Anomalously high activities of 137Cs in soils and vegetation on and near a diabase outcrop in La Sierra de Lema, Venezuela

Abstract

As a result of routine soil sampling to determine the 137Cs background activities country-wide in Venezuela, it was decided to further investigate El Mirador (Lookout) area at the base of the Sierra de Lema mountain range. In April 2003 (A), soil samples were collected at eight sites on and around the edge of the diabase outcrop to confirm that this area had anomalously high 137Cs activities. In July 2003 (B), not only soil samples were collected again, but also black mat, palm tree leaves and trunks, fruit bushes leaves and its fruit and fern leaves. The 137Cs content was measured by high resolution gamma-ray spectroscopy by a comparative method with reference materials. The 137Cs activity values range from 16.3 to 30.8 Bq·kg-1 in the soil samples collected in July 2003, 20.7–32.1 Bq·kg−1 for the black mat, 26.3–38.4 Bq·kg−1 for the palm leaves, 16.8–31.2 Bq·kg−1 for the palm trunks and 17.6–27.3 Bq·kg−1 for the fruit bush leaves, while, the 137Cs activity values for the whole fruit were between 23.4 and 30.7 Bq·kg−1; but, the value of the 137Cs activity in the center of the fruit (the edible part) was 51.6 Bq·kg−1, and the value of the 137Cs activity for the fern leaves was 51.8 Bq·kg−1. Thus, most of the 137Cs activity values determined in the soil, black mat and vegetation samples from El Mirador (Lookout) were considered anomalously high with respect to those found near the equator and in other areas of Venezuela. Only the center of the fruit from the Clusia grandiflora bushes and the fern leaves had high activity ratios, about a factor of three and could be considered as biomonitors that concentrate and retain the 137Cs. Finally, these anomalously high 137Cs activities have been attributed not only to the rich organic soils, as sinks, but also due to the affect of the cloud forests.