Improved Box Model for Simulating Pesticide Transport in the Vadose Zone with Dispersive Flux through the Boundary Layer

Abstract

A box-type transport model can be a useful tool for screening-level, long-term predictions of contamination and the relevant environmental assessment, especially when data are scarce and quick decisions are needed. In most box-type transport models, only advective flux is simulated to account for the interaction between boxes. In reality, however, significant mass transfer may also occur through the boundary between two boxes as a result of dispersion. For volatile contaminants (e.g., pesticides), the mass transfer through the boundary may involve both dissolved and vapor phases. In this study, an improved box-type analytical pesticide transport model with dispersive flux through the boundary layer (APTM-DF) was developed for simulating fate and transport of three-phase pesticides in the vadose zone, which was further divided into the surface zone, plant root zone, and deep vadose zone. The model also simulated pesticide leaching, degradation, volatilization, sorption, partition between dissolved and vapor phases, plant root uptake, runoff, and erosion. Analytical solutions were derived for both instantaneous and continuous pesticide applications. The performance of APTM-DF was evaluated in an application by comparing it with a regular APTM (no dispersive flux) and an integrated pesticide transport model (IPTM-CS) that provided detailed spatial and temporal distributions of pesticides. The modeling results indicated that, without accounting for dispersive flux through the boundary layer, pesticide exposure levels in deeper soil were significantly underestimated.