Semidiscrete pesticide transport modeling and application

A time-continuous and space-discrete method is proposed for three-phase (dissolved, adsorbed, and vapor phases) pesticide transport modeling in the vadose zone and a generalized semidiscrete solution is derived under conditions of heterogeneous media, unsteady flow fields, and space-time-dependent physical and biochemical processes concerning pesticide environmental fate. The developed model also takes into account pesticide runoff and erosion and pesticide transport in the plant canopy zone. The model is able to deal with various pesticide application methods commonly used in practice, including over-canopy, under-canopy, or any combined foliar and soil surface spray, as well as soil-incorporated applications. The hybrid semidiscrete solution method, formulated herein, incorporates analytical and numerical methodologies into a uniform, flexible modeling framework, which makes the model suitable for either screening-level investigations as a lumped model or detailed studies as a distributed model. Furthermore, the semidiscrete pesticide transport model is applied in the Orestimba Creek Basin, California, for evaluating the vulnerability of the hydrosystem to diazinon contamination. In accordance with the observations, the simulation demonstrates that diazinon exposure levels in the creek frequently exceed criteria for aquatic life, and many peak pulses even exceed the water-quality standard for human health. In the subsurface environment, however, high concentrations of diazinon are generally limited within the shallow soil. It is also found that the magnitude and timing of pesticide application and rainfall/irrigation dominate exposure levels of diazinon in both subsurface and surface environments.