239,240Pu and inorganic substances in aerosols from the vicinity of a waste isolation pilot plant: the importance of resuspension.

Abstract

Aerosol samples were collected and analyzed to characterize the spatial and temporal variations in the concentrations of plutonium and selected inorganic substances in the atmosphere around the Waste Isolation Pilot Plant (WIPP). High-volume aerosol sampling was conducted at three sites: (1) On Site, (2) Near Field, and (3) Cactus Flats. 239,240Pu was determined by alpha spectrometry following chemical separations; mass loadings were determined gravimetrically. A separate set of low-volume aerosol samples was analyzed for major ions using ion chromatography and for trace elements by inductively-coupled plasma emission spectrometry and mass spectrometry. The average 239,240Pu activity concentrations in total suspended particle (TSP) samples (12 to 16 nBq m(-3)) were consistent with those previously reported, but they varied strongly with season, with the highest values generally in spring. Further, the 239,240Pu activity concentrations were comparable among the three sites, and therefore there was no evidence for elevated 239,240Pu activities due to WIPP operations. The fraction of the 239,240Pu activity concentrations in the PM10, samples (particles less than 10 microm diameter) relative to TSP was lower than the corresponding PM10/TSP ratios of either high-volume mass or several inorganics (sulfate, aluminum or lead), indicating that 239,240Pu tends to be on large particles. Aerosol mass loadings (microg m(-3)) and 239,240Pu activity concentrations were correlated for all sets of samples, but at On Site, the TSP samples showed higher mass to 239,240Pu ratios than the other sites. Thus activities or processes occurring at or near the WIPP site evidently produced aerosols that contributed to the mass loadings but contained less 239,244Pu than ambient aerosols. About 63% of the variability in 239,240Pu activity concentrations was explained by wind travel, sampling location, length of the sampling interval, and aerosol mass. 239,240Pu activity concentrations also were correlated with aluminum (an indicator of mineral dust), further implicating the resuspension of soils as an important determinant of 239,240Pu in aerosols. The 239,240Pu/Al ratios for the aerosols were higher than in soils, and this could be explained by the preferential binding of 239,240Pu to small soil particles that have large surface area to mass ratios and also have higher aluminum contents than larger particles.