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

Abstract

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 ∼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 μg/L) to 95th percentile concentrations of 320 and 1300 μg/L (median = 12 and 13 μg/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 μg/μg 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.