Variations in Disinfection By-Product Precursors Bromide and Total Organic Carbon Among U.S. Watersheds

Watersheds with relatively higher concentrations of disinfection by-product (DBP) precursors are of concern for public water systems (PWSs) seeking to control DBP exposure risk. In this study, the occurrence of bromide and total organic carbon (TOC), which are important DBP precursors, was evaluated in PWS source waters on a watershed basis at the hydrologic unit code 2 (HUC2) level. Variations were examined across source water type (e.g., surface water or groundwater) and watershed, and temporally based on seasonality as well as differences between the late 1990s and the years 2018–2020. The median bromide concentrations were higher in groundwater systems compared with surface water (by 25 μg/L), whereas the median TOC concentrations were higher in surface water systems compared with groundwater (by 1.7 mg/L). Surface water sources in multiple midwestern and western watersheds had a combination of elevated bromide and TOC (relative to median values of 21 μg/L and 2.5 mg/L, respectively), which was especially pronounced in the Texas-Gulf and Rio Grande watersheds. None of the HUC2 watersheds had elevated levels of both bromide and TOC in groundwater sources (median values of 50 μg/L and 1.0 mg/L, respectively). Source water bromide median watershed concentrations for very large systems were ∼20% lower in 2018–2020 than in 1998, however, more than 40% higher at the 90th percentile, whereas source water TOC median watershed concentrations for very large systems were ∼25% higher in 2018–2020 than in 1998 but less than 10% higher at the 90th percentile. These temporal differences may be related to the influence of climate change and other anthropogenic effects. In addition, many individual PWSs showed substantial change in bromide and TOC concentrations between those time periods. For example, about 7% of the PWSs had at least double the bromide concentrations, and 5% at least double the TOC concentrations, in 2018–2020 than in 1998.