Characterization of radon levels in soil and groundwater in the North Maladeta Fault area (Central Pyrenees) and their effects on indoor radon concentration in a thermal spa

Exploring statistical and machine learning techniques to identify factors influencing indoor radon concentration

Radon is a radioactive gas with a carcinogenic effect. The malign effect on human health is, however, mostly influenced by the level of exposure. Dangerous exposure occurs predominantly indoors where the level of indoor radon concentration (IRC) is, in its turn, influenced by several factors. The current study aims to investigate the combined effects of geology, pedology, and house characteristics on the IRC based on 3132 passive radon measurements conducted in Romania. Several techniques for evaluating the impact of predictors on the dependent variable were used, from univariate statistics to artificial neural network and random forest regressor (RFR). The RFR model outperformed the other investigated models in terms of R2 (0.14) and RMSE (0.83) for the radon concentration, as a dependent continuous variable. Using IRC discretized into two classes, based on the median (115 Bq/m3), an AUC-ROC value of 0.61 was obtained for logistic regression and 0.62 for the random forest classifier. The presence of cellar beneath the investigated room, the construction period, the height above the sea level or the floor type are the main predictors determined by the models used.

Radon concentration in spring water as an indicator of seismic activity: a case study of the Muzaffarabad Fault in Pakistan

Radon and its progenies found in water indicate the existence of seismically active faults in the region. However, exposure to high levels of radon can also result in radiation-related health risks. This study focuses on radon-based active tectonic studies along the Muzaffarabad Fault in the core of the Hazara-Kashmir Syntaxis (HKS), NW Himalayas, Pakistan. In this study, spring water samples were collected along roadside of Jhelum Valley and in close proximity to the Muzaffarabad Fault in Pakistan using Radon Thoron Monitor (RTM1688-2). The results of the study showed that the radon concentrations in the water samples ranged from 1.895 to 17.097 Bq/l. The study found that the highest radon concentration was observed in the samples collected closest to the fault, while the lowest concentration was observed in the samples collected further away. The statistical analysis between the radon concentration and the distance from the fault showed a strong inverse relationship (R2=0.73). The study also found that 68% of the sampling sites had radon concentrations that exceeded the maximum contamination level (MCL) set by the US Environmental Protection Agency (EPA). The higher radon concentrations in the springs water suggest the probability of earthquake, which in turn poses potential health risks for the local population. The findings suggest that the measurement of radon concentration in water can be used as a tool for identifying seismically active faults in the region.

Application of radon (222Rn) as an environmental tracer in hydrogeological and geological investigations: An overview

Radon (²²²Rn) is a colourless, odourless, inert, and radioactive noble gas (t_1/2 = 3.8 days) that emanates from rocks and soils as a result of the alpha decay of its parent, radium (²²⁶Ra) in the decay series of uranium-238, is the focus of this study. Radon is produced in the crystal lattice of the minerals and emanates out through alpha recoil. It dissolves in water, and is also found in soil and air. Its distribution in water is more pertinent for scientific investigations. It can be measured by various methods. Certain properties of radon enable it to serve as an ideal tracer, viz., short-half life, inertness, high abundance in groundwater than surface water, preferential partitioning, sensitivity to sudden changes in subsurface conditions, non-invasiveness etc. This paper reviews the state-of-the-art techniques on the measurement of dissolved radon in water and its potential applications as a tracer and precursor in several hydrogeological and geological applications like understanding the surface water - groundwater interactions, hydrograph separation of streams, estimation of Submarine Groundwater Discharge (SGD), study of hydrodynamics and water balance of lakes, earthquake predictions, locating geological structures (faults/lineaments), geochemical explorations, NAPL contamination studies etc. Among the various applications presented, radon based approach is found to be more reliable in water resources domain than seismic precursory studies. The interpretations based on radon study in the above applications will pave the way for the improved understanding of the hydrological processes, and thus, help the planners and water managers for the sustainable development and management of water resources.

Role of lithology in the presence of natural radioactivity in drinking water samples from Tarragona province

One hundred and ninety-six drinking water samples from the different regions of Tarragona province (Catalonia, Spain) were analysed to determine the gross alpha and beta activity. Individual alpha emitting isotope activities were also determined to evaluate a possible relationship between their radiological content and the lithological and hydrogeological formations present in the studied area. The results obtained showed that approximately 23% of the analysed samples, mainly from five of the evaluated regions, had a gross alpha index exceeding the parametric value of 0.1 Bq/L for waters intended for human consumption according to the current legislation. This could be related to the presence of natural radionuclides in these water samples. The differences between the radiological content in these samples could be related to the different lithological conditions of the areas included in this study. High activity levels of ²³⁴U, ²³⁸U, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra were detected in specific samples, mainly from granitic and carbonate areas. This research also focuses on evaluating the radiological risk associated with water ingestion. In this regard, consuming 95.5% of the drinking water samples analysed would not imply a health risk to the population as the annual effective doses calculated were below 0.1 mSv/year. There was only one sample that exceeded this level with a value of 0.33 mSv/year. ²²⁶Ra activity concentration was the radionuclide that mainly contributed to this dose.

Occurrence of 222Rn and 226,228Ra in underground water and 222Rn in soil and their mutual correlations for underground water supplies in southern Greater Poland

European Union Council Directive 2013/51/EURATOM recently sets out so-called indicator parameters for: radon, tritium and indicative dose of water intended for human consumption. The aim of this research was to elaborate an effective procedure for determination of radon and radium ^226,228Ra isotopes (which are potentially the main contributors to the internal dose from drinking and cooking water) and to find the possible relationships between these radionuclides in underground water reservoirs and ²²²Rn concentration in the soil gas in their vicinity. The research was performed by applying a non-volatile and water-immiscible scintillation cocktail based on a pure diisopropylnaphthalene (Ultima Gold F: UGF), which allow for efficient radon extraction from 0.5 dm³ of water samples to 20 cm³ of scintillation phase and its direct determination with a detection limit of 5 × 10^-3 Bq dm^-3. The further preliminary concentration of 3 dm³ of crude water samples by evaporation to 0.5 dm³ samples led to the removal of all unsupported ²²²Rn activity and allowed the ²²⁶Ra determination via equivalent ²²²Rn detection after one-month samples storage using a low-background Triathler liquid scintillation counter in the α/β separation counting mode. Together with determination of ²²⁶Ra isotope in water samples, the simultaneous measurements of ²²⁸Ra and ²²²Rn radionuclides concentrations in water as well as ²²²Rn activity in the soil gas around the water supply sites were performed. The achieved limit of ²²⁶Ra detection was at a very low level of 10^-3 Bq dm^-3. The measured values of ²²⁶Ra concentration in 50 public underground water supply units for the Kalisz district of Poland were relatively low and ranged from below detection limit to 28.5 × 10^-3 Bq dm^-3 with arithmetic mean and median values of 12.9 and 12.2 × 10^-3 Bq dm^-3, respectively. Weak correlations were observed between activity concentrations of ²²⁶Ra and ²²²Rn in the crude water samples (R² = 0.31) and ²²²Rn in water and its concentration in the nearby soil gas (R² = 0.48).

The results of long-term simultaneous measurements of radon exhalation rate, radon concentrations in soil gas and groundwater in the fault zone

The regular monthly radon measurements were carried out in the fault zone on the Western slope of the Beshtau magmatic massif (North Caucasus). The radon exhalation rate from the soil surface, as well as radon concentrations in soil gas at a depth of 0.5 m and in groundwater discharged at a spring located nearby have simultaneously been measured. High seasonal fluctuations in radon exhalation and radon concentration in soil gas, characterized by highs in summer and lows in winter, were registered. In summer, the radon exhalation reached 23.8 Bq m^-2s^-1, and the radon concentration in the soil gas reached 166 kBq m^-3. In winter, both the radon exhalation and the radon concentration in the soil dropped to 0.025 Bq m^-2s^-1 and <3 kBq m^-3, respectively. The concentration of radon in ground water varied over the year in a relatively narrow range (100-210 Bq l^-1), and there were no seasonal fluctuations. A sharp increase in soil radon and radon exhalation in spring and a fall in autumn are timed to the moments when the temperature of the atmospheric air becomes, respectively, higher and lower than the temperature of the rock massif. Both the soil radon concentration and the radon exhalation show a close correlation with the temperature of atmospheric air, but in the first case the relationship is linear, and in the second - exponential. The obtained data confirm the assumption that the seasonal radon variations are caused by atmospheric air circulation in the shallow area of the fault due to the temperature difference between the atmosphere and the rock massif.

A Review of Radon Equilibrium Factors in Underground Mines, Caves, and Thermal Spas

Radon equilibrium factor Feq is an important factor in radon progeny dose assessment. A review of published measurements of Feq shows a range of values from 0.1 to 1.0 reported in studies from more than 26 countries measured in 173 underground mines, and 136 show caves, tourist mines, and thermal spas. The average values of Feq are 0.38 in underground mines and 0.39 for show caves, tourist mines, and thermal spas. The wide range of Feq in those special workplaces suggests that location-, environment-, and operation-specific values are more appropriate than a recommended average value in the calculation of lung bronchial dose. This is especially important in mines or other typically high radon exposure locations because Feq can be used for recording an individual's occupational radon exposure or dose.

High seasonal variations of the radon exhalation from soil surface in the fault zones (Baikal and North Caucasus regions)

The seasonal variations of radon exhalation rate from soil surface were studied in two seismically active regions of the Russian Federation - the Baikal rift and the North Caucasus. In each region, monthly measurements of the radon exhalation have been carried out at two relatively proximal sites, one of which was located within the active fault zone and the other outside of the fault zone. The Open Charcoal Chamber Method was used. Very high radon exhalation rate values were found in the fault zones at both regions. At the Baikal rift, the radon exhalation reached 1.4 Bq m^-2 s^-1, and at the Caucasian region in some periods it even achieved 24 Bq m^-2 s^-1, which is an extremely high value. The same pattern of seasonal variations of radon levels with abnormal high radon exhalation rate values in summer and extremely low in winter were observed in both the Baikal and Caucasus regions. Clear correlation between radon exhalation and air temperature were also revealed. The obtained data and simulation results indicate that seasonal fluctuations in the radon exhalation rate are caused by the inversion of the direction of convective air flow in the fractured zones of the rock massif. In summer, the convective air flow is directed from the rock massif to the atmosphere and in winter, vice versa, from the atmosphere to the rock massif. This phenomenon is similar to the well-known "chimney effect", i. e. in winter there is a direct draft in the system of fractures, and in summer - the reverse one. Thus, the detected radon anomalies are due to near-surface convective air circulation in permeable zones of the mountain ranges and most probably are not associated with deep crustal or mantle degassing. Seasonal thermally induced radon anomalies should be taken into account both in the radon risk mapping and in the application of radon as a tracer of natural processes in various fields of geology and geophysics.

An Assessment of Groundwater Contamination Risk with Radon Based on Clustering and Structural Models

There is currently some controversy in the scientific community regarding the efficiency of the water–rock interaction process in the contamination of radon in groundwater. In this study, some difficulties were found in the sampling phase. Many of the water collection points are used for human consumption. As such, some municipalities did not want to collaborate. When this natural contaminant is undetectable to the human sense and may cause pulmonary neoplasms in the long term, it is difficult to obtain collaboration from the municipalities concerned. To overcome this controversy, it is important to understand that geogenic, climatic, hydrological, and topographic features may contribute to the effective transfer of radon from rocks to groundwater. In brief, this new approach combines the radon transfer from the geological substrate to the groundwater circulation through hierarchic agglomerative clustering (HAC) and partial least squares-path modeling (PLS-PM) methods. The results show that some lithologies with higher radon production may not always contribute to noticeable groundwater contamination. In this group, the high-fracturing density confirms the recharge efficiency, and the physical-chemical properties of the hydraulic environment (electric conductivity) plays the main role of radon unavailability in the water intended for human consumption. Besides, the hydraulic turnover time of the springs can be considered an excellent radiological indicator in groundwater. In the absence of an anomalous radioactive source near the surface, it means that the high-turnover time of the springs leads to a low-radon concentration in the water. Besides linking high-risk areas with a short period required to free local flow discharges, this study exposes the virtues of a new perspective of a groundwater contamination risk modeling.

A New Framework for the Management and Radiological Protection of Groundwater Resources: The Implementation of a Portuguese Action Plan for Radon in Drinking Water and Impacts on Human Health

In general, this study was developed to assess the radon contamination in groundwater intended for human consumption, to raise awareness among policy-makers to implement a legal framework for drinking water management and the radiological protection of groundwater resources. Thus, we analyzed with parallel coordinate visualization (PCV) plots what features may influence the water–rock interaction and promote high-radon concentrations in water intended for human consumption. The results show that in granitic areas composed by biotite granites (Group V), although there is a higher radon production in the rocks, the radon transfer to groundwater was not effective, mainly due to the physical and chemical properties of the water. The main conclusions show that in all springs sampled (n = 69) for the entire study area, there are only four springs that must have an immediate intervention, and 22 of them are above the limit imposed by the Portuguese legislation. These results are intended to promote a Portuguese Action Plan for Radon that can be framed in the guidelines on the management and protection of groundwater resources.

Radon emission from soil gases in the active fault zones in the Capital of China and its environmental effects

The release of radon in active fault zones is a sustained radioactive pollution source of the atmospheric environment. The species, concentration and flux of radon emitted in soil gas in active fault zones in the Capital of China were investigated by in-situ field measurements. Two main species of radon discharging from soil gas in active fault zones were identified, including radon diffused and dispersed from permeable soil, and upwelling from faults. Higher concentrations and flux of radon from faults were observed in the Bohai Bay Basin due to the accumulated uranium in the sandstone reservoirs and higher permeability of the strata and bed rocks. Increased radon released by strong earthquakes persists, with the max flux of 334.56 mBq m⁻² s⁻¹ observed in FN (Fengnan district) located at the epicenter of the 28 July, 1976 Tangshan M_S 7.8 earthquake. The level of radon released in 8 of 22 locations within the Basin and Range Province (to the west of Taihangshan piedmont fault Zone) reached level 2, and 13 of 14 locations within the Bohai Bay Basin reached levels 2–4, according to the Chinese Code (GB 50325–2001, 2006). Corresponding protective and safety measures should be in place to protect the health of nearby residents, due to their exposure to radon emitted from the faults. Also, the concentration of radon in active fault zones should be investigated to assess the possible risk, before land-use is planned.