A spatial survey of iodine and its long‐lived isotope, 129I, in 40 rivers of the USA, Canada, and western Europe, reveals that the ratio of 129I/I is a sensitive indicator for the influence of nuclear fuel reprocessing facilities. Rivers with point sources for 129I in their watersheds are drastically affected, while all rivers sampled show evidence for atmospherically transported 129I from the world's major nuclear fuel reprocessing facilities. Varying mixtures of oceanic cyclic salt and soil‐derived iodine account for the observed spatial variation in iodine concentrations. A comparison of 129I concentrations in river and rainwater reveals concentration effects due to evapotranspiration.

129 I/ 127 I ratios measured in meteoric water and epiphytes from the continental United States are higher than those measured in coastal seawater or surface freshwater and suggest long-range atmospheric transport of 129 I from the main source for the earth's surface inventory, viz., nuclear fuel reprocessing facilities. The median ratio for 14 meteoric water samples is 2100 × 10 -12 , corresponding to a 129 I concentration of 2.5 × 10 7 atoms/L, whereas 9 epiphyte samples have a median ratio of 1800 × 10 -12 . Calculated deposition rates of 129 I in the continental United States reveal that a small but significant fraction of the atmospheric releases from the nuclear fuel reprocessing facilities at Sellafield, England, and Cap de La Hague, France, is deposited after distribution by long-range transport. The inferred dominant mode of transport is easterly, within the troposphere, mainly in the form of the organic gas methyl iodide.

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The watershed processes which control 129I/127I ratios, 129I and 127I concentrations, and speciation of iodine isotopes were studied through an investigation into the variability of these parameters in the Mississippi River near New Orleans, undertaken in 1996-1998. Analyses of suspended particulate matter (SPM) revealed a greater percent association of 127I than of 129I, resulting in lower 129I/127I ratios in SPM than in surrounding water. Furthermore, crossflow ultrafiltration showed that organo-iodine was the dominant form for both isotopes, with 70-85% of these isotopes found in the 0.45 microm filter-passing fraction associated with colloidal macromolecular organic matter. 129I showed a weak correlation, 127I no correlation, and 129I/127I ratios a strong inverse correlation with river flow rate. Inverse correlations between 129I/127I ratios and river flow rates can be best explained by rainwater and evapotranspiration dominated ratios at base flow and a lowering of the isotope ratios during higher flow due to extra inputs of 127I from soil weathering. We postulate that different equilibration times for 127I and 129I as well as for bomb-produced 129I and reprocessing-produced 129I are responsible for these fractionation effects and the differential mobilities of these isotopes in the Mississippi River watershed.