306-4 What Controls the Longitudinal Trends in 129I/I Ratios along the Upper Wabash River, IN, USA? Lithology, Nuclear Fallout, or Dietary Iodine in Wastewater?

Abstract:

Iodine-129 ratios (¹²⁹I/I) were measured in river samples collected longitudinally down the Upper Wabash River in northern IN, USA to investigate using ¹²⁹I/I to date baseflow, specifically to identify components of bomb-pulse recharge stored in the watershed. Based on a previous multi-isotope study, our hypothesis was that elevated ¹²⁹I/I bomb-pulse signals would be preserved and transported longitudinally along the river during baseflow conditions owing to the larger groundwater fraction in the baseflow and the presence of bomb-pulse ¹²⁹I/I in aquifers in central IN (ranging from 4000 × 10^-15 to 22,000 × 10^-15). The ¹²⁹I/I ratios measured along the river ranged between 2,000 × 10^-15 to 700,000 × 10^-15. The headwaters reach of the Wabash River has the lowest ¹²⁹I/I ratios and behaves differently from the rest of the river. The lowest ratio (2,203 × 10^-15 observed 142 km from the origin) represents dilution due to a combination of factors including dietary iodine from a wastewater treatment plant and possibly groundwater flow through a clay-rich, swampy area that is rich in organic matter. The higher ratios in the rest of the river likely indicate a discharge of a mixed groundwater signal that contains a component of elevated ¹²⁹I/I from nuclear fuel rod reprocessing and/or nuclear fallout. On average the river ¹²⁹I/I ratios were an order of magnitude higher than that of the aquifers in central IN and two orders of magnitude higher than the historically accepted background ¹²⁹I/I ratio of 1500 ± 150 × 10^-15. In comparison, ¹²⁹I/I ratios of monthly aggregated local precipitation samples collected from March 2025 to March 2026 had even higher ¹²⁹I/I concentrations ranging from 355,000 to 13,280,000 × 10^-15; however, the average ¹²⁹I/I in precipitation lies within the range of 4,000,000 to 8,000,0000 x 10^-15. This dataset shows that a mixture of ¹²⁹I sources related to nuclear weapons testing in the 1950s and 1960s, nuclear accidents and recent nuclear fuel rod reprocessing contribute to recharge and baseflow in the watershed, underscoring the complexities in the temporal variability of ¹²⁹I/I in modern precipitation which makes it challenging to use ¹²⁹I/I to date surface and groundwater in the Midwest.

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