Total residual chlorine as a regulatory tool

Inherent in the choice of total residual chlorine (TRC) as a basis for establishing water quality criteria or effluent guidelines for chlorine are assumptions that components of TRC are roughly equivalent in toxicity to aquatic species and that the toxicities of these components are additive. Most of the studies of this assumption involved comparisons of mixtures of residual chlorine components and thus led to conflicting conclusions. In addition, studies designed to avoid the mixture problem omitted one or more of the most likely products of chlorination from the comparison(s). On the other hand, the assumption of additive toxicity of residual chlorine species has largely been ignored. In view of the continuing controversy that focused on the levels of residual chlorine which can be discharged without deleterious impact on receiving waters, it is important that these assumptions be evaluated comprehensively. New data on the toxicities to the mosquitofish, Gambusia affinis, of inorganic monochloramine, inorganic dichloramine, a mixture of these two compounds, and two mixtures with free chlorine are reported. These and other data on the toxicity of residual chlorine species to mosquitofish are then used to examine the questions of toxicity equivalence and additivity of species of residual chlorine. (ERB)