in those treatments, the loss was mainly in the early stage of rainfall, within the first20min after runoff generation.(6)Cd、Cu、Ni and Zn in runoff transported mainly in sediment particulate form, the ratios of particulate to total heavy metals>89%under different rainfall intensities、slope gradients and lime addition amounts conditions. Cd> Cu and Zn transported mainly in sediment particulate form (the ratios of particulate to total heavy metals>50%) in different rice straw coverage treatments. Ni transported mainly in sediment particulate form under bare land (0%) and low coverage (40%) conditions, while it transported in dissolved form under high rice straw coverage (100%) conditions.(7)The sediment enrich coefficient (ER) of four tested heavy metals increased significantly with the increase of rainfall intensity, the raise of slope gradient, and the increase of rice straw coverage. ER in lime addition treatments was higher than that of other treatments. There was a high degree of similarity about the change characteristic between ER and total concentration of four tested heavy metals.