ESTIMATING LOSSES OF EFFICACY DUE TO PESTICIDE BIODEGRADATION IN SOIL- MODEL SIMULATIONS

A model was developed for describing rates of pesticide-substrate biod egradation, accounting for bioavailability and microbial growth. The m odel was used to simulate losses of efficacy for soil-applied pesticid es. The model requires rate constants for rapid sorption-desorption to and from soil surfaces (k(1)/k(-1) = K-d1); diffusion into and out of soil aggregates-organic matter particles (k(2)/k(-2) = K-d2); microbi al growth [yield (Y), maximum growth rate (mu(max)), half-saturation g rowth constant (K-S), and initial biomass concentration (X-0)]; initia l mass of substrate (S-0); and gravimetric water content (theta(g)). S imulations of microbial growth and substrate depletion were conducted assuming no sorption (aqueous solution), sorption to soil surfaces onl y, and sorption in conjunction with diffusion. The time required to ac hieve a soil solution concentration of 1 mu g mL(-1) was defined as a hypothetical loss of efficacy (LE1). Certain relationships were consis tently observed, regardless of sorption or diffusion: LE1 was found to be related to K-S linearly, to X-0 logarithmically, to mu(max) geomet rically, and to initial pesticide-substrate concentration (S-0) nonlin early. Sorption to soil surfaces resulted in decreased equilibrium soi l solution concentration (S-c), depending on the magnitude of theta(g) and K-d1. Rates of biodegradation-growth were a function of S-c, as o pposed to total (soluble + sorbed) concentration. Sorption coupled wit h diffusion decreased both S-c and time-dependent availability, result ing in slower rates of biodegradation. In general, larger values of S- 0 resulted in faster rates of biodegradation, i.e., decreased the time required for a loss of efficacy.