Is Genotoxicity Associated with Disinfection Byproducts in Potable Reuse and Conventional Drinking Water Additive?

We have previously shown that the Chinese hamster ovary (CHO) cell chronic cytotoxicity of disinfection byproducts (DBPs) in a mixture of DBPs is additive, supporting the use of the sums of measured concentrations of DBPs weighted by metrics of cytotoxic potency to compare the DBP-associated cytotoxicity of disinfected waters. Researchers applied this approach to estimate the genotoxicity of disinfected waters using the metrics of genotoxic potency derived from the CHO cell acute genotoxicity assay, but the additivity of DBP genotoxicity in a mixture has not been determined experimentally. In this study, we analyzed the CHO cell acute genotoxicity of five defined mixtures containing regulated and unregulated DBPs at the relative concentrations measured in two drinking waters and three potable reuse waters. The genotoxicity of DBPs in mixtures is less than additive, indicating antagonism. We observed antagonism in the subset mixtures containing only regulated DBPs or unregulated DBPs. Regulated DBPs were predicted to contribute substantially to the overall genotoxicity under the additivity assumption. However, quantitative bioassay results indicated that regulated DBPs are minor contributors to the genotoxicity of the mixtures compared with unregulated DBPs. Conducting quantitative genotoxicity analyses of water samples should be encouraged rather than only employing estimates of the calculated genotoxicity.