What factors complicate the patterns of occurrence?
Occurrence is complex because of mixtures or the “co-occurrence” of many compounds in individual samples. More than 75 percent of source- and treated-water samples contained five or more organic compounds, often including parent compounds and degradates. The common occurrence of compound mixtures means that the total combined toxicity in source water may be greater than that of any single compound that is present. Continued research is needed because human-health benchmarks are based on toxicity data for individual compounds, and the additive or synergistic effects of mixtures of compounds at low levels are not well understood.
