不同类型耕地紫色土中3,5,6-三氯-2-吡啶醇迁移试验与模拟

Abstract

3,5,6-trichloro-2-pyridinol (TCP), the main degradation product of pesticide chlorpyrifos and herbicide triclopyr, exhibits strong anti-degradation ability, high water solubility and high migration capability, which would easily lead to soil and water pollution. This situation would be worse in the purple soil distribution areas, most in the upstream regions of Yangtze River, due to soils with low organic matter content, large pores with high water conductivity and erosion. In this study, three typical farmland soil samples were collected based on major land use of purple soil from Chinese Academy of Agricultural Agro Ecological Experimental Station (Yanting), including a sloping field sample located at higher relative elevation, a paddy field sample at lower relative elevation and a vegetable field sample close to the residential areas, corresponding to the crop rotation of wheat-corn, rice-rape and radish-cabbage, respectively. Physical properties of these soil samples were measured, including particle-size fraction, organic matter content, bulk density, pH value and content of clay minerals (montmorillonite and kaolinite). Then, the batch equilibrium method was used to determine the liquid - solid partition coefficient of the TCP to reveal the absorbing characteristics of these purple soil samples to TCP; meanwhile, miscible displacement experiment was carried out under steady-state flow condition (12 mL/h) with Br- as a conservative tracer (50 mg/L), from which dispersion coefficient for TCP transport could be estimated by inverse simulation. Finally, the appropriate models were selected to simulate the physical and chemical processes of TCP in purple soil columns. The results were as follows: 1) The absorbing characteristics of purple soil to TCP were well described by the linear form of Freundlich Model, and the adsorption coefficients of sloping field, paddy field and vegetable field were 1.94, 1.22 and 1.02 L/kg, respectively. The content of clay and its components (montmorillonite and kaolinite) were the main factors affecting the adsorption coefficients; 2) The equilibrium outflow concentration of TCP in the outflow was 77%, 84% and 92% (relative concentration to the TCP outflow) respectively and the equilibrium time was at 2.88 PV, 4 PV and 6.5 PV, suggesting high pollution risk to deep soil layer, surface water and groundwater. Especially in the slope field, TCP displayed more easily and faster to enter the groundwater through infiltration and rivers through the surface runoff and lateral flow at the slope. 3) The two sites non-equilibrium convection-diffusion model could well simulate the transportation of TCP through purple soil columns, with a determination coefficient larger than 0.90 and the mean square errors (MSE) ranging from 0.009 to 0.021. These hydrodynamic and absorbing parameters from the experiment or reverse simulation revealed the reasons why TCP transportation was different among the three farmland purple soils: in the transient-absorption-dominated uniform soil environment, the main factor affecting dispersion coefficient was the flow velocity, which was mainly controlled by soil porosity or bulk density under the condition of stead flow without pressure; and dispersion coefficient and first-order kinetic rate constant showed an increase trend from vegetable field to paddy field, and then sloping field, while fractal coefficient displayed a decrease trend. These results are important to disclose TCP transfer mechanism in purple soil, and can also provide valuble information for preventing pollution caused by the pesticide and herbicide, especially their degradation productions, to the soil and water environment in the upstream regions of Yangtze River.