Deposition of ²³⁶U from atmospheric nuclear testing in Washington state (USA) and the Pechora region (Russian Arctic)

Stratospheric fallout-derived ²³⁶U has been detected by sector field ICPMS at two field locations for which our laboratory possessed available archived samples: A) four soil cores from Washington state (northwestern USA) and B) sediment cores from three small lakes in the Pechora region (Russian Arctic). Four Washington state soil cores exhibit ²³⁶U inventories of 8.1 ± 1.3, 11.1 ± 0.9, 18 ± 2, and 30.2 ± 3.9 Tatoms/m²; the respective ²³⁹Pu contents are 52.9 ± 3.5, 67 ± 3, 71 ± 2, and 151 ± 2 Tatoms/m². A ²³⁶U/²³⁹Pu atom ratio of 0.19 ± 0.04 (1 SD) has been determined from the Washington state soil cores. The three Pechora region lake cores each exhibit coincident maxima in their ²³⁶U and ²³⁹Pu atom concentration profiles. The ²³⁶U/²³⁸U atom ratios are controlled by two independent factors; ²³⁶U is from fallout deposition and ²³⁸U concentrations are a property of the geochemical distribution of naturally occurring U. A ²³⁶U/²³⁸U atom ratio as high as 8.9 × 10^-6 has been observed for acid-leached soils containing Pu solely derived from bomb-test fallout. Accordingly, a non-zero ²³⁶U background from stratospheric fallout must be recognized and taken into account when detectable ²³⁶U is used to infer specific local or regional influences of reactor-irradiated U.