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Two‐Site/Two‐Region Models for Pesticide Transport and Degradation: Theoretical Development and Analytical Solutions
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1989
Year
Soil PropertyChemical EngineeringEnvironmental ChemistryPhase DegradationTransport ModelsEngineeringPesticide-residue AnalysisEnvironmental EngineeringPesticide TransportLiquid PhaseEnvironmental PollutionSoil PollutionSoil ContaminationTransport PhenomenaTwo‐site/two‐region ModelsEnvironmental FateSoil PhysicTheoretical Development
The study presents analytical solutions for two convection‑dispersion transport models to analyze simultaneous pesticide sorption and degradation. The authors derive solutions for a two‑site sorption model with kinetic and equilibrium fractions and a two‑region mobile‑immobile transport model, both incorporating degradation in solution and sorbed phases, and show that one degradation parameter can be removed under certain assumptions. The analytical solutions for the two‑site and two‑region models are identical and involve up to six independent dimensionless parameters, including Peclet number, retardation factor, partitioning coefficient, rate coefficient, and two degradation coefficients.
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.