Two‐Site/Two‐Region Models for Pesticide Transport and Degradation: Theoretical Development and Analytical Solutions

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Abstract
Analytical solutions are presented for two convection‐dispersion type transport models useful for studying simultaneous pesticide sorption and degradation. One solution is for the familiar two‐site sorption model in which adsorption‐desorption proceeds kinetically on one fraction of the sorption sites, and at equilibrium on the remaining sites. Another solution holds for two‐region (or mobile‐immobile liquid phase) transport appropriate for aggregated or fractured media. The transport models account for degradation in both the solution and sorbed phases. The dimensionless analytical solutions for the two‐site and two‐region models are shown to be identical; they contain up to six independent dimensionless parameters: a column Peclet number, a retardation factor, a coefficient partitioning the soil/chemical system in equilibrium and nonequilibrium parts, a rate coefficient, and two dimensionless degradation coefficients. One of the two independent degradation coefficients may be eliminated when the solution and sorbed phase degradation rate coefficients are assumed to be identical, or when, with additional but reasonable assumptions, adsorbed phase degradation is assumed to be negligible.

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