ABSTRACT
Public and legislative concern for the magnitude and effects of nonpoint source pollution (NPS) evolved during the decade of the 1970s. The historical development of water quality runoff models to analyze NPS problems has emphasized the integration of hydrologic transport processes, pollutant sources, and chemical fate transformations. The soil consists of particulate matter to which chemicals may adsorb or be otherwise attracted, and water and gas phases in which the chemical may be dissolved or mixed. Ecosystem organisms also provide a compartment in which the chemical may reside. Molecules which absorb light in the range of wavelengths of the solar spectrum are potentially affected by photolysis in natural systems. Runoff, erosion and leaching can represent a significant portion of chemical nonpoint pollution contributions from agricultural sources due to use and application of pesticides and fertilizers. This chapter indicates "ideally" because in many applications existing data will not support performance of all steps.
