Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India

Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India John Samson M, Manikanda Bharath K, Vidyasakar A, Ajith Kumar K, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8281563/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Apr, 2026 Read the published version in Environmental Geochemistry and Health → Version 1 posted 13 You are reading this latest preprint version Abstract The short-lived progenies of radon, thoron (²²⁰Rn), and radon (²²²Rn) are major global sources of ionizing radiation exposure and require continuous environmental monitoring because of their inhalation-related health risks. This study investigated the spatial distribution of soil-gas radon and thoron, their mass and surface exhalation rates, and the mineralogical controls on beach sands along the Chennai megacity coast in Southeast India. Twenty-four intertidal soil samples were analyzed using a RAD7 radon–thoron monitor (Durridge Co., USA), an electrostatic solid-state alpha detector optimized for thoron measurements. Radon mass exhalation rates ranged from 2 to 12 mBq/kg/h (mean: 2.42 mBq/kg/h), whereas thoron surface exhalation rates varied widely from 162 to 31,623 Bq/m²/h (mean: 3,688.08 Bq/m²/h). The highest soil-gas radon levels (4.5–12 Bq/m³) were observed in Kokkilamedu, a placer-rich zone. Exhalation rates were inversely related to grain size, with finer sediments releasing more radon and thoron than coarser sediments. However, higher heavy-mineral content, greater bulk density, and finer grain fractions also restricted exhalation, indicating strong mineralogical and textural control of radionuclide mobility. The sediments contained up to 12.5% heavy minerals, including ilmenite, zircon, and monazite, contributing to gamma radiation levels of up to 7.5 µR/h. The annual effective radon doses ranged from 0.12 to 0.45 mSv/y, remaining below the global safety limits. Elevated thoron exhalation in monazite-rich areas highlights the need for regular radiological surveillance, particularly among placer-deposit workers. These results provide baseline data on coastal sediment radiology and provenance to support environmental risk management and public health planning in the Chennai coastal zone. Soil-gas radon Thoron concentration Radon/thoron exhalation rate Heavy mineral beach placers Geological setting RAD7 alpha detector Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Naturally occurring radioactive gases, in particular, the radon and thoron, are a notable category of emerging contaminants of geogenic sources that pose meaningful implications to the environmental exposure and human health (Haman et al., 2025). Their formation and spatial distribution are controlled mainly by the geological and geochemical properties of soils and sediments, in particular, the occurrence of uranium-, thorium-, and radium-minerals (Popoola et al., 2025). Heavy minerals like monazite, zircon and ilmenite found in mineralized beach sands are known to release significant quantities of radon and thoron, which are important ecological processes, environmental behaviour of radionuclides and health risks of communities in the One Health environment (Nebin et al., 2025). Radon and thoron move along multiple environmental paths, such as rocks, unconsolidated sediments, soil gas, groundwater, surface water, and ambient air, and have complex physicochemical and bioprocess pathways (Malik et al., 2025). Although these gases are usually diluted to non-hazardous levels in open and well-ventilated environments, they may become concentrated in space-constrained and highly poorly-ventilated environments to cause increased exposure and risks of respiratory diseases and lung cancer (Caridi et al., 2025). Their ecological behavior, mobility, and exhalation nature is thus important to quantify ecological safety and eliminate environmental health risks (Ali et al., 2025). These problems are magnified in urban settings, especially in megacities that are growing very fast, where land-use alterations, coastal development, and soil disturbances, as well as human-made changes in natural substrates, occur (Hanfi et al., 2025). Urban soils are usually the reservoirs of naturally occurring radionuclides and other contaminants and, therefore, they contribute to outdoor gamma radiation fields and risks of long-term exposures (Assefa & Arbese (2025)). Radon and thoron as natural terrestrial radiations, make a significant portion of the radiation dose on the earth and a measurement of their levels in ecologically sensitive and densely populated environments is important in overall environmental-health risk management (Elmehdi et al., 2025). The Chennai megacity coastline in the southeast coast of India is a dynamic ecological environment of heavy-mineral-laden beach sands, high urbanization and growing human-environment contact. Although its importance is significant to the environment, there has been paucity of systematic evaluation of the soil-gas radon and thoron exhalation in this area. This knowledge gap is referred to by the paper, which discusses the spatial distribution of radon and thoron emissions of mineral-enriched beach sediments using the RAD7 electrostatic alpha detector, which is an established method of measuring short-lived radionuclides in the environmental matrices. In keeping with the objectives of Environmental Radioactivity, the following research will attempt to: (i) describe the background radiations surrounding the Chennai coastal urban system; (ii) determine spatial distribution and environmental radiocarbon behaviour of radon and thoron in beach sediments; (iii) investigate the effects of mineralogical composition, such as heavy minerals and rare earth elements, on exhalation of radionuclides; and (iv) evaluate possible environmental and human health hazards in relation to the radionuclide exposure in a high-populated megacity on the coast. Through a combination of geochemical, ecological, and environmental-health, the results offer baseline information that is critical in environmental management of the coastal environment as well as urban mitigation of health risks and future research of One Health oriented studies on the coastal ecosystems. 2. Location of the study area The study area is a coastal stretch of about 150 km long, the location of the Palar River on the south and Pulicat Lake on the north, of Kadalur Kuppam and Pulicat Lake at a distance of about 150 km respectively. This area is geographically located between 12°9' and 13°9' within the N and 80°12’ to 80˚19’ within the E, a region within a low-lying coastal plain on the south eastern border of the Indian subcontinent which comprises the Chengalpattu, Chennai and Tiruvallur districts its shown in the Fig. 1 . Enclosed on the east by the Bay of Bengal, and spanning almost 200 km 2 , the coastline is an environmentally sensitive, but highly urbanized zone, that is still in the process of being transformed at a rapid rate, environmentally and infrastructurally. The coastal zone accommodates various industrial and service industries, among them an automotive production industry, healthcare, information technology services, and high-volume port, which have an effect on the land-use patterns, redistribution of sediment and movement of naturally occurring contaminants (Karuppasamy et al., 2024). The climate of the region is tropical monsoon climate during the period of June to September, northeast monsoon climate during the period of October to January that brings most of the annual rainfall and finally semi-monsoon climate between the months of February and May. These seasonal changes have a very strong impact on the sediment moisture regimes, coastal processes as well as the environmental behavior of radon and thoron in beach sands. Morphodynamically, the Chennai coastline is active and this occurs due to the recurrent wave action, tidal forces and longshore sediment transportation. In particular, the middle and northern stretches are affected by variation in littoral drift and waves that cause periods of erosion and accretion. The anthropogenic interventions such as harbour’s, breakwaters, jetties, groynes, and networks of canals have also caused profound changes in the sediment pathways and also lead to a localized geomorphic instability. These artificial structures, in combination with the natural hydrodynamic forces determine the distribution of heavy-mineral-rich sands, which are considered sources of geogenic radionuclides in the area. The lower parts of large city rivers like the Coovam, Adyar, Ennore, Palar which flow through fragmented wetlands and estuarine systems before reaching the coast are also within the study area. Their acts on the coastal environment also cause heterogeneity in the sediments, geochemical variability and the differing conditions of exposure. All of this creates a multiplier effect between the rapid urbanization, the growth of industries, the natural processes of the coasts, and the man-made interventions of the coastlines to create this coastal sector that is a critical landscape to evaluate the spatial variability of environmental radioactivity and resultant ecological and human health hazards in an integrated framework of One Health. 2.1 Sampling and laboratory procedures To determine the environmental behavior of naturally occurring radionuclides, soil-gas radon and thoron exhalation rates were determined using mineral-rich beach sediments collected within the intertidal area of the south east coast of India. The sampling was done on a depth of about 1 m to reduce the effects of short-term atmospheric changes like variation in air pressure, temperature, and humidity which can largely affect the near surface concentrations of gases its sown in the Fig. 2 . In every sampling site, approximately 1 kg of the subsurface sediment was collected using clean plastic bags and only undisturbed soil was collected. The surface materials were avoided on purpose since the radium particles containing radium are subject to energy and motion of wind and waves, which may cause biased measurements of the release of radon and thoron. Minute gravel and coarse fragments were avoided to ensure uniformity and analysis. All the samples were sealed, labelled appropriately, and taken to the laboratory where they were air-dried thoroughly to get all the moisture out of them, which would have affected the emanation rates. The dried sediments were then homogenized and sifted using a mechanical sieve shaker to sift through 1 mm mesh before instrumental analysis was done to give homogenized grain-size fraction. 2.2. Data Collection and experimental setup Active monitoring of radon and thoron was done in this study by the application of two complementary measurements to the evaluation of the soil-gas concentrations and exhalation rates of the coastal sediments within Chennai region. The main equipment utilized was the RAD7 electronic radon detector which was used to measure the mass exhalation of radon, surface exhalation of thoron and active soil-gas concentrations in the sampled sediments its shown in the Fig. 3 . Radon-222 and thoron-220 were the analytical objects; inert radioactive gases produced in the decay sequence of uranium-238 and thorium-232. Their vastly different half-lives 3.8 days of radon and 55 seconds of thoron have a major role to play in defining their emanation effects, their behaviour in porous beach sediments and their ultimate atmospheric dispersal. As radium is emitted by mineral grains, the radon isotopes that escape into the air-filled pores move to the surface of the soil, the concentration and the rate of exhalation may be measured using the RAD7 (RAD7 Radon Detector 2015). The experimental apparatus used to measure soil-gas radon was the circulation of air in a closed sample chamber into the RAD7 detection cell as shown in Fig. 3 . In the detector, the radon-222 breaks down to give positively charged polonium-218 ions which are attracted to the active surface of the detector electrostatically. The emitted alpha particles cause electrical pulses with energies that are equivalent to the expected alpha emissions of particular isotopes. These energy-specific pulses are amplified and classified by RAD7, and the alpha signatures of radon and thoron may be discriminated accurately. The instrument is used in SNIFF mode, where real-time, fast detection is expected, and use of majorly polonium-218 to measure radon concentration and polonium-216 to measure thoron concentration and inhibit other longer-lived progeny. This design provides a rapid reaction to temporary changes in the concentration and avoids signal saturation in case of high concentrations. Radon disintegrates in a clear sequence, which is, polonium-218 to lead-214 to bismuth-214 to polonium-214, then lead-210 which emits the characteristic radiation, yet RAD7 selects alpha emissions to give stable radon readings. Thoron measurement works with the same principle except that its half-life is extremely short which allows it to be detected mostly near the source. Thoron is unstable and breaks down to stable lead-208 via polonium-216, lead-212, bismuth-212 and polonium- 212 with high-energy alpha particle emissions occurring at various steps. Although thoron does not have a long half-life, it is nonetheless useful in the study of the environment where thorium is present in high amounts, e.g. in heavy-mineral beach sands on the Chennai coast. Measurement data of all the RAD7 were then processed and analyzed through the CAPTURE software of DURRIDGE to maintain quality control, consistency in the analysis, and standard reporting of radon and thoron activity. 2.3. Measurement of radon thoron exhalation rate An active soil-gas extraction technique was used to determine radon-thoron exhalation rate of coastal sediments, in which a hollow stainless-steel probe was drilled to a depth of 25 cm to sample undisturbed subsurface air (Kansal & Mehra (2013)). A dust filter followed by a desiccant tube containing calcium sulfate (CaSO 4 ) was used to dry the probe and eliminate moisture and dust particles to maintain the relative humidity under 10% level, which is the limit needed to guarantee optimal performance of the detector (UNSCEAR, 2011). The samples of soil-gas were gathered in grab mode within 5 min followed by 5-min decay period to enable dissipation of the thoron, which has a short half-life. The measurements of the four 5-minute cycles followed in a 20-minute period were then obtained. To minimize the possibility of intrusion of ambient air at time of sampling, care was taken and before sampling at each new location the RAD7 detector was carefully purged to minimize the residual background activity and offer consistency between the dataset samples. The accumulation chamber method was used to determine the rate of radon exhalation (J) of the soil surface: \(\:J=\frac{\left(C-{C}_{0}\right)V}{A\times\:t}\:\:\) mBqm −2 s −1 ………..(1) where C is measured radon concentration at the surface of the soil (Bq m -3), C 0 is background radon concentration (assumed zero in air that is radon-free), V is sum of effective volume of the chamber and the accumulation chamber (m 3 ), A is area of the soil covered by the chamber (m 2), and t is the time during which the accumulation took place (s). To assess possible health consequences in the population, the annual attributable dose of radon outside of residences was estimated by means of: $$\:AED={C}_{Rn}\times\:F\times\:O\left(DCF\right)$$ 2 …………. In which C Rn is the concentration of radon in the out air (Bq m − 3 ), F is the global average equilibrium factor of the radon and its progeny (0.6), O is the outdoor occupancy factor (1760 h − 1 ) and DCF is the dose conversion factor (9 nSv h − 1 per Bq m − 3 ). These are computations that are made based on recommended international standards and additional methodological information can be found in the U.S. EPA guidance document Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 402-R-92-004) that still continues to serve as a standard reference of environmental radon and thoron measurements. 3. Results and Discussion 3.1. Radon and Thoran concentration in soil gas Radon, thoron and their sources are also regularly checked indoors and outdoors because these gases have a significant health hazard in case of inhaling them. The rate of radon mass exhalation and the rate of the surface exhalation (thoron) were assessed on varying grain-size fractions in soil in the current study it shown in the Figs. 4 and 5 . The exhalation rate of radon mass measured 2–12 mBq kg − 1 h − 1 and thoron surface exhalation rate was measured 162 − 31,623 mBq m − 2 h − 1 . Radon and thoron had minimum detectable activities of 2.34 mBq kg − 1 h − 1 and 162 Bq m − 2 h − 1 respectively. The findings confirmed that the radon and thoron exhalation rates were the greatest in samples that had smaller sizes of particles, especially those with diameters of approximately 150µm, which implied that smaller particles enhanced diffusion and emission of radioactive gases. The highest concentration of soil-gas radon was registered between 4.5 and 12 mBq kg − 1 h − 1 at Kokkilamedu and this can be attributed to the existence of heavy minerals as a result of the placer deposits of local source rocks. The average radon, thoron and progeny annual concentrations and the dose rates were found to be within the stipulated limits of UNSCEAR (2011). The average radon exhalation rates of past researchers, which were 23 Bq m − 2 h − 1 in sand, 17 Bq m − 2 h − 1 in sandy loam and 15 Bq m − 2 h − 1 in clay loam are comparatively evidence that, natural soils are mostly higher in radon exhalation than most building materials. This is mainly explained by the fact that they are more porous and also by the heightened mobility of soil-gas radionuclides. When the Earth was formed several billion years ago, several naturally existing elements of radioactive content were incorporated into the Earth, and some of them; uranium-235, uranium-238, and thorium-232 are today present in measurable amounts (Jegede et al., 2025). They have extremely long half-lives, which are in the billions of years range, therefore the nuclides remain in nature and hold the first places in the natural radioactive decay series. The radioactive elements are not stable and they will eventually decay into other elements emitting radiations (Popoola et al., 2025). Even though it is impossible to predict what time exactly will be the time of the decay of one single atom, it is known that there are certain probabilities of the probability of the decay at a certain period of time. The measurement of radon gas in the atmosphere air and soil gas as well as building materials (Kansal et al., 2014; Ahmad et al., 2014) is thus necessary to determine the behaviour and diffusion of radon gas in the environment. The current study has proven that the rates of radon and thoron exhalations become decreasing with the increasing grain size of the soil, which indicates that the smaller particles play a bigger role in the gas emanation compared to the larger ones. These radionuclides include ²²²Rn and ²²⁰Rn (radon and thoron), which arise from the decay chains of ²³⁸U and ²³²Th, respectively. They also include ²²⁶Ra and ²²⁴Ra, formed through the decay of their direct parent nuclides, ²³⁰Th and ²²⁸Ra. These parent radionuclides are primarily concentrated in rocks, uranium-bearing minerals, and soil (Zakaly et al., 2024). The consequent decay of radon and thoron produce radioactive isotopes of polonium, bismuth, lead and thallium, elements which are considered heavy metals (Kanse et al., 2016). These decay products can easily be attached to aerosol particles suspended in the atmosphere and hence affect their dispersion, residence time and possible radiological effect on human being and environment (Pinto et al., 2020). 3.2. Geospatial variation of the rare earth elements (REE) bearing minerals Deposits of beaches on the south eastern coast of India are majorly made up of quartz, feldspar and a collection of economically meaningful heavy minerals. India is estimated to have 348 Mt of ilmenite, 107 Mt of garnet, 21 Mt of zircon, 18 Mt of rutile, 8 Mt of monazite and 130 Mt of sillimanite, in its coastal placer deposits, which comprise almost 35% of the world ilmenite reserves, 10% of the world rutile reserves, 14% of the world zircon reserves, and an exemplary 71.4% of The investigation of these heavy mineral placers has been highlighted due to the ilmenite being the main raw material of producing white pigments, but monazite is the key raw material of thorium and rare earth elements (REE). Another mineral rich in REE is xenotime that contains a significant amount of yttrium, a common phosphor in displays and in high-technology materials like superconductors and special alloys. Industrial Garnet is also part of the industrial value of Garnet as an abrasive and in sandblasting operations, particularly following limitations on the use of quartz sand following silicosis problems. With all the strategic and economic significance, the distribution of heavy minerals along the coastal area is crucial to the regulation and management of the resources as well as monitoring controls. The separation of heavy minerals in this study was done through the methods of gravity concentration whereby bromoform was the heavy liquid. The samples of the beach sand had significantly elevated heavy mineral contents when compared to the sampling sites in the inland areas. The level of in-situ gamma radiations was recorded using scintillometer and the background radiation values were deducted to get the corrected values. The total heavy mineral content was 12.5% and the gamma radiation levels reached up to 7.5 µR/h which was significantly higher than the levels obtained in other places. There is a great deal of variability in the abundance of heavy mineral as indicated in the spatial distributions depicted in the Figs. 6 and 7 . The ilmenite was present with a range of between 17.48–76.15 showing an average of 47.37 percent of total heavy mineral fraction in the beach sands. Microscopic work also indicated a mixed mixture of heavy minerals comprising of zircon, sillimanite, kyanite, rutile, garnet, hornblende and tourmaline. The mineralogical patterns and compositional changes point towards a polygenetic origin, mainly of metamorphic origin rocks, and with the contribution of metasedimentary units of minor importance. The combined analysis of heavy mineral composition and gamma radiations in the south eastern coastal strip proves that the level of gamma radiations in the Kalanji Beach is indeed quite high, and this can be explained by the presence of zircon and monazite in the area. This was the locality that also had the highest concentration of heavy minerals compared to all other sampling points. These minerals have been attributed to the provenance of the Proterozoic rock formations located in the southern part of the study area that are the major enriches of the REE-bearing and radioactive minerals in the coastal sediments. 3.3. Geological setting and Provenance of the Beach Placers Figure 7 shows the geological structure of the study site based on Bhukosh spatial data platform of Geological Survey of India (GSI). Based on the geological map of Tamil Nadu (on a scale of 1: 50,000), the beach sands in this coastal stretch are thought to be depositions that occurred between the Quaternary and the Recent epochs. These are unconsolidated sediments, which are running between the older geology formations, which comprise Miocene-Pliocene Cuddalore Sandstone, Upper Gondwana Sandstone, and massive Archean crystalline complexes. The Archean basement is mainly charnockites and gneisses and the related basic intrusives which are together the dominant hard rock provenance of the heavy mineral assemblies identified in the coastal sediments. The mineralogical and geochemical indicators of the beach placers indicate that it has a mixed provenance with metamorphic and meta-sedimentary terrains providing the bulk of heavy minerals (Pinto et al., 2020). Charnockites and related granulite facies rocks that are rich in minerals like ilmenite, garnet, zircon, and monazite are some of the rocks that play significant roles in the sediment load in the regional river systems in the area, including the Coovam, Adayar, Ennore, and Palar rivers. Fluvial processes promote the erosion, transportation and sorting of the heavy minerals which are later concentrated in the coastline by waves, tides, longshore current and littoral drift. The environment is a dynamic coast which facilitates the deposition of valuable economically significant heavy mineral placers (Mahamood et al., 2020). The increased occurrence of ilmenite, zircon and monazite in areas like the Kalanji region, in particular, is in line with a provenance to the provenance of the Proterozoic and Archean rocks in the southern area of the study region. These sources of geology explain the patterns of enrichment of REE-bearing and radioactive minerals in the beach sediments. 4. Conclusion This research paper will give a detailed evaluation of the rate of radon and thoron exhalation in 24 samples of beach sands in Chennai coast, Tamil Nadu, India. The findings prove that the grain size of soil is a decisive factor determining the behaviour of exhalation of gases, and as grain size decreases, the rates of radon and thoron exhalation increase and as the size increases, the rates decline. The results also suggest that an increment in the heavy mineral content, the bulk density and grain size relationship is associated with a reduction in the rate of exhalation, which underscores the multifariousness of the association between the sediment texture and the radionuclide mobility. Radon gas itself is not very dangerous to human health but when it does decay, it gives off alpha radiation and forms a significant part of the natural radiation received by humans. Such offspring are able to stick to the aerosols and land on lung tissue when they are inhaled, which poses a possible long-term health risk. Mineral resource estimation of the area under study has shown that ilmenite comprises about 43 percent of the total economic mineral reserves and the other ones are zircon and monazite which comprise 4 and 1 percent respectively. These minerals are of particular importance especially monazite as they contain REE and thorium. The results of this study form the necessary background data on the management of coastal resources, radiation safety, and occupational health priorities. Employees operating on the extracting, processing, and managing of monazite-laden beach sands are at a risk of getting high amounts of radon/thoron and their daughters. Thus, adoption of proper measures of protection, constant monitoring and maintenance of radiological guidelines is of vital importance in reducing radiological risks to the workers and the local population. The findings establish the foundation of a line of future geochemical, environmental, and radiological research that would be essential in the management of the coastal placer deposits in the sustainable way. Declarations CRediT authorship statement John Samson M (JSM): Conceptualization, Methodology, Data curation, Software, Formal analysis, Visualization, Writing – Original draft preparation. Manikanda Bharath K (MBK) : Writing – review & editing, Conceptualization, Methodology, Software, Formal analysis. Vidyasakar A (VA): Writing – review & editing, Supervision. Ajith Kumar K: Data curation. V Arun Bharathi - Data curation. Satyanarayan Bramha (SB) : Writing – review & editing, Methodology, Data curation. Chandrasekaran S (CS): Writing – review & editing. Kannaiyan Neelavannan (KN) : Visualization, Data curation. Gopal V: Methodology, Validation, Writing – Review and Editing. Acknowledgement The Corresponding authors gratefully acknowledge the Chief Minister Research Grant 2024-2025 (CMRG2400865) of Tamil Nadu. The authors express their sincere gratitude to the Group Head, Health Physics Section, Health and Industrial Safety Division, , Safety, Quality and Resource Management Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India, for facilitating access to essential equipment and supporting the instrumentation analyses critical to this study. The authors also express their gratitude to Aqua Forge Robotics Pvt. Ltd. Chennai – 600113, India, for the support provided during sample collection. The authors also extend their thanks to the Director, Centre for Earth and Atmospheric Sciences, Sathyabama Institute of Science and Technology, Jeppiaar Nagar, Rajiv Gandhi Salai, Chennai – 600119, Tamil Nadu, India, for providing software support and assistance with map preparation, which significantly strengthened the analytical components of this work. 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Chem. 324(3), 949–961. DOI:10.1007/s10967-020-07133-5. Additional Declarations No competing interests reported. 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17:08:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":3727352,"visible":true,"origin":"","legend":"\u003cp\u003eSpatial distribution of the Soil Thoron exhalation rate in the study area\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/548542554360976eadf22a3b.png"},{"id":98510295,"identity":"30a1dd2b-6985-4584-a839-20ce493b97c5","added_by":"auto","created_at":"2025-12-18 11:32:29","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":3775141,"visible":true,"origin":"","legend":"\u003cp\u003eSpatial distribution map of the total heavy minerals number weight percentage\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/9f790cd62aa3eef013667a03.png"},{"id":98624835,"identity":"9ee031c5-fdff-4520-8976-f5e4ef3ee1e1","added_by":"auto","created_at":"2025-12-19 17:08:45","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":917192,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 6. Spatial distribution and Beach maophology based Individual heavy minerals number weight percentage\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/21c923e5f7fe4980619983e1.png"},{"id":98624971,"identity":"b7f92ff9-e398-48e4-902d-ac8e88880003","added_by":"auto","created_at":"2025-12-19 17:08:52","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":886566,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 7. Geological Setting of the Study Area\u003c/p\u003e","description":"","filename":"8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/764964da0dffe72ac6bbd7a9.jpeg"},{"id":107351069,"identity":"e4d0a4b6-a3bb-4131-9400-0ce6537c43fe","added_by":"auto","created_at":"2026-04-20 16:08:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":14575949,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/cea62829-1714-436e-97a0-2820522cfac8.pdf"},{"id":98774847,"identity":"d75024a2-3ca5-4429-b8de-3c1e113391ee","added_by":"auto","created_at":"2025-12-22 12:15:44","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":339551,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical Abstract\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-8281563/v1/2ce335b89971ab20ad8710f6.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eNaturally occurring radioactive gases, in particular, the radon and thoron, are a notable category of emerging contaminants of geogenic sources that pose meaningful implications to the environmental exposure and human health (Haman et al., 2025). Their formation and spatial distribution are controlled mainly by the geological and geochemical properties of soils and sediments, in particular, the occurrence of uranium-, thorium-, and radium-minerals (Popoola et al., 2025). Heavy minerals like monazite, zircon and ilmenite found in mineralized beach sands are known to release significant quantities of radon and thoron, which are important ecological processes, environmental behaviour of radionuclides and health risks of communities in the One Health environment (Nebin et al., 2025). Radon and thoron move along multiple environmental paths, such as rocks, unconsolidated sediments, soil gas, groundwater, surface water, and ambient air, and have complex physicochemical and bioprocess pathways (Malik et al., 2025). Although these gases are usually diluted to non-hazardous levels in open and well-ventilated environments, they may become concentrated in space-constrained and highly poorly-ventilated environments to cause increased exposure and risks of respiratory diseases and lung cancer (Caridi et al., 2025). Their ecological behavior, mobility, and exhalation nature is thus important to quantify ecological safety and eliminate environmental health risks (Ali et al., 2025). These problems are magnified in urban settings, especially in megacities that are growing very fast, where land-use alterations, coastal development, and soil disturbances, as well as human-made changes in natural substrates, occur (Hanfi et al., 2025). Urban soils are usually the reservoirs of naturally occurring radionuclides and other contaminants and, therefore, they contribute to outdoor gamma radiation fields and risks of long-term exposures (Assefa \u0026amp; Arbese (2025)).\u003c/p\u003e \u003cp\u003eRadon and thoron as natural terrestrial radiations, make a significant portion of the radiation dose on the earth and a measurement of their levels in ecologically sensitive and densely populated environments is important in overall environmental-health risk management (Elmehdi et al., 2025). The Chennai megacity coastline in the southeast coast of India is a dynamic ecological environment of heavy-mineral-laden beach sands, high urbanization and growing human-environment contact. Although its importance is significant to the environment, there has been paucity of systematic evaluation of the soil-gas radon and thoron exhalation in this area. This knowledge gap is referred to by the paper, which discusses the spatial distribution of radon and thoron emissions of mineral-enriched beach sediments using the RAD7 electrostatic alpha detector, which is an established method of measuring short-lived radionuclides in the environmental matrices.\u003c/p\u003e \u003cp\u003eIn keeping with the objectives of Environmental Radioactivity, the following research will attempt to: (i) describe the background radiations surrounding the Chennai coastal urban system; (ii) determine spatial distribution and environmental radiocarbon behaviour of radon and thoron in beach sediments; (iii) investigate the effects of mineralogical composition, such as heavy minerals and rare earth elements, on exhalation of radionuclides; and (iv) evaluate possible environmental and human health hazards in relation to the radionuclide exposure in a high-populated megacity on the coast. Through a combination of geochemical, ecological, and environmental-health, the results offer baseline information that is critical in environmental management of the coastal environment as well as urban mitigation of health risks and future research of One Health oriented studies on the coastal ecosystems.\u003c/p\u003e"},{"header":"2. Location of the study area","content":"\u003cp\u003eThe study area is a coastal stretch of about 150 km long, the location of the Palar River on the south and Pulicat Lake on the north, of Kadalur Kuppam and Pulicat Lake at a distance of about 150 km respectively. This area is geographically located between 12\u0026deg;9' and 13\u0026deg;9' within the N and 80\u0026deg;12\u0026rsquo; to 80˚19\u0026rsquo; within the E, a region within a low-lying coastal plain on the south eastern border of the Indian subcontinent which comprises the Chengalpattu, Chennai and Tiruvallur districts its shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Enclosed on the east by the Bay of Bengal, and spanning almost 200 km\u003csup\u003e2\u003c/sup\u003e, the coastline is an environmentally sensitive, but highly urbanized zone, that is still in the process of being transformed at a rapid rate, environmentally and infrastructurally. The coastal zone accommodates various industrial and service industries, among them an automotive production industry, healthcare, information technology services, and high-volume port, which have an effect on the land-use patterns, redistribution of sediment and movement of naturally occurring contaminants (Karuppasamy et al., 2024). The climate of the region is tropical monsoon climate during the period of June to September, northeast monsoon climate during the period of October to January that brings most of the annual rainfall and finally semi-monsoon climate between the months of February and May. These seasonal changes have a very strong impact on the sediment moisture regimes, coastal processes as well as the environmental behavior of radon and thoron in beach sands.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMorphodynamically, the Chennai coastline is active and this occurs due to the recurrent wave action, tidal forces and longshore sediment transportation. In particular, the middle and northern stretches are affected by variation in littoral drift and waves that cause periods of erosion and accretion. The anthropogenic interventions such as harbour\u0026rsquo;s, breakwaters, jetties, groynes, and networks of canals have also caused profound changes in the sediment pathways and also lead to a localized geomorphic instability. These artificial structures, in combination with the natural hydrodynamic forces determine the distribution of heavy-mineral-rich sands, which are considered sources of geogenic radionuclides in the area. The lower parts of large city rivers like the Coovam, Adyar, Ennore, Palar which flow through fragmented wetlands and estuarine systems before reaching the coast are also within the study area. Their acts on the coastal environment also cause heterogeneity in the sediments, geochemical variability and the differing conditions of exposure. All of this creates a multiplier effect between the rapid urbanization, the growth of industries, the natural processes of the coasts, and the man-made interventions of the coastlines to create this coastal sector that is a critical landscape to evaluate the spatial variability of environmental radioactivity and resultant ecological and human health hazards in an integrated framework of One Health.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Sampling and laboratory procedures\u003c/h2\u003e \u003cp\u003eTo determine the environmental behavior of naturally occurring radionuclides, soil-gas radon and thoron exhalation rates were determined using mineral-rich beach sediments collected within the intertidal area of the south east coast of India. The sampling was done on a depth of about 1 m to reduce the effects of short-term atmospheric changes like variation in air pressure, temperature, and humidity which can largely affect the near surface concentrations of gases its sown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. In every sampling site, approximately 1 kg of the subsurface sediment was collected using clean plastic bags and only undisturbed soil was collected. The surface materials were avoided on purpose since the radium particles containing radium are subject to energy and motion of wind and waves, which may cause biased measurements of the release of radon and thoron. Minute gravel and coarse fragments were avoided to ensure uniformity and analysis. All the samples were sealed, labelled appropriately, and taken to the laboratory where they were air-dried thoroughly to get all the moisture out of them, which would have affected the emanation rates. The dried sediments were then homogenized and sifted using a mechanical sieve shaker to sift through 1 mm mesh before instrumental analysis was done to give homogenized grain-size fraction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Data Collection and experimental setup\u003c/h2\u003e \u003cp\u003eActive monitoring of radon and thoron was done in this study by the application of two complementary measurements to the evaluation of the soil-gas concentrations and exhalation rates of the coastal sediments within Chennai region. The main equipment utilized was the RAD7 electronic radon detector which was used to measure the mass exhalation of radon, surface exhalation of thoron and active soil-gas concentrations in the sampled sediments its shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Radon-222 and thoron-220 were the analytical objects; inert radioactive gases produced in the decay sequence of uranium-238 and thorium-232. Their vastly different half-lives 3.8 days of radon and 55 seconds of thoron have a major role to play in defining their emanation effects, their behaviour in porous beach sediments and their ultimate atmospheric dispersal. As radium is emitted by mineral grains, the radon isotopes that escape into the air-filled pores move to the surface of the soil, the concentration and the rate of exhalation may be measured using the RAD7 (RAD7 Radon Detector 2015).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe experimental apparatus used to measure soil-gas radon was the circulation of air in a closed sample chamber into the RAD7 detection cell as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. In the detector, the radon-222 breaks down to give positively charged polonium-218 ions which are attracted to the active surface of the detector electrostatically. The emitted alpha particles cause electrical pulses with energies that are equivalent to the expected alpha emissions of particular isotopes. These energy-specific pulses are amplified and classified by RAD7, and the alpha signatures of radon and thoron may be discriminated accurately. The instrument is used in SNIFF mode, where real-time, fast detection is expected, and use of majorly polonium-218 to measure radon concentration and polonium-216 to measure thoron concentration and inhibit other longer-lived progeny. This design provides a rapid reaction to temporary changes in the concentration and avoids signal saturation in case of high concentrations. Radon disintegrates in a clear sequence, which is, polonium-218 to lead-214 to bismuth-214 to polonium-214, then lead-210 which emits the characteristic radiation, yet RAD7 selects alpha emissions to give stable radon readings.\u003c/p\u003e \u003cp\u003eThoron measurement works with the same principle except that its half-life is extremely short which allows it to be detected mostly near the source. Thoron is unstable and breaks down to stable lead-208 via polonium-216, lead-212, bismuth-212 and polonium- 212 with high-energy alpha particle emissions occurring at various steps. Although thoron does not have a long half-life, it is nonetheless useful in the study of the environment where thorium is present in high amounts, e.g. in heavy-mineral beach sands on the Chennai coast. Measurement data of all the RAD7 were then processed and analyzed through the CAPTURE software of DURRIDGE to maintain quality control, consistency in the analysis, and standard reporting of radon and thoron activity.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Measurement of radon thoron exhalation rate\u003c/h2\u003e \u003cp\u003eAn active soil-gas extraction technique was used to determine radon-thoron exhalation rate of coastal sediments, in which a hollow stainless-steel probe was drilled to a depth of 25 cm to sample undisturbed subsurface air (Kansal \u0026amp; Mehra (2013)). A dust filter followed by a desiccant tube containing calcium sulfate (CaSO\u003csub\u003e4\u003c/sub\u003e) was used to dry the probe and eliminate moisture and dust particles to maintain the relative humidity under 10% level, which is the limit needed to guarantee optimal performance of the detector (UNSCEAR, 2011). The samples of soil-gas were gathered in grab mode within 5 min followed by 5-min decay period to enable dissipation of the thoron, which has a short half-life. The measurements of the four 5-minute cycles followed in a 20-minute period were then obtained. To minimize the possibility of intrusion of ambient air at time of sampling, care was taken and before sampling at each new location the RAD7 detector was carefully purged to minimize the residual background activity and offer consistency between the dataset samples. The accumulation chamber method was used to determine the rate of radon exhalation (J) of the soil surface:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:J=\\frac{\\left(C-{C}_{0}\\right)V}{A\\times\\:t}\\:\\:\\)\u003c/span\u003e \u003c/span\u003emBqm\u003csup\u003e\u0026minus;2\u003c/sup\u003es\u003csup\u003e\u0026minus;1\u003c/sup\u003e \u0026hellip;\u0026hellip;\u0026hellip;..(1)\u003c/p\u003e \u003cp\u003ewhere C is measured radon concentration at the surface of the soil (Bq m -3), C 0 is background radon concentration (assumed zero in air that is radon-free), V is sum of effective volume of the chamber and the accumulation chamber (m\u003csup\u003e3\u003c/sup\u003e), A is area of the soil covered by the chamber (m 2), and t is the time during which the accumulation took place (s). To assess possible health consequences in the population, the annual attributable dose of radon outside of residences was estimated by means of:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:AED={C}_{Rn}\\times\\:F\\times\\:O\\left(DCF\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;.\u003c/p\u003e \u003cp\u003eIn which C\u003csub\u003eRn\u003c/sub\u003e is the concentration of radon in the out air (Bq m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), F is the global average equilibrium factor of the radon and its progeny (0.6), O is the outdoor occupancy factor (1760 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and DCF is the dose conversion factor (9 nSv h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e per Bq m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e). These are computations that are made based on recommended international standards and additional methodological information can be found in the U.S. EPA guidance document Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 402-R-92-004) that still continues to serve as a standard reference of environmental radon and thoron measurements.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Radon and Thoran concentration in soil gas\u003c/h2\u003e \u003cp\u003eRadon, thoron and their sources are also regularly checked indoors and outdoors because these gases have a significant health hazard in case of inhaling them. The rate of radon mass exhalation and the rate of the surface exhalation (thoron) were assessed on varying grain-size fractions in soil in the current study it shown in the Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The exhalation rate of radon mass measured 2\u0026ndash;12 mBq kg \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h \u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and thoron surface exhalation rate was measured 162\u0026thinsp;\u0026minus;\u0026thinsp;31,623 mBq m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Radon and thoron had minimum detectable activities of 2.34 mBq kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 162 Bq m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively. The findings confirmed that the radon and thoron exhalation rates were the greatest in samples that had smaller sizes of particles, especially those with diameters of approximately 150\u0026micro;m, which implied that smaller particles enhanced diffusion and emission of radioactive gases.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe highest concentration of soil-gas radon was registered between 4.5 and 12 mBq kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at Kokkilamedu and this can be attributed to the existence of heavy minerals as a result of the placer deposits of local source rocks. The average radon, thoron and progeny annual concentrations and the dose rates were found to be within the stipulated limits of UNSCEAR (2011). The average radon exhalation rates of past researchers, which were 23 Bq m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in sand, 17 Bq m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in sandy loam and 15 Bq m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in clay loam are comparatively evidence that, natural soils are mostly higher in radon exhalation than most building materials. This is mainly explained by the fact that they are more porous and also by the heightened mobility of soil-gas radionuclides.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhen the Earth was formed several billion years ago, several naturally existing elements of radioactive content were incorporated into the Earth, and some of them; uranium-235, uranium-238, and thorium-232 are today present in measurable amounts (Jegede et al., 2025). They have extremely long half-lives, which are in the billions of years range, therefore the nuclides remain in nature and hold the first places in the natural radioactive decay series. The radioactive elements are not stable and they will eventually decay into other elements emitting radiations (Popoola et al., 2025). Even though it is impossible to predict what time exactly will be the time of the decay of one single atom, it is known that there are certain probabilities of the probability of the decay at a certain period of time. The measurement of radon gas in the atmosphere air and soil gas as well as building materials (Kansal et al., 2014; Ahmad et al., 2014) is thus necessary to determine the behaviour and diffusion of radon gas in the environment. The current study has proven that the rates of radon and thoron exhalations become decreasing with the increasing grain size of the soil, which indicates that the smaller particles play a bigger role in the gas emanation compared to the larger ones.\u003c/p\u003e \u003cp\u003eThese radionuclides include \u0026sup2;\u0026sup2;\u0026sup2;Rn and \u0026sup2;\u0026sup2;⁰Rn (radon and thoron), which arise from the decay chains of \u0026sup2;\u0026sup3;⁸U and \u0026sup2;\u0026sup3;\u0026sup2;Th, respectively. They also include \u0026sup2;\u0026sup2;⁶Ra and \u0026sup2;\u0026sup2;⁴Ra, formed through the decay of their direct parent nuclides, \u0026sup2;\u0026sup3;⁰Th and \u0026sup2;\u0026sup2;⁸Ra. These parent radionuclides are primarily concentrated in rocks, uranium-bearing minerals, and soil (Zakaly et al., 2024). The consequent decay of radon and thoron produce radioactive isotopes of polonium, bismuth, lead and thallium, elements which are considered heavy metals (Kanse et al., 2016). These decay products can easily be attached to aerosol particles suspended in the atmosphere and hence affect their dispersion, residence time and possible radiological effect on human being and environment (Pinto et al., 2020).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Geospatial variation of the rare earth elements (REE) bearing minerals\u003c/h2\u003e \u003cp\u003eDeposits of beaches on the south eastern coast of India are majorly made up of quartz, feldspar and a collection of economically meaningful heavy minerals. India is estimated to have 348 Mt of ilmenite, 107 Mt of garnet, 21 Mt of zircon, 18 Mt of rutile, 8 Mt of monazite and 130 Mt of sillimanite, in its coastal placer deposits, which comprise almost 35% of the world ilmenite reserves, 10% of the world rutile reserves, 14% of the world zircon reserves, and an exemplary 71.4% of The investigation of these heavy mineral placers has been highlighted due to the ilmenite being the main raw material of producing white pigments, but monazite is the key raw material of thorium and rare earth elements (REE). Another mineral rich in REE is xenotime that contains a significant amount of yttrium, a common phosphor in displays and in high-technology materials like superconductors and special alloys. Industrial Garnet is also part of the industrial value of Garnet as an abrasive and in sandblasting operations, particularly following limitations on the use of quartz sand following silicosis problems. With all the strategic and economic significance, the distribution of heavy minerals along the coastal area is crucial to the regulation and management of the resources as well as monitoring controls.\u003c/p\u003e \u003cp\u003eThe separation of heavy minerals in this study was done through the methods of gravity concentration whereby bromoform was the heavy liquid. The samples of the beach sand had significantly elevated heavy mineral contents when compared to the sampling sites in the inland areas. The level of in-situ gamma radiations was recorded using scintillometer and the background radiation values were deducted to get the corrected values. The total heavy mineral content was 12.5% and the gamma radiation levels reached up to 7.5 \u0026micro;R/h which was significantly higher than the levels obtained in other places. There is a great deal of variability in the abundance of heavy mineral as indicated in the spatial distributions depicted in the Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe ilmenite was present with a range of between 17.48\u0026ndash;76.15 showing an average of 47.37 percent of total heavy mineral fraction in the beach sands. Microscopic work also indicated a mixed mixture of heavy minerals comprising of zircon, sillimanite, kyanite, rutile, garnet, hornblende and tourmaline. The mineralogical patterns and compositional changes point towards a polygenetic origin, mainly of metamorphic origin rocks, and with the contribution of metasedimentary units of minor importance. The combined analysis of heavy mineral composition and gamma radiations in the south eastern coastal strip proves that the level of gamma radiations in the Kalanji Beach is indeed quite high, and this can be explained by the presence of zircon and monazite in the area. This was the locality that also had the highest concentration of heavy minerals compared to all other sampling points. These minerals have been attributed to the provenance of the Proterozoic rock formations located in the southern part of the study area that are the major enriches of the REE-bearing and radioactive minerals in the coastal sediments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Geological setting and Provenance of the Beach Placers\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the geological structure of the study site based on Bhukosh spatial data platform of Geological Survey of India (GSI). Based on the geological map of Tamil Nadu (on a scale of 1: 50,000), the beach sands in this coastal stretch are thought to be depositions that occurred between the Quaternary and the Recent epochs. These are unconsolidated sediments, which are running between the older geology formations, which comprise Miocene-Pliocene Cuddalore Sandstone, Upper Gondwana Sandstone, and massive Archean crystalline complexes. The Archean basement is mainly charnockites and gneisses and the related basic intrusives which are together the dominant hard rock provenance of the heavy mineral assemblies identified in the coastal sediments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe mineralogical and geochemical indicators of the beach placers indicate that it has a mixed provenance with metamorphic and meta-sedimentary terrains providing the bulk of heavy minerals (Pinto et al., 2020). Charnockites and related granulite facies rocks that are rich in minerals like ilmenite, garnet, zircon, and monazite are some of the rocks that play significant roles in the sediment load in the regional river systems in the area, including the Coovam, Adayar, Ennore, and Palar rivers. Fluvial processes promote the erosion, transportation and sorting of the heavy minerals which are later concentrated in the coastline by waves, tides, longshore current and littoral drift. The environment is a dynamic coast which facilitates the deposition of valuable economically significant heavy mineral placers (Mahamood et al., 2020). The increased occurrence of ilmenite, zircon and monazite in areas like the Kalanji region, in particular, is in line with a provenance to the provenance of the Proterozoic and Archean rocks in the southern area of the study region. These sources of geology explain the patterns of enrichment of REE-bearing and radioactive minerals in the beach sediments.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis research paper will give a detailed evaluation of the rate of radon and thoron exhalation in 24 samples of beach sands in Chennai coast, Tamil Nadu, India. The findings prove that the grain size of soil is a decisive factor determining the behaviour of exhalation of gases, and as grain size decreases, the rates of radon and thoron exhalation increase and as the size increases, the rates decline. The results also suggest that an increment in the heavy mineral content, the bulk density and grain size relationship is associated with a reduction in the rate of exhalation, which underscores the multifariousness of the association between the sediment texture and the radionuclide mobility.\u003c/p\u003e \u003cp\u003eRadon gas itself is not very dangerous to human health but when it does decay, it gives off alpha radiation and forms a significant part of the natural radiation received by humans. Such offspring are able to stick to the aerosols and land on lung tissue when they are inhaled, which poses a possible long-term health risk. Mineral resource estimation of the area under study has shown that ilmenite comprises about 43 percent of the total economic mineral reserves and the other ones are zircon and monazite which comprise 4 and 1 percent respectively. These minerals are of particular importance especially monazite as they contain REE and thorium.\u003c/p\u003e \u003cp\u003eThe results of this study form the necessary background data on the management of coastal resources, radiation safety, and occupational health priorities. Employees operating on the extracting, processing, and managing of monazite-laden beach sands are at a risk of getting high amounts of radon/thoron and their daughters. Thus, adoption of proper measures of protection, constant monitoring and maintenance of radiological guidelines is of vital importance in reducing radiological risks to the workers and the local population. The findings establish the foundation of a line of future geochemical, environmental, and radiological research that would be essential in the management of the coastal placer deposits in the sustainable way.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJohn Samson M (JSM):\u003c/strong\u003e Conceptualization, Methodology, Data curation, Software, Formal analysis, Visualization, Writing – Original draft preparation. \u003cstrong\u003eManikanda Bharath\u003c/strong\u003e K \u003cstrong\u003e(MBK)\u003c/strong\u003e: Writing – review \u0026amp; editing, Conceptualization, Methodology, Software, Formal analysis. \u003cstrong\u003eVidyasakar A (VA):\u003c/strong\u003e Writing – review \u0026amp; editing, Supervision. \u003cstrong\u003eAjith Kumar K:\u0026nbsp;\u003c/strong\u003eData curation. \u003cstrong\u003eV Arun Bharathi\u003c/strong\u003e- Data curation.\u003cstrong\u003e\u0026nbsp;Satyanarayan Bramha (SB)\u003c/strong\u003e: Writing – review \u0026amp; editing, Methodology, Data curation. \u003cstrong\u003eChandrasekaran S (CS):\u003c/strong\u003e Writing – review \u0026amp; editing. \u003cstrong\u003eKannaiyan Neelavannan\u003c/strong\u003e \u003cstrong\u003e(KN)\u003c/strong\u003e: Visualization, Data curation. \u003cstrong\u003eGopal V:\u003c/strong\u003e Methodology, Validation, Writing – Review and Editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Corresponding authors gratefully acknowledge the Chief Minister Research Grant 2024-2025 (CMRG2400865) of Tamil Nadu. The authors express their sincere gratitude to the Group Head, Health Physics Section, Health and Industrial Safety Division, , Safety, Quality and Resource Management Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India, for facilitating access to essential equipment and supporting the instrumentation analyses critical to this study. The authors also express their gratitude to Aqua Forge Robotics Pvt. Ltd. Chennai – 600113, India, for the support provided during sample collection. The authors also extend their thanks to the Director, Centre for Earth and Atmospheric Sciences, Sathyabama Institute of Science and Technology, Jeppiaar Nagar, Rajiv Gandhi Salai, Chennai – 600119, Tamil Nadu, India, for providing software support and assistance with map preparation, which significantly strengthened the analytical components of this work. Finally, the authors appreciate the substantial institutional support provided by the Department of Geology, Periyar University Centre for Postgraduate and Research Studies, Dharmapuri – 635205, which was instrumental in the successful completion of this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHaman, F., Guembou Shouop, C. J., Kpeglo Okoh, D., Bongue, D., Tiomene, D. F., Degbe, P. L., Gondji, D.S., Moyo, M.N., \u0026amp; Kwato Njock, M. G. (2025). Soil and air radon/thoron exhalation rates and radon activity concentrations in the vicinity of Lake Monoun, West region of Cameroon, Int. J. Environ. Anal. Chem. 105 (6), 1279\u0026ndash;1296, DOI:10.1080/03067319.2023.2288641\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePopoola, O. J., Olubi, O. E., Bamidele, S. E., \u0026amp; Adepoju, A. O. (2025). Geochemical distribution, pollution evaluation, and radiological health hazards of naturally occurring radionuclides in soil and stream sediments from Idanre area, Southwest Nigeria. Discov. Geosci. 3(1), 1\u0026ndash;29, DOI:10.1007/s44288-025-00243-1.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNebin, R. A., Bramha, S., Chitra, N., Chandrasekaran, S., Yardily, A., Krishnaveni, M., \u0026amp; Wesley, S. G. (2025). Distribution of naturally occurring radioactive materials at three coastal areas of Tamil Nadu, India: a comparative study. J. Radioanal. Nucl. Chem. 1\u0026ndash;12, DOI:10.1007/s10967-025-10137-8. [4] Malik, G., Parkash, R., Bhutani, M., Hooda, B., Panghal, A., Malik, P. S., ... \u0026amp; Kaur, P. (2025). Radon and Thoron exhalation rates in soil near coal mines in Shahdol, Madhya Pradesh. Appl. Radiat. Isot. 111994, DOI:10.1016/j.apradiso.2025.111994. [5] Caridi, F., Paladini, G., Gregorio, F., Lanza, S., Lando, G., Sfacteria, M., Tuccinardi S., Venuti M., Cardiano P., Majolino D., \u0026amp; Venuti, V. (2025). Natural Radioactivity Content and Radon Exhalation Rate Assessment for Building Materials from the Archaeological Park of Tindari, Sicily, Southern Italy: A Case Study. Int. J. Environ. Res. Public Health, 22(3), 379. DOI: 10.3390/ijerph22030379 [6] Ali, K., Abbady, A. E. B., Abu-Taleb, A., \u0026amp; Harb, S. (2025). Radiological behavior and health risk assessment of radon gas in Lake Nasser sediments, Egypt: implications for natural hazards. Environ. Geochem. Health. 47(9), 354. DOI:10.1007/s10653-025-02657-9 [7] Assefa, N. A., \u0026amp; Arbese, Y. (2025). A Study of Radium Content and Radon Exhalation Rates in Soil Samples from Abi-Adi Town, Ethiopia, Using LR-115 Type-II. Health Phys. 128(5), 365\u0026ndash;370. DOI: 10.1097/HP.0000000000001911 [8] Hanfi, M. Y., El-Gamal, H., Hussien, M. T., Khandaker, M. U., Alqahtani, M. S., \u0026amp; Hasabelnaby, M. (2025). Radiation doses assessment and radon exhalation rate from the soils of Albyda area, Yemen. Nucl Eng Technol. 57(3), 103227. DOI:10.1016/j.net.2024.09.030 [9] Elmehdi, H. M., Ramachandran, K., Al-Khalaileh, S. T., Ahmed, S. E. S., Daoudi, K., \u0026amp; Gaidi, M. (2025). Distribution of naturally occurring radioactive materials (NORMs) in Sharjah: Geological drivers and public health implications. Case Stud. Chem. Environ. Eng. 11, 101150. DOI:10.1016/j.cscee.2025.101150 [10] Karuppasamy, M. B., Natesan, U., Seethapathy, C., \u0026amp; Seshachalam, S. (2024). Environmental radioactivity, radiological hazards, and trace elements assessment of nearshore sediment in the Bay of Bengal. Int. J. Sediment Res. 39(1), 70\u0026ndash;82. DOI:10.1016/j.ijsrc.2023.12.002 [11] RAD7 Radon Detector (2015). Electronic Radon Detector User Manual, Durridge Company Inc., Billerica, MA 01821. [12] United Nations Scientific Committee on the Effects of Atomic Radiation. (2011). Report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2011. United Nations. [13] Jegede, D. O., Afolabi, T. A., Agunbiade, F. O., Afolabi, T. A., Ogundiran, O. O., Gbadamosi, M. R., ... \u0026amp; Varanusupakul, P. (2025). Spatial distribution and radiological hazards assessment of naturally occurring radionuclide materials in soil from quarry sites in Ogun State, Nigeria. Environ. Monit. Assess. 197(5), 575. DOI:10.1007/s10661-025-13988-6 [14] Kansal, S., \u0026amp; Mehra, R. (2013). Assessment of indoor radon concentration in air using RAD7 and radon exhalation rate measurement in soil samples (p. p. 186). Aggarwal College. [15] Popoola, O. J., Olubi, O. E., Adewalure, O. S., \u0026amp; Raphael, A. E. (2025). 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(2020). Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers. Radiat. Meas. 130, 106220. DOI:10.1016/j.radmeas.2019.106220. [19] Kanse, S. D., Sahoo, B. K., Gaware, J. J., Prajith, R., \u0026amp; Sapra, B. K. (2016). A study of thoron exhalation from monazite-rich beach sands of High Background Radiation Areas of Kerala and Odisha, India. Environ. Earth Sci. 75(23), 1465. DOI 10.1007/s12665-016-6279-9. [20] Mahamood, K. N., Divya, P. V., Vineethkumar, V., \u0026amp; Prakash, V. (2020). Dynamics of radionuclides activity, radon exhalation rate of soil and assessment of radiological parameters in the coastal regions of Kerala, India. J. Radioanal. Nucl. Chem. 324(3), 949\u0026ndash;961. DOI:10.1007/s10967-020-07133-5.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Soil-gas radon, Thoron concentration, Radon/thoron exhalation rate, Heavy mineral beach placers, Geological setting, RAD7 alpha detector","lastPublishedDoi":"10.21203/rs.3.rs-8281563/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8281563/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe short-lived progenies of radon, thoron (²²⁰Rn), and radon (²²²Rn) are major global sources of ionizing radiation exposure and require continuous environmental monitoring because of their inhalation-related health risks. This study investigated the spatial distribution of soil-gas radon and thoron, their mass and surface exhalation rates, and the mineralogical controls on beach sands along the Chennai megacity coast in Southeast India. Twenty-four intertidal soil samples were analyzed using a RAD7 radon–thoron monitor (Durridge Co., USA), an electrostatic solid-state alpha detector optimized for thoron measurements. Radon mass exhalation rates ranged from 2 to 12 mBq/kg/h (mean: 2.42 mBq/kg/h), whereas thoron surface exhalation rates varied widely from 162 to 31,623 Bq/m²/h (mean: 3,688.08 Bq/m²/h). The highest soil-gas radon levels (4.5–12 Bq/m³) were observed in Kokkilamedu, a placer-rich zone. Exhalation rates were inversely related to grain size, with finer sediments releasing more radon and thoron than coarser sediments. However, higher heavy-mineral content, greater bulk density, and finer grain fractions also restricted exhalation, indicating strong mineralogical and textural control of radionuclide mobility. The sediments contained up to 12.5% heavy minerals, including ilmenite, zircon, and monazite, contributing to gamma radiation levels of up to 7.5 µR/h. The annual effective radon doses ranged from 0.12 to 0.45 mSv/y, remaining below the global safety limits. Elevated thoron exhalation in monazite-rich areas highlights the need for regular radiological surveillance, particularly among placer-deposit workers. These results provide baseline data on coastal sediment radiology and provenance to support environmental risk management and public health planning in the Chennai coastal zone.\u003c/p\u003e","manuscriptTitle":"Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-18 11:32:24","doi":"10.21203/rs.3.rs-8281563/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-05T18:02:40+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-14T16:02:41+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-28T16:48:55+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-26T18:36:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"87305706995789475541912059797039985784","date":"2025-12-22T04:47:53+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"147551098632419353673069556179288381784","date":"2025-12-21T09:53:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"186199495280060539220072048953747918848","date":"2025-12-17T03:31:34+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"123661194759842849281453189772221435257","date":"2025-12-16T08:50:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"224761767775430645789302962420319527823","date":"2025-12-16T08:24:00+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-16T08:12:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-08T09:36:13+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-06T05:47:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Geochemistry and Health","date":"2025-12-04T16:59:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a88769bd-0ef1-4722-b65f-2e2c3dc17bbf","owner":[],"postedDate":"December 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T16:06:35+00:00","versionOfRecord":{"articleIdentity":"rs-8281563","link":"https://doi.org/10.1007/s10653-026-03169-w","journal":{"identity":"environmental-geochemistry-and-health","isVorOnly":false,"title":"Environmental Geochemistry and Health"},"publishedOn":"2026-04-14 15:59:24","publishedOnDateReadable":"April 14th, 2026"},"versionCreatedAt":"2025-12-18 11:32:24","video":"","vorDoi":"10.1007/s10653-026-03169-w","vorDoiUrl":"https://doi.org/10.1007/s10653-026-03169-w","workflowStages":[]},"version":"v1","identity":"rs-8281563","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8281563","identity":"rs-8281563","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}