Historical and Future Needs for Geospatial Iodide Occurrence in Surface and Groundwaters of the United States of America

While iodide (I-) is critical for biological systems, it can serve as a precursor to organic iodinated disinfection byproducts (I-DBPs) of human health concern during water treatment. Thus, understanding potential I- occurrence in fresh waters is critical. Although I- occurrence data are sparse in surface water (SW) or groundwater (GW) used for drinking water supplies, data exist for other locations. We analyzed historical I- occurrence for similar to 9200 SW and GW sampling locations in the United States to understand potential I- sources and also spatial and temporal variability. I- ranged from below detection limits (<1 mu g/L) to 95th percentile concentrations of 320 and 1300 mu g/L (median = 12 and 13 mu g/L), respectively, in SW and GW. I- appears to be influenced by halite basins, organic-rich shale/oil formations, saltwater intrusion, and rainfall, with median Br-/I- mass ratios of 10 and 17 mu g/mu g in SW and GW, respectively. Our results demonstrated considerable variability in iodine sources and speciation, which can impact I-DBP formation at WTPs. We advocate for occurrence studies to measure I- IO3- and total iodine in raw and finished drinking waters to fill critical data gaps necessary to understand the potential formation of I-DBPs that impact public health.