Detection of past underground nuclear events by geochemical signatures in soils

Abstract

This paper describes a research program to determine if a geochemical signature is retained in surface and sub-surface soils over a well-contained underground nuclear explosion. Samples of soil at surface and about 10–15 cm below were collected on orthogonal traverses up to about 1 km from ground zero (GZ, surface projection of blast point) at three locations in central (Ville) and southern Yucca Flat (Laredo and Presidio) on the Nevada Test Site (NTS). They were analyzed for total element content by instrumental neutron activation (INAA) and, using selective leaches, for partial element content, namely that amount bound to amorphous Fe oxyhydroxide and that amount bound to Mn oxide. Forty-two elements were determined in these leaches of both surface and sub-surface soils, by inductively coupled plasma mass spectrometry (ICP-MS), ICP emission spectrometry (ICP-ES) and hydride generation quartz tube atomic absorption spectrometry (HG-QTAAS). These partial leaches were chosen on the premise that the extreme temperature and pressure conditions created by an underground blast would volatilize elements in the surrounding rock and transport them along with fission products to the surface where they would be trapped by these phases in the soil. The INAA data generally show consistent concentrations of major, minor and trace elements within each location. In contrast, the selective leach data show distinct geochemical anomalies in numerous elements. Iodine, As, Mo, and U demonstrate particularly consistent positive anomalies at and around GZ at all three locations by both leaches. The amplitudes of these anomalies can exceed two orders of magnitude over background, particularly for I. This positive response is shown by more elements associated with Mn oxide than associated with Fe oxyhydroxide at all three locations; furthermore these patterns are more coherent and intense for some elements. The Mn oxide phase itself is more consistently distributed than is the Fe oxyhydroxide phase at the sites within each location. In most cases, significant depletion of Fe as amorphous Fe oxyhydroxide coincides with the multiple positive anomalies in this phase. The number and group of elements displaying distinct patterns is dependent upon the location as well as on the leach. The concentration ranges of elements extracted from the Fe oxyhydroxide or Mn oxide phase are similar for sites at Laredo and Presidio but dissimilar from those at Ville, probably a reflection of the differing geological settings. Whereas some anomalies are spatially quite compact, extending only about 50 m from GZ (e.g. Rb, Cs, Th), others such as I and Sb extend up to 300 m. The geochemical behaviour of the surface soil samples, although slightly noisier, is similar to that of the sub-surface suite. These results indicate that analysis of specific geochemical phases in soils over a suspected underground nuclear test has an important role in the on-site inspection component of the verification of the Comprehensive Test Ban Treaty.