Effective Radium Concentration Research Articles

Measurement of the radon exhalation rate and effective radium concentration in soil samples of southern Sakarya, Turkey

In the present study, radon exhalation rates in terms of mass and area, as well as the effective radium concentration in soil samples collected simultaneously from different districts of southern Sakarya have been measured by Sealed Can technique using LR-115 type-II detectors. Mass and areal radon exhalation rates in soil samples vary from 35.76 ± 1.5 to 253.15 ± 3.8 mBqkg−1h−1with an average value of 112.53 ± 2.7 mBqkg−1h−1and 0.73 ± 0.2 to 5.18 ± 0.6 Bqm−2h−1with an average value of 2.30 ± 0.6 Bqm−2h−1, respectively. The effective radium content was found to vary in the range 3.77 ± 0.5 to 26.69 ± 1.3 Bqkg−1with an average value of 11.86 ± 0.9 Bqkg−1. The area exhalation rate was also used to calculate indoor radon concentration contributed by radon exhalation from soil, and to estimate annual effective dose equivalent. While the indoor radon concentration contributed by radon exhalation from soil varies from 2.93 ± 0.9 to 20.73 ± 2.3 Bqm−3with an average value of 9.22 ± 1.5 Bqm−3, the estimated effective dose equivalent varies from 0.09 to 0.61 mSvy−1with an average value of 0.27 mSvy−1.

Effective radium concentration in topsoils contaminated by lead and zinc smelters

Trace elements (TE) are indicative of industrial pollution in soils, but geochemical methods are difficult to implement in contaminated sites with large numbers of samples. Therefore, measurement of soil magnetic susceptibility (MS) has been used to map TE pollutions, albeit with contrasted results in some cases. Effective radium concentration (ECRa), product of radium concentration by the emanation factor, can be measured in a cost-effective manner in the laboratory, and could then provide a useful addition. We evaluate this possibility using 186 topsoils sampled over about 783km2 around two former lead and zinc smelters in Northern France. The ECRa values, obtained from 319 measurements, range from 0.70±0.06 to 12.53±0.49Bq·kg−1, and are remarkably organized spatially, away from the smelters, in domains corresponding to geographical units. Lead-contaminated soils, with lead concentrations above 100mg·kg−1 <3km from the smelters, are characterized on average by larger peak ECRa values and larger dispersion. At large scales, away from the smelters, spatial variations of ECRa correlate well with spatial variations of MS, thus suggesting that, at distance larger than 5km, variability of MS contains a significant natural component. Larger ECRa values are correlated with larger fine fraction and, possibly, mercury concentration. While MS is enhanced in the vicinity of the smelters and is associated with the presence of soft ferrimagnetic minerals such as magnetite, it does not correlate systematically with metal concentrations. When multiple industrial and urban sources are present, ECRa mapping, thus, can help in identifying at least part of the natural spatial variability of MS. More generally, this study shows that ECRa mapping provides an independent and reliable assessment of the background spatial structure which underlies the structure of a given contamination. Furthermore, ECRa may provide a novel index to identify soils potentially able to fix leached components.

Effective radium concentration in agricultural versus forest topsoils

Effective radium-226 activity concentration (ECRa), the radon-222 source term, was measured in the laboratory with 724 topsoil samples collected over a ∼110 km2 area located ∼20 km south of Paris, France. More than 2100 radon accumulation experiments were performed, with radon concentration measured using scintillation flasks, leading to relative uncertainties on ECRa varying from 10% for ECRa = 2 Bq⋅kg−1 to less than 6% for ECRa > 5 Bq⋅kg−1. Small-scale dispersion, studied at one location with 12 samples, and systematically at 100 locations with three topsoils separated by 1 m, was of the order of 7%, demonstrating that a single soil sample is reasonably representative. Agricultural topsoils (n = 540) had an average (arithmetic) ECRa of 8.09 ± 0.11 Bq⋅kg−1, and a range from 2.80 ± 0.22 to 19.5 ± 1.1 Bq⋅kg−1, while forest topsoils (n = 184), with an average of 3.21 ± 0.14 Bq⋅kg−1 and a range from 0.45 ± 0.12 to 9.09 ± 0.55 Bq⋅kg−1, showed a clear systematic reduction of ECRa when compared with the closest agricultural soil sample. Large-scale organization of ECRa was impressive for agricultural topsoils, with homogeneous domains of several kilometers size, characterized by smooth variations smaller than 10%. These patches emerged despite heavy human remodeling; they are controlled by the main geographical units, but do not necessarily coincide with them. Valleys were characterized by larger dispersion and less organization. This study illustrates how biosphere and anthroposphere modify the soil distribution inherited from geological processes, an important baseline needed for the study of contaminated sites. Furthermore, the observed depletion of forest topsoils suggests an atmospheric radon signature of deforestation.

Study of Radium and Radon Exhalation Rate in Soil Samples from Areas of Northern Rajasthan

Abstract The health hazards of the radioactive gas radon on the humans are well known. Radon from soil gas is the main cause of radon problems. The “can technique” using LR-115 type II plastic track detectors have been used for the measurement of radon exhalation rate in soil samples from areas of northern Rajasthan, India. The radon exhalation values were calculated in terms of mass (EM) and area (EA). The mass exhalation rate in these samples vary from 8.27 to 23.19 mBq kg-1 h-1 with an average of 14.96 mBq kg-1 h-1, whereas, surface exhalation rate vary from 273.80 to 768.04 mBq m-2 h-1 with an average of 495.32 mBq m-2 h-1. The radium concentration ranges from 6.88 to 19.31 Bq kg-1 with an average value of 12.45 Bq kg-1. The observed values of effective radium concentration in all the soil samples are less than the permissible value of 370 Bq kg-1 as recommended by Organization for Economic Cooperation and Development (OECD). Hence, the result shows that this area is safe as for as the health hazard effects of radium and radon exhalation rate are concerned.

The Syabru‐Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

AbstractThe Syabru‐Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30–62°C) water springs and cold (&lt;35°C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO2 and 850 radon flux measurements, with complementary self‐potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m−2 d−1 and 38.5 ± 8.0 Bq m−2 s−1, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d−1 and 140 ± 30 MBq d−1, respectively. Soil‐gas radon concentration is 40 × 103 Bq m−3 in GZ1–GZ2 and 70 × 103 Bq m−3 in GZ3–GZ4. Strong relationships between CO2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO2. CO2 carbon isotopic ratios (δ13C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA &lt; 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

The Syabru‐Bensi hydrothermal system in central Nepal: 2. Modeling and significance of the radon signature

AbstractThe Syabru‐Bensi hydrothermal system (SBHS), located in the Nepal Himalayas, is characterized by numerous hot (&gt;30°C) springs and the release of dry, cold (&lt;35°C) CO2 associated with radon‐222, detailed in the companion paper. In the SBHS, CO2 and radon fluxes on the ground vary over 5–6 orders of magnitude, reaching exceptional mean values of 100 kg m−2 d−1 and 12 Bq m−2 s−1, respectively. This paper extends the companion paper by developing three quantitative models for the radon signature of CO2 based on measurements of radon and radium concentrations in the spring waters and effective radium concentration of rocks and soils. The first model considers near‐surface radon and CO2 degassing from water, considered unlikely unless there exist currently unidentified large discharges of hydrothermal water. The second model considers CO2, arising from deeper hydrothermal sources, incorporating radon from shallow radium sources as it percolates upward toward the surface, considered more likely as a percolation depth of 100 m is sufficient to account for the observed radon discharge. The third model considers the observed peak radon concentrations in the gas zones and assumes that gaseous CO2 can be transported from kilometer‐scale depths through a fault network connected to the zones. This latter model affords the possibility that variations of physical parameters at depths associated with earthquake nucleation might be detectable at the surface. Gas‐dominated transport might operate in other locations in Himalayas and elsewhere and may be an important aspect of the coupled mechanisms associated with seismically active orogens.

Effective radium concentration across the Main Central Thrust in the Nepal Himalayas

Effective radium concentration (ECRa) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured in the laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to Eastern Nepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade metamorphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence (GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. ECRa values range from 0.03±0.03 to 251.6±4.0Bqkg−1, and appear to be representative of the formation and clearly related to the local lithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350 available ECRa values, LHS rocks are characterized by a mean value of 5.3±1.3Bqkg−1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69±0.11Bqkg−1, thus leading to a LHS/GHS ECRa ratio of 7.8±2.2. This behavior was systematically confirmed by other transects (ratio of 7.9±2.2 in all other sites), with a threshold ECRa value, separating LHS from GHS, of 0.8Bqkg−1, thus bringing forward a novel method to characterize, within the MCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks, occurred in several transects and were characterized by high ECRa values (17.9±4.3Bqkg−1), easy to distinguish from the GHS gneisses, characterized by low ECRa values at the bottom of the GHS, thus providing a further argument to locate the MCT. The measurement of ECRa data, thus, provides a cost-effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurement of ECRa provides additional information to discriminate different geological formations, and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial.

Measuring effective radium concentration with large numbers of samples. Part I – experimental method and uncertainties

Effective radium concentration ECRa, product of radium concentration and radon emanation, is the source term for radon release into the pore space of rocks and the environment. To measure ECRa, we have conducted, over a period of three years, more than 5500 radon-222 accumulation experiments in the laboratory with scintillation flasks, and about 700 with integrating solid state nuclear track detectors, leading to experimental values of ECRa for more than 1570 rock and soil samples. Through detailed systematic checks and intercomparison between various repeated experiments, the experimental uncertainty has been assessed, and ranges from 30% (1 σ) for ECRa values smaller than 0.2 Bq kg−1 to about 8–10% for ECRa values larger than 50 Bq kg−1. The detection limit, defined as the 90% probability for obtaining a non-zero experimental ECRa value at 68% confidence level, depends on the mass of the sample with respect to the volume of the accumulation volume, and typically varies between 0.04 and 0.09 Bq kg−1. To measure ECRa from large numbers of samples with sufficient accuracy and uncertainty for our purpose, i.e. for the most natural objects encountered in the environment, the accumulation method with scintillation flask emerged as particularly useful and robust. Properties of ECRa and interpretations inferred from this large data set are presented in the companion paper.

Measuring effective radium concentration with large numbers of samples. Part II – general properties and representativity

Effective radium concentration ECRa, product of radium concentration and radon emanation, is the source term for radon release into the pore space of rocks and the environment. Over a period of three years, we performed more than 6000 radon-222 accumulation experiments in the laboratory with scintillation flasks and SSNTDs and we obtained experimental ECRa values from more than 1570 rock and soil samples. With this method, which allowed the measurement of ECRa from large numbers of samples with sufficient accuracy and uncertainty, as detailed in the companion paper, the dependence of the emanation factor on temperature and moisture content is revisited. In addition, with such a large ECRa dataset, dispersion of ECRa can be studied at sample-scale (cm to dm) and at scarp-scale (m to tens of m). Furthermore, we are able to discuss the representativity of obtained ECRa values at field-scale, and to investigate the spatial variations of ECRa over kilometric scales, within geological formations and across formations and faults. This experimental study opens new perspectives in the understanding of radium geochemistry and illustrates the importance of studying the radon source term with large numbers of samples for the modelling of geological and environmental processes, and also for the assessment of the radon health hazard.

Estimating the importance of factors influencing the radon-222 flux from building walls

Radiation hazard in dwellings is dominated by the contribution of radon-222 released from soil and bedrock, but the contribution of building materials can also be important. Using a simple air mixing model in a 2-story house with an attic and a basement, it is estimated that a significant risk arises when the Wall Radon exhalation Flux (WRF) exceeds 10×10−3Bq·m−2·s−1. WRF is studied using a multiphase advection–diffusion 3-layer analytical model with advective flow, possibly induced by a pressure deficit inside the house compared with the outside atmosphere. To first order, in most circumstances, the WRF is proportional to the wall thickness and to the radon source term, the effective radium concentration ECRa, which is the product of the radium-226 concentration by the emanation coefficient E. The WRF decreases with increasing material porosity and exhibits a maximum for water saturation of about 50%. For ECRa=10Bq·kg−1, in many instances, WRF is larger than 10×10−3Bq·m−2·s−1 and, therefore, ECRa=10Bq·kg−1 can be considered as the typical limit not to be exceeded by building materials. An upper limit of the WRF is obtained in the purely advective regime, independent of porosity or moisture content, which can thus be used as a robust safety guideline. The sensitivity of WRF to temperature, due to the temperature sensitivity of ECRa or the temperature sensitivity of radon Henry constant can be larger than 5% for the seasonal variation in the presence of slight pressure deficit. The temperature sensitivity of ECRa is the dominant effect, except for moist walls. Temperature and moisture variation effects on the WRF potentially can account for most observed seasonal variations of radon concentration in houses, in addition to seasonal changes of air exchange, suggesting that the contribution of walls should be considered when designing remediation strategies and studied with dedicated experiments.

Measuring effective radium concentration with less than 5 g of rock or soil

Radon generation in natural systems and building materials is controlled by the effective radium concentration ECRa, product of the radium concentration CRa and the emanation factor E. An experimental method is proposed to measure ECRa in the laboratory by radon accumulation experiments using less than 5 g of sample inserted in 125 mL scintillation flasks. Accumulation curves with fine temporal resolution can be obtained, allowing the simultaneous determination of the effective leakage rate. The detection limit, defined as the ECRa value giving a probability larger than 90% for a determination with a one-sigma uncertainty better than 50%, is moderate, varying from 2 to 5 Bq kg−1 depending on the conditions. Obtained punctual uncertainties on ECRa vary from about 10 to 20% at 10 Bq kg−1 to less than 3% for ECRa larger than 500 Bq kg−1. The representativity of small samples to estimate meaningful values at site or system level is, however, a definite limitation of the method, and the sample dispersion needs to be considered carefully in every case. Nevertheless, the value obtained with 5 g or less differs on average by 9 ± 13% from the value given by standard methods using 100 g or more, thus is sufficiently reliable for most applications. When ECRa is sufficiently large, the temperature sensitivity of ECRa can be measured reliably with this method, with obtained mean values ranging from 0.39 ± 0.05% °C−1 for Compreignac granite, to 2.8 ± 0.2% °C−1 for La Crouzille pitchblende, both from the centre of France. This method is useful to study dedicated problems, such as the small scale variability of ECRa, and in circumstances when only a small amount of sample is available, for example from remote areas or from precious materials such as historical building stones.

Soil characterization using patterns of magnetic susceptibility versus effective radium concentration

Abstract. Low-field magnetic susceptibility χm and effective radium concentration ECRa, obtained from radon emanation, have been measured in the laboratory with 129 soil samples from Nepal. Samples along horizontal profiles in slope debris or terrace scarps showed rather homogeneous values of both χm and ECRa. One sample set, collected vertically on a lateritic terrace scarp, had homogeneous values of ECRa while χm increased by a factor of 1 to 10 for residual soils and topsoils. However, for a set of samples collected on three imbricated river terraces, values of ECRa, homogeneous over a given terrace, displayed a gradual increase from younger to older terraces. By contrast, χm showed more homogeneous mean values over the three terraces, with a larger dispersion, however, for the younger one. Similarly, Kathmandu sediments exhibited a large increase in ECRa from sand to clay layers, while χm increased moderately. The combination of χm and ECRa, thus, provides a novel tool to characterize quantitatively various soil groups and may be of interest to distinguish modes of alteration or deposition histories.

Heterogeneous temperature sensitivity of effective radium concentration from various rock and soil samples

Abstract. Temporal variations of radon concentration, or spatial variations around geothermal systems, are partly driven by the effect of temperature on the radon source term, the effective radium concentration (ECRa). ECRa from 12 crushed rock and 12 soil samples from Nepal was measured in the laboratory using the radon accumulation method and Lucas scintillation flasks at three temperatures: 7, 22 and 37 °C. For each sample and at each temperature, 5 or 6 measurements were carried out, representing a total of 360 measurements, with an ECRa average varying from 1.1 to 75 Bq kg−1. While the effect is small, ECRa was observed to increase with temperature in a significant and sufficiently reproducible manner. The increase was approximately linear with a slope (temperature sensitivity, TS) expressed in % °C−1. We observed a large heterogeneity of TS with average values (range min-max) of 0.79 ± 0.05 (0.16–2.0) % °C−1 and 0.61 ± 0.05 (0.10–2.0) % °C−1, for rock and soil samples, respectively. While this range overlaps with the results of previous studies, our values of TS tend to be smaller. The observed heterogeneity implies that the TS, rather poorly understood, needs to be assessed by dedicated experiments in every case where it is of consequence for the interpretation.

Measurements of radon emanation from soil samples in triple-junction of North and East Anatolian active faults systems in Turkey

Using the closed-can technique, radon exhalation rate, radon concentration and effective radium content have been carried out for soil samples collected from triple-junction of the North Anatolian Fault System and East Anatolian Fault System. The measured maximum values of the exhalation rate and radon concentration in the system were 400.7 (mBq m −2 h −1) and 8.10 Bq kg −1, respectively. However, effective radium concentration in soil samples has been found to vary from 0.02 to 0.80 Bq kg −1. Linear correlation was observed between soil-gas radon concentration, effective radium content and radon exhalation rate. The linear correlation coefficient between radium content and radon concentration was found to be 0.91. Nevertheless, it was found that there is a linear correlation ( R 2 = 0.99 ) between the radon concentration and exhalation rate.