All humans are receiving radon generated by Naturally Occurring Radioactivity Material (NORM) in soil, air, water, and food in both outdoor as well as indoor environments. Because of the long half-life of Radon (222Rn), it has a much better chance of escaping from the material in which it was formed. It enters the interior environment as a soil gas from the earth, as well as through walls, ceilings, and other construction materials utilized in dwellings (Kumar et al., 2013). Radium (226Ra) is the primary source of radon gas, formed by the disintegration process of the 238U series. Contribution to the dose in human from some of the other natural radionuclides (235U series, 87Rb, 138La, 147Sm, and 176Lu) are very small as these natural radionuclides also exist in the environment (Anamika et al., 2020). The bulk of buildings' radon comes from soil gas (Li et al., 1992; Nero & Nazaroff, 1984). A complete understanding of the generation and transportation of radon through soil is necessary to comprehend the radon flow and entrance rates into the structures (Ramola, Choubey, et al., 2008; Ramola & Choubey, 2003). The activity concentration of 222Rn in dwellings is governed largely by exhalation from building materials, soil, and other sources, and secondly by environmental factors such as ventilation rate, temperature, pressure, and humidity (Ramola et al., 2006; Singh et al., 2023). The radioactive gas radon reaches the environment through two processes: emanation and exhalation. Radon gas (222Rn) escapes from grains of solid mineral to air-filled pores in the soil matrix by the process of emanation and then it is transported from the soil matrix to the earth's environment through the processes of exhalation (Ahamad et al., 2021).
Exhalation and emanation rate of 222Rn in soil sub-surfaces depend on several factors such as; 226Ra content in soil or rocks, temperature, humidity, and most importantly by regional geology and climate (Johner HUU et al. 2001; Prasad et al. 2008). Exhalation and emanation are the primary contributors, whereas humidity, temperature, pressure, ventilation rate, as well as other variables also affect the transport of radon (222Rn) concentration level in the dwelling from the outdoor environment. (Semwal et al., 2018). Other governing factors for 222Rn concentration are the type of house (mud, stone, cement), building materials (rocks, sand, wood), wind speed (usually high to low pressure), and even the routine of the individuals living in the house (Abd El-Zaher, 2013; Martz et al., 1991; Subba Ramu et al., 1988). The Radon (222Rn) exhalation rate cannot be calculated directly from 226Ra activity concentration rather than radon concentration of surface and mass should be measured first, and then a simple calculation yields the radon exhalation rate. Natural radiation accounts for approximately eighty-seven percent of the total radiation dosage received by the inhabitant, with the remainder contributing from manmade radiation (UNSCEAR 2008). The average value of Annual Effective Dose (AED) per capita to humans is 2.4 mSvy− 1 worldwide (UNSCEAR. 1993;2000). However, significantly larger levels of primordial radionuclides in the soil possess a risk to human health (Anamika et al., 2020).
According to United Nations for Scientific Committee on the Effect of Atomic Radiation (UNSCEAR), Radon is the second leading cause of lung cancer after smoking (UNSCEAR 2000). Nearly 52% of the overall dosage from ionizing radiation is contributed by radon (222Rn), thoron(220Rn), and their decay products, which has an impact on the general population (UNSCEAR 2000; Semwal et al. 2018). Therefore, the radon (222Rn) activity concentration and its exhalation rate from both mass and surface in soil have received special attention to analyzing the effect of radon in the atmosphere. In the current study, measurement of radon concentration of soil gas at various depths and exhalation rate (surface and mass) of radon in the soil samples of different locations with the gamma dose rate of each sample was carried out around the Main Central Thrust (MCT) of Garhwal Himalayan region, India.
Geology of the study area
The present study area extends from 30.74⁰ to 30.81⁰ N and 78.57⁰ to 78.62⁰ E (Fig. 1). Mass and surface exhalation rates were estimated for the soil samples of 61 grids out of the whole area of Sainj, Lata, Malla, and nine other villages of Bhatwari block, Uttarkashi district in Uttarakhand. The Garhwal Himalaya stretches from north of the Indo-Gangetic Plains to the higher Himalayas. It also consists of the outer and lesser Himalayas (Ramola, Prasad, et al., 2008; Singh et al., 2023). The Southern Tibet Detachment is located between the High Himalayan Crystalline (HHC) rocks and overlying Tethyan sedimentary rocks which is generally known as the North Himalayas. Based on their research, (Burchfiel et al., 1992; Gapais et al., 1992) concluded that the High Himalayan Crystalline was created by a significant deformation incident that diverted the Indian plate crust above the MCT. It is a thrust fault that connects the lesser Himalayan to the higher Himalayan having low and high-grade metamorphic rocks, respectively (Gansser, 1964; Heim & A. Gansser, 1939). It is a tectonically active zone causing several natural calamities, such as landslides, earthquakes, rock falls, etc. The main source of earthquakes in this area is the movement of the Indian plate (about 50 mm year− 1) towards the Eurasian plate. One of the main causes of change in its geological parameters is the formation of weak plains and several active faults in this area having frequent tectonic activities (earthquakes, volcanoes, etc.) per year. MCT is separating the Garhwal group of Gamri quartzite from the Central Crystallines, which can be found in the Uttarkashi region along the Kumalti stream and extends southeast up to Siab (Gupta, 1977). The main central thrust zone of the study area contains a large number of other active faults(Bourai et al., 2013).
Gneiss, schists, and meta basics rocks are found in the High-grade Central Crystalline Group metamorphic as well as slates, dolomites, and quartzites from the Garhwal Group of Himalayas. These rocks have been sheared and worn extensively (Valdiya, 1980). Soil samples collected as well as studied from the focused area for mass and surface exhalation are red, black, dark black, chocolaty brown, and sand types in colors (Singh et al., 2023; Yadav et al., 2016). Many studies were done previously in the present study area one of them was done for a limited number of samples at a large distance (Bourai et al., 2013) but in the present study, a comprehensive study with a grid pattern was done around the MCT zone.