Influence of Lake Trophic Structure on Iodine-131 Accumulation and Subsequent Cumulative Radiation Dose to Trout Thyroids

Iodine-131 is a major component of the atmospheric releases following reactor accidents, and the passage of ¹³¹I through food chains from grass to human thyroids has been extensively studied. By comparison, the fate and effects of ¹³¹I deposition onto lakes and other aquatic systems have been less studied. In this study we: (1) reanalyze 1960s data from experimental releases of ¹³¹I into two small lakes; (2) compare the effects of differences in lake trophic structures on the accumulation of ¹³¹I by fish; (3) relate concentrations in fish and fish tissues to that in the water column using empirically estimated uptake (L kg⁻¹ d⁻¹) and loss (d⁻¹) parameters; and (4) show that the largest concentrations in the thyroids of trout (Oncorhynchus mykiss) may occur from 8 to 32 days after initial release. Iodine-131 concentration in trout thyroids at 30-days post release may be >1000 times that in the water. Estimates of cumulative radiation dose (mGy) to thyroids computed using an anatomically-appropriate model of trout thyroid structure within the Monte Carlo N-particle modeling software predicted cumulative thyroid doses that increased approximately linearly after the first 8 days and resulted in 32-day cumulative thyroid doses that ranged from 6 mGy g⁻¹ to 18 mGy g⁻¹ per 1 Bq mL⁻¹ of initial ¹³¹I in the water depending upon fish size. The majority of this dose is due to beta emissions, and the dose varies with positions in the thyroid tissue. - Highlights: • ¹³¹I introduced into lakes is concentrated through food webs. • Highest concentrations in trout thyroids are from 8 to 32 days after release. • Continuing high concentrations causes linearly increasing cumulative doses. • The majority of the dose to thyroids is due to beta emissions. • Cumulative doses at 32 days may be >6 mGy g⁻¹ per initial ¹³¹I mBq mL⁻¹