An accurate estimate of the environmental concentration of a chemical substance resulting from its manufacture, use, and disposal is essential to any organized hazard assessment program. This estimate of dose can then be used in combination with results from laboratory-derived toxicity tests to assess the associated risks to aquatic species. In following this approach, one must consider what the environment does to the pollutant rather than the reverse. Chemical transformations of a pollutant can take many routes, several of the more significant being ionization as typified by simple acid-base equilibria, hydrolysis, photolysis, microbial degradation, volatilization, and partitioning (sorption). In general, first-order equations can be used to estimate each of these reaction rates using expressions that involve a term for the aqueous concentration and a function of some property of the environment such as pH, turbulence, or microbial population size. These major processes are identifiable, experimentally manageable, and can be arranged easily into differential equations expressing the net rate of change under given conditions. By specifying realistic ranges of these environmental properties, an analysis of pollutant changes can be carried out for a wide variety of aquatic environments.