Assessment of indoor radon concentration levels in Serule, Botswana

Abstract

Discoveries of huge uranium deposits along the Letlhakane and Serule area in Botswana, led and motivated various researchers to investigate the long term effects of these discov eries. In addition to economic benefits, the possibility of environmental pollution is also high. This study assesses the indoor radon concentration levels in Serule area. Radon ( 222Rn) is of interest because, it is a daughter radionuclide of Radium-226 (226Ra) from the Uranium-238 (238U) decay series and can cause adverse health problems such as lung cancer when inhaled. It is odourless, tasteless, colourless and there is no way of knowing how much of it you are exposed to unless you measure it. The indoor radon concentration levels were measured in 10 houses in Serule using passive radon monitors called the E-PERMS (Electret-Passive Environmental Radon Monitors) and 2 houses in Palapye were used as the control. Of the 10 houses in Serule, 6 houses were within the proposed mining area and 4 outside the mining area. The weighted average indoor radon concentration in Serule area from the sampled houses, was found to be 23.4 ± 2.1 Bq/m3 and from the control houses it was 7.0 ± 0.3 Bq/m3 . The results obtained are below the recommended limit of 300 Bq/m3 as per the IAEA regulations. It was also observed that, the highest radon concentration levels were recorded in the summer and winter seasons. The building materials used in the construction of the sampled houses along with the ventilation habits of the inhabitants also influenced the amount of radon concentrations recorded. The main contributor however, was the uranium rich underlying rocks as the radon concentration was higher in areas within the mining area than those vioutside. The average annual effective dose received by the Serule residents in the sampled houses from inhalation of radon was calculated to be 0.67 ± 0.23 mSv and only 0.1764 ± 0.0071 mSv was received in the control houses. The obtained results are below the 3 - 10 mSv dose limit recommended by the ICRP. Soil samples from the vicinity of the sampled houses were collected and counted using a High Purity Germanium (HPGe) detector at iThemba LABS. Gamma spectrometry was used in this case to obtain the activity concentration of each radionuclide contained in a given sample. The activity concentrations of 238U, 232Th and 40K obtained ranged from 6 - 658 Bq.kg−1 . The highest activity concentration of 238U was found in areas within the mining area, and the activity concentration of 232Th was generally low across all areas sampled. The activity concentration of 40K showed high levels in sampled areas outside the mining area and this might be related to potassium rich fertilizers used for agricultural productions in their homes. The radiological hazard assessment indicated that the highest radium equivalent (Raeq) value calculated was 164.1 ± 12.2 Bq.kg−1 from SSH3 which was within the mining area. The results for all other Raeq values obtained in this study are below the 370 Bq.kg−1 limit set by IAEA. Two sampled locations within the mining area (SH3 and SH4) had the highest absorbed dose rate of 75.0 ± 12.5 nGy.h−1 and 69.6 ± 10.4 nGy.h−1 respectively, which were higher than world average value of 57 nGy.h−1 . The calculated external and internal hazard indices of all the twelve soil samples were below 1, with the highest value being 0.7 meaning the risk due to exposure is generally low. The heavy metals concentration in the soil samples were determined using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Only nine of the heavy metals were of interest, that is, chromium, nickel, arsenic, copper, zinc, cobalt, lead, mercury and cadmium. Soil sample SSH7 had the highest concentration of heavy metals, with the nickel and cadmium concentrations being higher than the maximum allowable limit according to WHO/FAO guidelines of 50 mg/kg and 100 mg/kg with values of 109.282 mg/kg and 158.213 mg/kg respectively. Soil samples collected in Serule had slightly higher heavy metals concentration compared to those from the control area