A combined soil-atmosphere model for evaluating the fate of surface- applied pesticides. 2. The effect of varying environmental conditions

Simulations using the one-dimensional nonisothermal model (Reichman et al. Environ Sci. TechnoL 2000, 34,1313-1320). this issue) for pesticides that have been applied to the soil surface revealed that diurnal variation of climatic factors, soil type (silty clay and sand), initial soil-moisture distribution, and pesticide type (parathion, dieldrin, and trifluralin) potentially have major influences on pesticide volatilization rates and persistence. The diurnal variation of water evaporation, which is closely related to the diurnal variation in solar radiation, has potentially significant effect on the diurnal variation of parathion volatility and mobility (low volatility and high mobility). However, the volatilization rates of low-water-soluble pesticides (e.g., trifluralin and dieldrin) are only slightly affected by water evaporation rate. Their migration downward via vapor-phase diffusion reduces their concentration in the soil surface layer and decreases their actual volatilization rates. The limiting factor for these pesticides is their movement downward in the soil. The volatilized masses of parathion and dieldrin from a silty clay soil are higher than from sandy soil. However, trifluralin's volatilization rate, volatilized mass, and penetration depth are lower for the silty clay than for the sandy soil due to adsorption. Higher volatilization rates were obtained from initially dry soils for trifluralin and from initially wet soils for parathion and dieldrin. Comparison between isothermal and nonisothermal models revealed that use of the former might lead to erroneous conclusions with respect to the relative volatility of pesticides especially those with a low Henry constant and high water solubility.