Transport of 3,5,6-trichloro-2-pyrdionl (a main pesticide degradation product) in purple soil: Experimental and modeling

Purple soil, a loamy entisol dominating the hilly central Sichuan basin in the upper reaches of Yangtze River, is low in organic matter content and abundant in macropores that form the rapid transport network for water and solutes including pesticides. However, effects of soil's macroporosity on transport of pesticides has not been quantitatively explored and simulated at the column scale. In this study, we explore the transport of 3,5,6-trichloro-2-pyrdionl (TCP), a main pesticide degradation product, in purple soil through experiments and simulations on typical farmland categories, including rice-rape and vegetable farmland. Soil water retention characteristics (SWRCs) and physical properties are measured to learn the hydraulic parameters and porous structures of the soil. Batch experiments are conducted to obtain the adsorption coefficient of TCP on soil, and breakthrough curves of Br− (a conservative tracer) in undisturbed soil columns are applied to evaluate the hydrodynamics of TCP transport. The two-region model is used to simulate the transport processes, and the parameters are obtained by inversion simulation. It is found that TCP transport in purple soil exhibited complex patterns and is strongly affected by both chemical and physical non-equilibrium. A higher pollution risk on water body of rice-rape field is due to the stronger preferential flow and better pore connectivity despite the larger macroporosity observed in vegetable field. These results can provide useful data and modeling approach for evaluating the risk of pesticide leaching toward groundwater.