A multimedia fate and chemical transport modeling system for pesticides: I. Model development and implementation

We have combined the US EPA MM5/MCIP/SMOKE/CMAQ modeling system with a dynamic soil model, the pesticide emission model (PEM), to create a multimedia chemical transport model capable of describing the important physical and chemical processes involving pesticides in the soil, in the atmosphere, and on the surface of vegetation. These processes include: agricultural practices (e.g. soil tilling and pesticide application mode): advection and diffusion of pesticides, moisture, and heat in the soil: partitioning of pesticides between soil organic carbon and interstitial water and air: emissions from the soil to the atmosphere: gas-particle partitioning and transport in the atmosphere: and atmospheric chemistry and dry and wet deposition of pesticides to terrestrial and water surfaces. The modeling system was tested by simulating toxaphene in a domain that covers most of North America for the period from 1 January 2000 to 31 December 2000. The results show obvious transport of the pesticide from the heavily contaminated soils in the southern United States and Mexico to water bodies including the Atlantic Ocean, the Gulf of Mexico and the Great Lakes, leading to significant dry and wet deposition into these ecosystems. The spatial distributions of dry and wet depositions differ because of their different physical mechanisms: the former follows the distribution of air concentrations whereas the latter is more biased to the North East due to the effect of precipitation.