Wednesday, 1 September 2021

Estimating the risks presented by radon gas from geological sources in Perak State, Malaysia.

Radon is a naturally occurring radioactive noble gas produced as part of the decay chain of the readionuclides uranium²³⁸, thorium²³³, and uranium²³⁵. The most abundant isotope of radon is radon²²², which is a part of the uranium²³⁸ decay chain, which leads eventually to lead²⁰⁶, although there are several further stages in the decay chain between radon²²² and lead²⁰⁶, all of which are reactive and radioactive heavy metals. Radon²²² is naturally given off by many granitic rocks, and unlike most other radionuclides, can be easily inhaled, making it responsible for about 50% of all Human exposure to ionising radiation globally, and the leading cause of lung cancer after smoking internationally, according to the International Agency for Research on Cancer, with prolonged exposure to even relatively low levels of the gas presenting potential health problems. Radon is particularly prone to accumulating in buildings built upon rocks or soil which emit the gas, which makes assessment of the risks presented by radon an important part of the planning process in areas where it can be a problem.

There are essentially two ways to assess the risks presented by radon in any given area; either measure the accumulation of the gas within existing buildings, or assess the dangers presented by the local geology. The latter method is generally preferable, and may be done directly measuring radon gas in the soil, by measuring gamma radiation in the area, or by measuring the uranium and/or radon levels in rocks and soils remotely during aerial surveys.

The potential of a soil or geological formation to release radon into the environment is known as its Geogenic Radon Potential, which is often used to assess the risks of radon accumulating within buildings, although sometimes a combination of radon activity concentration and soil permeability is used to construct hazard maps.

Relatively few measurements of radon in soil in Malaysia have been undertaken, although those that have produced readings as high as 375.42 kBq/m³ (readings of above 300 375.42 kBq/m³ are generally considered harmful). No data appears to have been collected on the hazards presented by radon in Perak State on Peninsula Malaysia, despite the presence of extensive granite outcrops, considered to present the highest geologic risk of radon release.

In a paper published in the journal PLoS One on 28 July 2021, Habila Nuhu of the Department of Physics at Universiti Teknologi Malaysia and the Department of Science at Plateau State Polytechnic, Suhairul Hashim also of the Department of Physics, and of the Ibnu Sina Institute for Scientific and Industrial Research at Universiti Teknologi Malaysia, Muneer Aziz Saleh of the Nuclear Engineering Programme at Universiti Teknologi Malaysia, Mohamad Syazwan Mohd Sanusi, again of the Department of Physics at Universiti Teknologi Malaysia, Ahmad Hussein Alomari of the Energy and Minerals Regulatory Commission in Amman, Jordan, and Mohamad Hidayat Jamal, Rini Asnida Abdulla, and Sitti Asmah Hassan, of the Faculty of Engineering at the Universiti Teknologi Malaysia, present the results of a field study of radon activity concentration and soil permeability in Perak State, Malaysia, undertaken with a view to creating a Geogenic Radon Potential map of the state, which should be of use to future planners.

Perak province is located in the northwest of Penninsula Malaysia. The peninsula is made up of two tectonic blocks, the Sibumasu Block to the west and the Sukhotai Arc (or Eastern Malaya Block) to the east. The two blocks are separated by the Bentong-Raub Suture Zone, which formed when they converged in the Triassic, to form part of the core of the Sundaland Continental Block, which comprises much of modern Southeast Asia. Much of the Sibumasu Block is covered by Peninsula Malaysia's Main Granite Range. The area is divided into four different geological regions, Quaternary succession of continental and shallow marine sediments, a Triassic-Jurassic succession of deeper marine sediments with volcanic tuff (ash) beds, a Silurian succession of sediments and metamorphic rocks, and a suite of intrusive rocks, mainly granite. Location sites for sampling were chosen at random across the state, but attempting to include each major rock and soil type; private land or land where permits were needed to carry out investigative work were also excluded. At each test site the local geology was recorded, as was any previous sampling work by other researchers. A total of 70 sites were visited.

Geological map of Perak state. Nuhu et al. (2021).

Nuhu et al. detected radon activity concentrations of between 0.11 and 434.5 kBq/m³ at localities across Perak State, with an average reading of 18.96 kBq/m³. The majority of the readings (81%) were low (defined as below 20 kBq/m³), while 13% were high (between 20 and 50 kBq/m³) and 6% very high (more than 50 kBq/m³). Soils derived from granite rocks, and young aluvial (river) sediments (which probably also contained material derived from granites, were found to have elevated radon activity, with average radon activity concentration values of between 4.28 and 44.48 kBq/m³. The highest value recorded in the study, of 434.5 kBq/m³, was obtained from the most western granitic region of the state. Since radon is associated with granite rock, these findings were in line with predictions. Moderately high readings were also associated with soils from mined and urban areas, as well as soils derived from sedimentary rocks. In these areas radon activity concentration values ranged from 8.48 to 9.62 kBq/m³. The lowest values were obtained from soils derived from marine sources on the coastal plains. Here the highest reading was 27.6 kBq/m³.

Map of radon activity concentration in soil gas Perak state. Nuhu et al. (2021).

Both soils derived from Quaternary sediments and those from Silurian rocks had average radon activity concentration values below the overall for the province, with Quaternary rock-derived soils producing an average reading of 1.13 kBq/m³, and Silurian rock-derived soils producing an average reading of 1.4 kBq/m³. Soils derived from the Triassic-Jurassic rock sequence were far closer to the average for the state, with an average reading of 1.97 kBq/m³ from Triassic-Jurassic rock-derived soils and an average of 1.98 kBq/m³ for the state as a whole. Readings in these soils ranged from 1.0 to 106.5 kBq/m³. Readings from areas with intrusive igneous rocks were the highest, ranging from 9.81 to 434.5 kBq/m³.

Box plot of radon activity concentration in geological formations of Perak state. Nuhu et al. (2021).

The release of radon gas into the environment is also partially controlled by soil permeability, with higher permeability soils allowing more easily the upward migration of the gas. Nuhu et al. divided soils into three classes (High, Medium, and Low). Soils derived from Silurian and Quaternary sources tended to have the lowest permeability, with those of Triassic-Jurassic rock-derived soils being intermediate, and the highest permeability being found in rocks derived from granites. Geographically, the least permiable soils were found on the coastal plains of the west of the state, median values were found in the northern, central, and southern areas, and the most permeable soils were found in the eastern part of the state, where the rocks are primarily granitic.

Soil permeability map of Perak state. Nuhu et al. (2021).

Finally, Nuhu et al. calculated the geogenic radon potential for each area of the state, by combining the data on radon production and soil permsability, dividing the state into low-, medium-, and high-risk areas. This tended to follow the local geology, with high risk zones primarily occurring in areas with granitic bedrocks, medium risk zones in areas with Triassic-Jurassic bedrocks, and low risk zones found in areas with Silurian or Quaternary bedrocks.

Geogenic radon potential map of Perak state Malaysia. Nuhu et al. (2021).

Nuhu et al. produced a number of maps outlining the risks presented by geogenic radon gas in Perak State, Malaysia. The highest risks were found to be in the central and eastern parts of the state, where soils tend to be derived from intrusive granitic rocks, with the lowest risks being found in the southeast and north. However, this is not an absolute rule, and there are areas with higher risks than would be predicted by simply extrapolating from the geology, and it is possible that other higher risk pockets exist undetected in the areas classified by Nuhu er al. as low risk. These maps produced are intended be useful as a base for authorities monitoring radon control and mitigation, in dwellings and workplaces.

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