Iodine in drinking water in Denmark is bound in humic substances

OBJECTIVE: The iodine intake level is important for the occurrence of thyroid disorders in a population. We have previously found that iodine in drinking water is related to iodine excretion but whether iodine is present as iodide or bound in other molecules remains unknown. DESIGN: We measured iodine in drinking water from 22 locations in Denmark. Six locations were selected by iodine content for further tap water analysis (Skagen 140 micro g/l, Samsoe 56 micro g/l, Nykoebing S. 50 micro g/l, Nakskov 40 micro g/l, Ringsted 38 micro g/l, Copenhagen 19 micro g/l). METHODS: HPLC size exclusion before (Skagen) and after (all sites) freeze drying and measurement of absorbance (280 nm) and iodine in fractions, and fluorescence spectroscopy of bulk organic matter in Skagen drinking water. RESULTS: Iodine content was unaltered after 3 Years (P=0.2). All samples contained organic molecules with characteristics similar to humic substances. Most iodine eluted with humic substances (Skagen 99%, Ringsted 98%, Nykoebing S. 90%, Copenhagen 90%, Samsoe 75%, Nakskov 40%). Changing pH and ionic strength and preincubation with iodide indicated that iodine was bound in humic substances. Humic substances may affect thyroid function but differ with geology. Geological and geochemical data agree with tap water humic substances having been released from marine deposits. Iodine is abundant in the marine environment and marine deposits are particularly rich in iodine. Correlation analysis (r=0.85, P=0.03) conform to iodine in drinking water, suggesting marine humic substances at the source rock. CONCLUSION: Iodine in Danish drinking water varied considerably. In drinking water with a high iodine content, the iodine mainly eluted with humic substances derived from marine source rock. We hypothesize that iodine in drinking water in general suggests coexisting humic substances of marine origin.