| Pesticide leaching and runoff lead to contamination of both surface and subsurface environments. The intrinsic connection between surface and ground water, in particular, increases the vulnerability of the entire hydrosystem to pesticide contamination. This study is aimed at modeling the physical and biochemical processes related to three-phase pesticide transport and transformation and quantifying the spatial and temporal variability of pesticide residues in the hydrosystem so as to evaluate their potential adverse impacts and eventually identify water-quality management strategies.; Integrity of a hydrosystem highlights the importance of integrated modeling of pesticide, water, and sediment transport. As an initial effort, a physically-based analytical integrated pesticide transport model (A-IPTM) is proposed for a simplified vadose zone-aquifer-river system and compared with a numerical counterpart that couples widely-used PRZM2 and MT3D. The A-IPTM is suitable for quickly obtaining an overall insight into pesticide pathways and conducting a screening-level environmental evaluation. Furthermore, an integrated pesticide transport model (IPTM) is developed for more sophisticated characterization of both spatial and temporal distributions of pesticide residues and their pathways in a coupled surface and subsurface system. The IPTM essentially comprises a set of internally-linked models simulating water flow and three-phase pesticide transport and transformation in the plant canopy zone, overland, vadose zone, aquifer, and river water column and bed. As one of the distinct features of this study, a hybrid semidiscrete solution method is proposed, which incorporates analytical and numerical methodologies into a uniform, flexible modeling framework. Implementation of a set of linear and nonlinear numerical schemes and flexibility in the model structure particularly enhance the capability of the IPTM. Efforts are also made to test the IPTM by means of theoretical analysis, comparison with standard numerical and analytical counterparts, and application to real-world problems. In a case study, the IPTM is applied to the Orestimba Creek basin for evaluating the environmental fate of diazinon. It is found that timing of rainfall and pesticide application as well as irrigation scheduling and methods often dominate pesticide exposure levels in surface and subsurface environments. The results also emphasize the importance of the integrated pesticide-water-crop management. |
