Excess plutonium in soil near the Nevada Test Site, USA

摘要

Two soil profiles were collected from undisturbed areas near the Nevada Test Site (NTS). The activity of ~(137)Cs in the surface layer of the downwind Queen City Summit profile is three times higher than at the upwind site at Searchlight, NV (41.1 +- 0.6 mBq/g vs. 13.0 +- 0.4 mBq/g), and the ~(239, 240)Pu activity is 100 times greater (51 +- 2 mBq/g vs. 0.52 +- 0.03 mBq/g). An examination of the literature suggests that the ~(137)Cs/~(239, 240)Pu and the ~(239, 240)Pu/~(238)pu activity ratios in soils and sediments from the northern hemisphere, clue to fallout from atmospheric atomic weapons testing, have generalized values of 36 +- 4 and 30 +- 4, respectively (as of 1 July 1995). Deviations from these values may indicate possible contamination by sources other than fallout. Data from the surface soil of the downwind Queen City Summit profile yield a ~(137)Cs/~(239, 240)Pu ratio of 0.81 +- 0.02 and a ~(239, 240)Pu/~(238)Pu ratio of 78 +- 6. Clearly, an increase in ~(239, 240)Pu relative to ~(137)Cs or ~(238)Pu can account for these observations. There is compelling evidence that this "excess" ~(239, 240)Pu came from activities at the NTS during the aboveground testing of nuclear devices, more than likely from safety tests, some 40 years ago, and/or during the interim by the wind-driven resuspension of contaminated surface soil on the NTS and its transport off-site. Moreover, the two concentration profiles show that high percentages of both of these elements are retained for decades in the upper few centimeters of soil in Nevada's desert environment.