Analysis of trace neptunium in the vicinity of underground nuclear tests at the Nevada National Security Site

A high sensitivity analytical method for Np-237 analysis was developed and applied to groundwater samples from the Nevada National Security Site (NNSS) using short-lived Np-239 as a yield tracer and HR magnetic sector ICP-MS. The Np-237 concentrations in the vicinity of the Almendro, Cambric, Dalhart, Cheshire, and Chancellor underground nuclear test locations range from <4 x 10(-4) to 2.6 mBq/L (6 x 10(-17)-4.2 x 10(-13) mol/L). All measured Np-237 concentrations are well below the drinking water maximum contaminant level for alpha emitters identified by the U.S. EPA (560 mBq/L). Nevertheless, Np-237 remains an important indicator for radionuclide transport rates at the NNSS. Retardation factor ratios were used to compare the mobility of Np-237 to that of other radionuclides. The results suggest that Np-237 is less mobile than tritium and other non-sorbing radionuclides (C-14, (CI)-C-36, Tc-99 and I-129) as expected. Surprisingly, Np-237 and plutonium (Pu-239,Pu-249) retardation factors are very similar. It is possible that Np(IV) exists under mildly reducing groundwater conditions and exhibits a retardation behavior that is comparable to Pu(IV). Independent of the underlying process, Np-237 is migrating downgradient from NNSS underground nuclear tests at very low but measureable concentrations. (C) 2014 The Authors. Published by Elsevier Ltd.