The effects of pesticides¹ on water quality commonly are assessed by comparing measured concentrations of individual pesticide compounds in the environment with concentrations that have been determined to have potential adverse effects on humans, aquatic organisms, or other beneficial uses of water. Direct evaluation of the adverse effects of every pesticide present in a given hydrologic system is beyond the scope and budget of most water-quality studies. Many studies rely on standards or guidelines set by federal or state agencies or other institutions to indicate what concentrations may have adverse effects on human health, aquatic organisms, or wildlife. Such standards and guidelines generally are based on laboratory or field studies that document the effects of individual pesticides on specific aspects of water quality. Single-species toxicity tests (using a single species of a test organism) under various laboratory conditions are the most common type of study, whereas artificial ecosystem studies (using multiple species of a test organism) and field studies are relatively uncommon. Such studies rarely consider the effects of exposure to more than one chemical at a time. Technical information from such studies has been used by the U.S. Environmental Protection Agency (USEPA)² in issuing national standards, such as drinking-water regulations (for example, see USEPA 1991a) and guidelines, such as ambient water-quality criteria for the protection of human health and aquatic organisms (for example, see USEPA 1980a) to meet its statutory requirements under the Safe Drinking Water Act (SDWA) and the Clean Water Act (CWA). The Food and Drug Administration (FDA) also has used its authority under the Federal Food, Drug, and Cosmetic Act (FFDCA) to set action levels (enforceable regulatory limits) for unavoidable residues of pesticides in foods (FDA 1990).