Natural Radioactivity In Soils Of The Walvis Bay – Henties Bay Coastal Area, Namibia

ABSTRACT

Naturally occurring radionuclides such as 238U, 232Th and 40K are present in the soil where they disintegrate spontaneously releasing ionizing radiation which could pose health hazards to the inhabitants of a given location especially when the concentrations of the radionuclides are high. In Namibia, the Walvis Bay – Henties Bay coastal area is known to have many mineral resources including uranium and therefore the concentrations of the radionuclides in the soils and the associated radioactivity may be high in some towns in the area. It is therefore important to study the concentrations of radionuclides and the radioactivity in the soils of the major towns in the area. In this study, the concentrations of the radionuclides 238U, 232Th and 40K and their progeny, and their contribution to natural radioactivity in the soils of six major towns in the Walvis Bay – Henties Bay coastal area have been determined. These towns are Usakos, Arandis, Swakopmund, Walvis Bay, Wlotzkasbaken and Henties Bay.

A total of 305 soil samples were collected across the six towns and dried under laboratory temperature for 72 hours. These samples were pulverised and passed through a 2 mm mesh screen and sealed in 500 ml polythene bottles. The specific activities of the radionuclides 238U, 232Th and 40K in the samples were subsequently analysed using a well-shielded high-purity germanium (HPGe) detector. The average concentration for 238U varies from a minimum of 14.5 ± 2.5 Bq kg-1 in Walvis Bay to a maximum of 104.1 ± 28.1 Bq kg-1 in Wlotzkasbaken, with a mean of 52.6 ± 24.5 Bq kg-1 for the six towns. Also, the average concentration for 232Th varies from a minimum of 18.0 ± 3.0 Bq kg-1 in Walvis Bay to a maximum of 334.4 ± 86.9 Bq kg-1 in Arandis, with a mean of 104.3 ± 85.9 Bq kg-1 while the average concentration for 40K varies from a minimum of 363.3 ± 41.3 Bq kg-1 in Walvis Bay to a maximum of 1336.5 ± 142.0 Bq kg-1 in Usakos, with a mean of 778.7 ± 204.6 Bq kg-1 for the six towns. These values were used to calculate the mean absorbed dose rate and the annual effective dose. The average annual effective dose varies from a minimum of 0.04 ± 0.01 mSv in Walvis Bay to a maximum of 0.35 ± 0.07 mSv in Arandis, with a mean of 0.15 ± 0.08 mSv for the six towns. These values are lower than the maximum permissible limit of 1.0 mSv recommended by the International Commission on Radiological Protection (ICRP) for the public so that the towns do not have high natural radioactivity.