ESTIMATION OF LOSS OF EFFICACY DUE TO PESTICIDE BIODEGRADATION IN SOIL BY SIMULATION

Abstract

The widespread use of pesticides in agriculture resulted in the transfer of part of these chemicals into soil and water. The degradation of these pesticides solution in soil by number of microorganisms is an important and useful method by which these compounds are removed from the environment. On the other hand, bacteria, fungi occupy a unique position in biological cycles and are essential for plant growth and soil fertility. Therefore the models, describing pesticide biodegradation in soil are necessary and useful. Several transport phenomena models for insitu biodegradation of contaminants in a soil bed were developed and reported in the literature. These differ from each other in respect of complexity of physical processes assumed in modeling and require large computation time for numerical simulation. In these models inhibitory substrates kinetics were not used; only Monod kinetics is used to represent the growth of microorganisms. Sensitivity analysis of the model was also not carried out to study the effects of different parameters on the biodegradation rate. Shelton and Doherty (1997) proposed a model for describing rates of pesticide-substrate biodegradation in soil, which is relatively very simple. This model accounted the pesticide-substrate in soil solution, sorption to soil surfaces, diffusion into the internal matrix of soil organic matter and/or aggregates, and microbial growth. In this model rates of sorption and diffusion are approximated by first order kinetics while microbial growth is approximated by Monod kinetics. In the thesis, this model has been modified by incorporating the effect of toxicity of pesticides on microorganisms. The developed model utilizes endogenous kinetics in the microorganism growth to include the effect of toxicity of pesticides, and considers Haldane kinetics instead of Monod kinetics for inhibitory pesticide-substrate for predicting the biomass growth. The model has been validated with the results available in the literature. Effects of change in variables on model predictions were studied. Further, helped us to the sensitivity of the biodegradation to individual parameters is also investigated. The new relationships are found: LE1 is related to death constant (kJ) almost linearly and to inhibition constant (K1) geometrically. The sensitivity analyses also identify two important parameters: Monod kinetic parameter maximum growth rate (gma.) and inhibition constant (KO. Initial substrate concentration (So) is moderately sensitive to biodegradation rate. Another Monod kinetic parameter half-saturation growth constant (K,), decay constant (kd), and initial biomass concentration (X0) are found almost insensitive. In conjunction with the estimation of loss of efficacy, the model may be useful to suggest the choice of microorganisms depending on the values of its characteristic parameters.