Topics in occurrence and mixture risk assessment supporting formulation and analysis of the stage II residual disinfectants/disinfection byproducts (D/DBP) rule.

摘要

The goal of the work presented in this dissertation is to characterize the implications of employing the sum of four trihalomethanes and five haloacetic acids as risk factors and compliance metrics for DBP risk assessment and regulation. This work has led to three main conclusions. First, concentrations of fully chlorinated trihalomethanes, haloacetic acids, and haloacetonitriles are not correlated with concentrations of species within these classes containing at least one bromine atom. Furthermore, these bromine-substituted DBPs are not correlated with class sum concentrations. Fully brominated DBPs are negatively correlated with fully chlorinated analogues and class sum concentrations. Thus, the use of class sum concentrations, such as HAA5 or THM4, may not be appropriate surrogate exposure markers for the health risk of chlorinated drinking water mixtures. Second, while epidemiological exposure assessment strategy advances have achieved improvements in internal validity of effect estimates for adverse reproductive outcomes due to DBP exposure, the implications for external validity are less clear. To improve external validity with readily available measurements, the bromine incorporation fraction should be reported and evaluated as a potential confounding risk factor in DBP epidemiology. Finally, although bromine-substituted DBPs are not correlated with DBP class sum concentrations, or fully chlorinated analogue concentrations, the bromine incorporation fraction is an appropriate surrogate exposure measure providing information about the DBP speciation in the mixture. The bromine incorporation fraction in distribution system samples for Information Collection Rule plants applying only free chlorine as a primary or residual disinfectant is predicted by water quality criteria readily measured in source water or through the treatment train. The bromine incorporation fraction is strongly controlled by the bromide to total organic carbon ratio at the first point of chlorine addition, alkalinity at the first point of chlorine addition, and specific UV absorption. These parameters suggest that location of chlorine addition relative to natural organic matter removal in drinking water treatment are treatment decisions with the most significant tradeoffs for bromine-substituted DBP formation.