Geogenic radon and thoron potential mapping in Johor State, Malaysia

Abstract

Public radiation exposure to natural ionizing radiation is due to radon and its progeny. Knowledge of natural radioactivity exposure level is significant for making policy regarding radiological protection of the environment and humans. This study aims at establishing baseline data and identifying areas with the probability of high radon (222Rn/220Rn) exposure in Johor State, Malaysia. Therefore, The RAD7 alpha detector coupled to air sampling accessories, soil gas probe and RAD-H2O was used to measure the activity concentrations of 222Rn/220Rn in outdoor air, 222Rn /220Rn in soil gas and 222Rn in water, respectively. The RAD7 recorded the average temperature and relative humidity during measurement of 222Rn/220Rn in soil gas. The data for soil gas permeability was obtained with RADON-Joke equipment. The terrestrial gamma dose rate was measured using a portable NaI (Tl) survey meter. The specific activity of 226Ra, 232Th, and 40K in the soil samples was determined using a high purity germanium detector (HPGe). The established data range from minimum detectable activity (MDA) to 127.25 ± 3.00 Bq L-1 for 222Rn and MDA to 159.07 ± 3.40 Bq L-1 for 220Rn in soil gas, respectively. The data for 222Rn and 220Rn in outdoor air range from MDA to 3850 ± 180 mBq L-1 and MDA to 600 ± 17 mBq L-1, respectively. The measured data categorized according to the study area's geological formations show that higher values of 222Rn /220Rn in both soil gas and outdoor air were obtained in regions underlain with Triassic and Intrusive rock geological formations. The soil gas permeability data has a mean value of 1.9 x 10-12 m2. The field data obtained from the measurement of 222Rn in soil gas and soil gas permeability were used to estimate the geogenic radon potential (GRP) of this study area. Three high categories of GRP values were identified (53.667, 53.252 and 47.826). Statistical correlation analysis indicates that the estimated GRP data is strongly correlated with the measured 222Rn/220Rn in soil gas and soil gas permeability. In contrast, an insignificant relationship was obtained between the measured 222Rn/220Rn in soil gas and the measured 226Ra/232Th in the surface soil. The recorded relative humidity was found to have a moderately negative correlation with 222Rn in soil gas. The measured data of 222Rn activity concentrations in water varies from 80 ± 110 to 5400 ± 1100 mBq L-1 in surface water and spring water source, respectively, with a mean value of 1227 mBq L-1 from all samples. The water samples measured activity concentration was found to be below the maximum permissible limit for 222Rn in water referring to United States Environmental Protection Agency (EPA) and World Health Organisation which is 1100 mBq L-1 and 105 mBq L-1, respectively. The mean activity concentration of 222Rn in spring water is five times higher than that of surface water. The mean values of the annual effective dose due to inhalation of 222Rn in spring water and surface water, were 2.15 |iSv y-1 and 0.423 |iSv y-1, respectively. Hence, the inhalation doses estimated were well below the recommended limit set by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) of 1260 |iSv y -1. The maps of spatial distribution of 222Rn/220Rn in soil gas and soil gas permeability are created and indicates that higher values of 222Rn and 220Rn were obtained from Ledang, Muar and Johor Bahru districts.