Resolving global versus local/regional Pu sources in the environment using sector ICP-MS

Abstract

Sector inductively coupled plasma mass spectrometry is a versatile method for the determination of plutonium activities and isotopic compositions in samples containing this element at fallout levels. Typical detection limits for ^{239 + 240}Pu are 0.1, 0.02 and 0.002 Bq kg⁻¹Pu for samples sizes of 0.5 g, 3 g, and 50 g of soil, respectively. The application of sector ICP-MS-based Pu determinations is demonstrated in studies in sediment chronology, soil Pu inventory and depth distribution, and the provenance of global fallout versus local or regional Pu sources. A sediment core collected from Sloans Lake (Denver, Colorado, USA) exhibits very similar ¹³⁷Cs and ^{239 + 240}Pu activity profiles; ²⁴⁰Pu/²³⁹Pu atom ratios indicate possible small influences from the Nevada Test Site and/or the Rocky Flats Environmental Technology Site. An undisturbed soil profile from Lockett Meadow (Flagstaff, Arizona, USA) exhibits an exponential decrease in ^{239 + 240}Pu activity versus depth; ²⁴⁰Pu/²³⁹Pu in the top 3 cm is slightly lower than the global fallout range of 0.180 ± 0.014 due to possible regional influence of Nevada Test Site fallout. The ^{239 ± 240}Pu inventory at Lockett Meadow is 56 ± 4 Bq m⁻², consistent with Northern Hemisphere mid-latitude fallout. Archived NdF₃ sources, prepared from Polish soils, demonstrate that substantial ^{239 + 240}Pu from the 1986 Chernobyl disaster has been deposited in north eastern regions of Poland; compared to global fallout, Chernobyl Pu exhibits higher abundances of ²⁴⁰Pu and ²⁴¹Pu. The ratios ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu co-vary and range from 0.186–0.348 and 0.0029–0.0412, respectively, in forest soils (²⁴¹Pu/²³⁹Pu = 0.2407·[²⁴⁰Pu/²³⁹Pu] − 0.0413; r² = 0.9924).