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Estimating Losses of Efficacy Due to Pesticide Biodegradation in Soil: Model Simulations

USDA-ARS Environmental Chemistry Lab.

Michael A. Doherty

USDA-ARS Weed Science Lab., Bldg. 007, BARC-West, 10300 Baltimore Ave., Beltsville, MD 20705-2350

*Corresponding author (dshelton{at}asrr.arsusda.gov).

ABSTRACT

A model was developed for describing rates of pesticide-substrate biodegradation, accounting for bioavailability and microbial growth. The model was used to simulate losses of efficacy for soil-applied pesticides. The model requires rate constants for rapid sorption-desorption to and from soil surfaces (k₁/k_-1 = K_d1); diffusion into and out of soil aggregates-organic matter particles (k₂/k_-2 = K_d2); microbial growth [yield (Y), maximum growth rate (µ_max), half-saturation growth constant (K_s), and initial biomass concentration (X₀)]; initial mass of substrate (s₀); and gravimetric water content (_g). Simulations of microbial growth and substrate depletion were conducted assuming no sorption (aqueous solution), sorption to soil surfaces only, and sorption in conjunction with diffusion. The time required to achieve a soil solution concentration of 1 µg mL^–1 was defined as a hypothetical loss of efficacy (LE₁). Certain relationships were consistently observed, regardless of sorption or diffusion: LE₁ was found to be related to K_s linearly, to X₀ logarithmically, to µ_max geometrically, and to initial pesticide-substrate concentration (S₀) nonlinearly. Sorption to soil surfaces resulted in decreased equilibrium soil solution concentration (S_e), depending on the magnitude of _g and K_d1. Rates of biodegradation-growth were a function of S_e, as opposed to total (soluble + sorbed) concentration. Sorption coupled with diffusion decreased both S_e and time-dependent availability, resulting in slower rates of biodegradation. In general, larger values of S₀ resulted in faster rates of biodegradation, i.e., decreased the time required for a loss of efficacy.

Received for publication September 25, 1995.

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