Distribution of neptunium and plutonium in New Mexico lichen samples ( Usnea arizonica ) contaminated by atmospheric fallout

The concentrations of 237 Np, 239 Pu and 240 Pu were determined in lichen samples ( Usnea arizonica ) that were collected from ten locations in New Mexico between 2011 and 2013 using isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS). The observed isotopic ratios for 237 Np/ 239 Pu and 240 Pu/ 239 Pu indicate trace contamination from global and regional fallout (e.g. Trinity test and atmospheric testing at the Nevada Test Site). The fact that actinide contamination is detected in recent lichen collections suggests continuous re-suspension of fallout radionuclides even 50 years after ratification of the Limited Test Ban Treaty.