Dissolved organic matter (DOM) composition influences the formation of disinfection byproducts (DBPs) during drinking water treatment, yet the role of DOM composition at the molecular level in forming both known and novel DBPs has not been established. We characterized the composition of DOM from drinking water utilities that draw from surface (n = 4) and groundwaters (n = 14), focusing on groundwater because the reactivity of its DOM is poorly understood. We quantified the formation of targeted DBPs, identified novel high molecular weight (HMW) halogenated DBPs, and analyzed DOM transformation after reaction with free available chlorine. Characterization by UV−visible spectroscopy and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) reveals a high degree of variability in DOM composition and reactivity with chlorine, particularly among groundwater samples. Despite the variability in DOM composition, novel DBPs with up to three halogens are compositionally similar among all waters and are positively correlated with trihalomethane and, to a lesser extent, haloacetonitrile formation. These relationships demonstrate that some low molecular weight DBPs and novel DBPs detected by FT-ICR MS share similar aromatic precursors, providing evidence that DBPs such as trihalomethanes are a useful proxy for the formation of their HMW counterparts.

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Halogenated estrogens are thought to be moderately potent endocrine-disrupting compounds that are formed during chlorine-based wastewater disinfection processes and may represent a significant fraction of the total amount of estrogen delivered from wastewater treatment plants to receiving waters. Yet we lack key information about the photochemical degradation of halogenated estrogens, a process that has important implications for UV-based wastewater treatment and environmental fate modeling. To better understand halogenated estrogen degradation in aquatic environments, we studied the direct photolysis of 17β-estradiol (E2), 2-bromo-17β-estradiol (monoBrE2), 2,4-dibromo-17β-estradiol (diBrE2), and 2,4-dichloro-17β-estradiol (diClE2) as well as the indirect photolysis of diBrE2 under natural solar irradiance. We found that direct photolysis rate constants increased with halogenation as pKa values decreased and molar absorptivity spectra shifted toward higher wavelengths. Compared to E2, quantum yields were threefold larger for monoBrE2, but 15-32% smaller for the dihalogenated forms. The rate of diBrE2 (pKa ∼ 7.5) photolysis was strongly influenced by pH. At pH 7, diBrE2 degraded on minute time scales due to the large red-shifted molar absorptivity values and greater quantum yields of the phenolate form. Degradation rates were only slightly different in the presence of Suwannee River Humic Acid (5 mg L-1), and quenching experiments pointed to excited triplet state dissolved organic matter (3DOM*) as the dominant reactive intermediate responsible for the indirect photolysis of diBrE2. Overall, our data suggest that halogenated estrogens are particularly susceptible to photochemical degradation at environmentally relevant pH values.