Radioactive microparticles related to the Woolsey Fire in Simi Valley, CA

In November 2018, the Woolsey Fire burned north of Los Angeles, CA, USA, potentially remobilizing radioactive contaminants at the former Santa Susana Field Laboratory, a shuttered nuclear research facility contaminated by chemical and radiochemical releases. Wildfire in radiologically contaminated zones is a global concern; contaminated areas around Chernobyl, Fukushima, Los Alamos, and the Nevada Nuclear Test Site have all experienced wildfires. Three weeks after the Woolsey Fire was controlled, sampling of dusts, ashes, and surface soils (n = 360) began and were analyzed by alpha- and beta-radiation counting. Samples were collected up to a 16 km radius from the perimeter of the laboratory. Controls and samples with activities 1σ greater than background were also examined by alpha and/or gamma spectroscopy or Scanning Electron Microscopy with Energy Dispersive X-ray analysis. Of the 360 samples collected, 97% showed activities at or close to site-specific background levels. However, offsite samples collected in publicly-accessible areas nearest to the SSFL site perimeter had the highest alpha-emitting radionuclides radium, thorium, and uranium activities, indicating site-related radioactive material has escaped the confines of the laboratory. In two geographically-separated locations, one as far away as 15 km, radioactive microparticles containing percent-concentrations of thorium were detected in ashes and dusts that were likely related to deposition from the Woolsey fire. These offsite radioactive microparticles were colocated with alpha and beta activity maxima. Data did not support a finding of widespread deposition of radioactive particles. However, two radioactive deposition hotspots and significant offsite contamination were detected near the site perimeter.