WRRCTR No.176 Chemical residuals transport in aggregated soils: Mathematical simulation by the linear system approach

Abstract

Groundwater contamination caused by residuals of agricultural chemicals and leachates from solid waste landfill sites is now one of the most critical environmental problems. Regulatory decisions regarding the use of agricultural chemicals and the control of the solid waste disposal require an understanding of the behavior and fate of residue chemicals in a subsurface environment. To meet this requirement, mathematical models that predict the pollution potential of groundwater contamination caused by various forms and amounts of waste inputs are commonly used as important management tools. Mathematical models have been traditionally formulated by following a physically based approach. The amount of chemicals within a transport volume is determined by mass flux into and out of the volume, and by kinetics of production and reduction within the volume. Relevant hydrodynamic characteristics and reaction kinetics are represented by specific model parameters. These parameters are then incorporated in a mathematical structure based on the mass conservation principle. The successful application of physically based models are often limited because of difficulties in solving the interrelated problems of parameter identification, system boundary definition, and mathematical solution. These problems can be largely alleviated by an alternative modeling approach which was developed based on a system theory. By following the system approach, dynamic relations between the chemical input at the soil surface and its subsequent downward movement are represented by system response functions that do not require knowledge of the intimate structure of relevant transport mechanisms. Techniques developed in this study were applied in the simulation of the transport of residuals of the pesticide DBCP in soils in the Kunia area of central Oahu, Hawaii, after an accidental spill of the chemical in April 1977. As a result, compatibility of the system modeling approach and a physically based modeling approach is demonstrated. This study also illustrates the computational superiority of the system modeling approach, especially for chemical residual transport in an aggregated soil where a bypassing phenomenon is obvious.