A Monte-Carlo Simulation Technique for Risk Assessment of Roadside Applied Herbicide.

A one-dimensional mathematical model of herbicide fate and transport in surface runoff along highway roadsides is developed and applied stochastically using a Monte-Carlo simulation technique to evaluate the management decisions concerning the spraying of herbicides on highway roadsides. The impact of various management decisions such as restricting herbicide application on the basis of site characteristics or changing other application practices on the risk reduction is also evaluated. Field surveys, laboratory sorption data, and literature data were used to generate probability distribution functions for model input parameters to allow extrapolation of the model to the regional scale. The approach was used for 33 herbicides with different decay and sorption properties and the impact of these properties on the magnitude of the risk is also investigated. Based on the frequencies of potential toxicity (FPTs) obtained by the model, selected herbicides were determined to be more environmentally acceptable than others in terms of acute toxicity and other documented environmental effects. Also it was found that eliminating application where the width of the grass adjacent to the highway is less than 2 m or where soil organic carbon content is less than 0.5% can lead to significant reductions in environmental risk for certain herbicides. This analysis creates a decision support system that can be used to evaluate the relative water quality impacts of varied herbicide application practices.