Subbasin-level discharge-intake flow index to assess potential iodine point source discharges upstream of surface drinking water sources

Drinking water sources can be downstream of pollution sources that affect their treatability. One example is the formation of disinfection byproducts (DBPs) when chemical disinfectants react with precursors such as organic matter, bromine (Br), and iodine (I) in drinking water treatment. There are currently no regulated iodine-containing DBPs (I-DBPs) in the U.S., although six iodinated trihalomethanes were on a draft list for potential future regulation. Iodine species and I-DBPs are not routinely monitored in surface waters or in wastewater discharges, making it difficult to assess drinking water risk associated with I-DBPs. The objective of this study was to investigate iodine-containing point source discharges across nine categories that are upstream of surface drinking water sources at the hydrologic unit code subbasin (8-digit HUC, or HUC8) level through the establishment of a discharge-intake flow index (DIFI). A DIFI score was calculated for each HUC8 in the contiguous U.S. based on the count of wastewater discharges potentially containing iodine (representing potential hazard) upstream of surface drinking water facility subwatersheds (HUC12) (representing potential exposure). Statistical differences in DIFI were evaluated at the hydrologic region (HUC2) level. Of the 1,154 HUC8 subbasins containing surface water facilities in the contiguous U.S., 651 (56%) contain upstream potential iodine discharges. While DIFI scores were geographically widespread, the highest overall scores were primarily located in the eastern U.S.: HUC02, HUC05, and HUC03. The methodology presented here enables identification of high-priority watersheds for further investigation, and results can inform water utility decision-making and future DBP research.