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

Abstract

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.