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 + 240Pu 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 + 240Pu 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 + 240Pu 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 ± 240Pu 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 + 240Pu 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).