EVALUATING ENVIRONMENTAL PERSISTENCE

Environmental persistence is an important consideration when assigning priorities to chemical contaminants. It has been proposed that pass/f ail half-life criteria be established for the media of air, water, soi l, and sediment: The feasibility of applying such criteria is discusse d. It is shown that there are severe problems in estimating these half -lives. Additionally, it is shown that the overall environmental half- life or persistence in a multimedia environment is also a function of the mode of entry of the chemical and its partitioning properties. Exa mples of potential misclassification are presented. An alternative two -tiered procedure is suggested in which the first tier considers only media half-lives and the second tier considers the overall half-life a s deduced by mass-balance models. Furthermore, the overall persistence calculated in the second tier should be given as a distribution rathe r than a single value. This procedure enables chemicals to be compared , avoids undue reliance on relatively unimportant half-lives, and is r egarded as a truer depiction of actual chemical persistence in the env ironment.