Radioactivity levels in Eniwetok soil.

Abstract

A radiological survey was performed to define the distributions of radioactivity within the soil on the islands in the Eniwetok Atoll, a former U.S. nuclear weapons test site in the Pacific. Soil samples were collected on each of the 40 islands within the atoll. Special emphasis, in terms of increased sample collections, was given to islands expected to have been the most affected by the testing program as well as those considered to be the most likely sites for future habitation. Approximately 3000 soil samples were analyzed by Ge(Li) gamma spectrometry and by wet chemistry. The predominant radionuclides observed in the samples were 90Sr, 137Cs, 239Pu and 60Co. In general, the activities appeared to approximate log-normal distributions. Geometric mean values were determined for each area exhibiting significantly different radiological characteristics. The southern islands showed the lowest levels of activity and they were distributed in a fairly uniform way over the surface of the islands. On the northern islands, where weapons testing was most intensive, the highest activities were observed toward the island interiors or in proximity to ground zero sites, and could usually be related directly to the surrounding vegetation density. The distribution of activity with soil depth shows a wide range of variations. On the southern islands the activities are low to the depth sampled. Within areas subjected to fallout, but excluding ground zeros and construction areas, the activities generally show a rapid decrease immediately below the surface and then level off. Beaches exhibit lower activities than the interiors and the depth distributions are essentially homogeneous. Profiles obtained in the ground zero and construction areas show highly variable depth distributions with layers of buried contamination indicated at some locations. The island of Yvonne is the most contaminated land area within the atoll. Particles containing as much as several milligrams of plutonium are randomly scattered on or near the surface over most of the northern part of the island. In addition, the northern tip of the island includes the highest external gamma levels (500μ750 μR/hr) due to soil radioactivity found on the atoll.