Spatial variability in Polonium-210 and Lead-210 activity concentration in New Zealand shellfish and dose assessment

The activity concentrations of Polonium-210 (²¹⁰Po) and Lead-210 (²¹⁰Pb) were determined in shellfish (Perna canaliculus and Paphies subtriangulata) sampled bimonthly from March 2018 to February 2019 from 14 sites around New Zealand. Activity concentrations of ²¹⁰Po ranged from 4.7 ± 1.1 to 324 ± 17 Bq. kg^-1 with a mean value of 57 ± 72 Bq. kg^-1 (wet weight). The activity concentrations of ²¹⁰Pb were lower than those for ²¹⁰Po (0.1 ± 0.4 and 1.9 ± 0.4 Bq. kg^-1, with a mean value of 0.7 ± 0.4 Bq. kg^-1, wet weight). The calculated ²¹⁰Po/²¹⁰Pb activity concentration ratios were higher than unity in all samples indicating that radionuclides are not in equilibrium in shellfish and most of the ²¹⁰Po was unsupported by its grandparent ²¹⁰Pb. No significant difference was noted in ²¹⁰Po activity concentration between different seasons, species or shellfish condition index. Significant spatial variability in ²¹⁰Po activity concentration was observed with elevated ²¹⁰Po activity concentration in two sampling sites: Ninety Mile Beach (mean 257 ± 47 Bq. kg^-1) and Maunganui Bluff (mean 127 ± 22 Bq. kg^-1). Elevated ²¹⁰Po is hypothesised to be related to an increase of ²¹⁰Po accumulation through diet. Individuals who consume large quantities of shellfish (10 g per day or more) from areas affected by elevated ²¹⁰Po activity concentration may be exposed to an annual committed effective dose from ²¹⁰Po in shellfish in excess of 1 mSv.