Pesticide Volatilization as Related to Water Loss From Soil

Abstract

AbstractResults of experiments to determine the relationship between pesticide volatilization and water loss from soil indicate that volatilization rate is controlled by diffusion of the pesticide and by mass flow of water to the soil surface. Volatilization of soil‐incorporated lindane and dieldrin was measured under conditions whereby the soil surface remained relatively moist while the humidity of the air passing over the soil was varied to regulate water loss from the soil. Water loss accelerated pesticide volatilization rate, but only after the soil surface had been depleted of the pesticide, indicating that the enhancement was due to the “wick effect,” whereby pesticides move to the soil surface by mass flow in water moving to the surface to replace that evaporated, and not due to the mechanism called “codistillation.” There was good agreement between measured increases in volatilization rates and calculated amounts of lindane or dieldrin moving to the soil surface in evaporating water. Lindane is more water soluble than dieldrin and its volatilization was more enhanced by concurrent evaporation of water from the soil surface. At very low relative humidities, dieldrin accumulated at the soil surface; this resulted in rapid volatilization when the surface was again moistened by exposure to a relative humidity of 100%. Results indicate that pesticides carried to the surface in evaporating water will volatilize, either immediately or after remoistening the soil surface following drying. Desorption isotherms relating soil pesticide concentrations to soil solution concentrations should be useful in predicting volatilization rates associated with evaporating water.