A guideline for the evaluation of the soil radon potential based on geogenic and anthropogenic parameters

Following a brief review about radon (²²²Rn) in soils, this study gives a compilation and evaluation of important parameters of the soil ²²²Rn potential. Because investigations of the potential are usually limited to rural areas, anthropogenic or urban effects have not attracted much attention so far. Measurements within the densely populated Ruhr district (Germany) present some new insights into geogenic and anthropogenic parameters on the soil ²²²Rn potential. According to their importance, seven parameters were strung in aranking system, which can be applied in both rural and urban areas: made grounds, geology, relief, vegetation cover, tectonics, soil sealing, traffic vibrations. While the first four parameters control the soil ²²²Rn potential on a more regional scale, the last three can modify it locally. Furthermore, an evaluation of the potential within a ten-point-system is proposed. The advantage of the system lies in the possibility to estimate the soil ²²²Rn potential of any site on a local scale. Common classifications and rankings, based on insufficiently scaled maps, can be included in the proposed system.     

象牙海岸Issia地区铌钽砂矿的研究(英文)

在西非的象牙海岸Issia地区分布着许多铌钽矿床,包括原生矿和砂矿。原生矿产在分带很好的伟晶岩中。这个地区发育着许多花岗岩和花岗伟晶岩。伟晶岩可分为:(1)分异不好的伟晶岩,无矿化;(2)含Be伟晶岩,分带较好,见绿柱石;(3)含Be-Nb-Ta的伟晶岩,分带很好,是矿体;(4)Be-Nb-Ta-Li伟晶岩,分带很好,是矿体。除了10多个原生矿外,还有许多砂矿。砂矿有两种类型:一种近原生矿的坡积砂矿,另一种是经河流冲刷搬运后的冲积砂矿。我们选择了Etienne-Meguhe砂矿作为近原生矿的坡积砂矿的代表,Bemadi矿作为河流冲积型的砂矿的代表,对这两种类型的砂矿的风化、搬运、沉积机理进行了比较研究。研究发现不同的搬运距离和搬运模式对铌钽砂矿的主要矿物——铌钽铁矿的化学性质没有什么影响,而其物理形态则发生了变化。坡积型砂矿的铌钽铁矿常与石英、长石等矿物连生,其颗粒大小与原生的Nb-Ta伟晶岩中的铌钽铁矿一致;经河流冲积而形成的砂矿中,铌钽铁矿颗粒变小、轮廓变得圆滑,几乎无连生矿物,但矿体集中,常在河流的冲积扇中分布,易开采。     

Factors controlling the groundwater transport of U, Th, Ra, and Rn

A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the²³⁸U and²³²Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay, α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the²³⁸U and²³²Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance ϰ_i, characteristic of each nuclide. Where ϰ_i is much longer than the aquifer length, (for²³⁸U,²³⁴U, and²³²Th), the activities grow linearly with distance. Where gKⁱ is short compared to the aquifer length, (for²³⁴Th,²³⁰Th,²²⁸Th,²²⁸Ra, and²²⁴Ra), the activities rapidly reach a constant or quasi-constant activity value. For²²⁶Ra and²²²Rn, the limiting activity is reached after 1 km. High δ²³⁴U values (proportional to the ratio^ɛ234Th/^W238U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios²³⁰Th/²³²Th,²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for²²⁸Ra/²²⁶Ra and²²⁴Ra/²²⁶Ra activity ratios less than unity. From the model, the highest²²²Rn emanation equals 2ɛ. This is in agreement with the hypothesis that²²²Rn activity can be used as a first approximation for input by recoil (Krishnaswamiet al 1982). However, high²²²Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.     

Dependence of radon exhalation on grain size of sedimentary waste

Tailings resulted from sulphuric acid leaching process of uranium from sedimentary rocks contain high concentrations of ²²⁶Ra and its daughters, the most important of which is ²²²Rn. Movement of radon gas out of the tailings is strongly influenced by the physicochemical characteristics of these tailings especially their radium content and the grain size. So, the tailing samples were size fractionated into four sizes (>?250, 250–125, 125–74 and <?74 µm). The natural radioactivity was investigated using hyper-pure germanium detector and solid-state nuclear track detectors (CR-39) for bulk size and after size fractionation. The activity concentrations of different radionuclides in size-fractionated tailing samples have been shown to be strongly dependent on the size of the particles. In the range of >?250 and <?74 µm, the activity concentrations of ²³⁰Th, ²²⁶Ra, ²¹⁴Pb, ²¹⁴Bi, ²¹⁰Pb, ²³²Th and ⁴⁰K increased throughout with decreasing particle size, while that of ²³⁸U, ²³⁴U and ²³⁵U have an opposite effect. The results revealed an inverse relationship between the radon exhalation rate and size fractionation. Also, the results showed a good correlation between radium activity concentration and radon mass exhalation rate.     

U-238 series radioactive disequilibrium in groundwaters: implications to the origin of excess U-234 and fate of reactive pollutants

The concentrations of ²³⁸U, ²³⁴Th, ²²⁶Ra, ²²²Rn and ²¹⁰Pb and ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios have been measured in several groundwater samples from Gujarat, India. In the aqueous phase the abundances of ²³⁴Th and ²¹⁰Pb are grossly deficient relative to their parents, ²³⁸U and ²²²Rn respectively. The deficiency is ascribable to the impact adsorption of ²³⁴Th and ²¹⁰Pb atoms onto particle surfaces which are very abundant in the groundwater regimes. The scavenging residence times for both these nuclides is about a day, suggesting that irreversible removal of ‘reactive’ metals and pollutants in groundwaters can occur on very short time scales. The fast removal of ²³⁴Th onto particles necessitates that in these groundwaters ²³⁴U ‘excess’ has to originate through leaching of soil grains rather than through in situ decay of dissolved ²³⁴Th in the water.     

Statistical analysis of the radon-222 potential of rocks in Virginia,U.S.A.

More than 3,200 indoor radon-222 (²²²Rn) measurements were made seasonally in an area of about 1,000 square kilometers of the Coastal Plain and Piedmont physiographic provinces in Virginia, U.S.A. Results of these measurements indicate that some geological units are associated, on the average, with twice as much indoor²²²Rn as other geological units, and that indoor²²²Rn varies seasonally. The Kruskal-Wallis test was used to test whether indoor²²²Rn concentrations for data gathered over the winter and summer seasons differ significantly by rock unit. The tests concluded that indoor²²²Rn concentrations for different rock units were not equal at the 5-percent significance level. The rocks associated with the highest median indoor²²²Rn concentration are specific rocks in the Mesozoic Culpeper basin, including shale and siltstone units with Jurassic diabase intrusives, and mica schists in the Piedmont physiographic province. The pre-Triassic Peters Creek Schist has the highest ranking in terms of indoor²²²Rn concentration. The rocks associated with the lowest indoor²²²Rn concentrations include coastal plain sediments, the Occoquan Granite, Falls Church Tonalite, Piney Branch Mafic and Ultramafic complex, and unnamed mafic and ultramafic inclusions, respectively. The rocks have been ranked according to observed²²²Rn concentration by transforming the average rank of indoor²²²Rn concentrations to z scores.     

液体闪烁测222Rn法及野外应用

本文介绍了一种新的野外测²²²Rn法——液体闪烁测²²²Rn法。该法取样时间短,操作简单,灵敏度高,重复性好,成本低廉,并能有效地克服钍射气的干扰。野外应用取得了较好的效果。     

The transport of U- and Th-series nuclides in sandy confined aquifers

Abundances of ²³⁸U, ²³⁴U, ²³²Th, ²²⁶Ra, ²²⁸Ra, ²²⁴Ra, and ²²²Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that ²³²Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of ²³²Th and ²³⁰Th on aquifer grains. Uranium in the aquifer waters has very high [²³⁴U/²³⁸U] ∼ 9 and low ²³⁸U concentrations. These levels can be explained by low weathering rates in the aquifer (w_238U ∼ 2 × 10⁻¹⁸ to 2 × 10⁻¹⁷s⁻¹) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The ²²²Rn and ²²⁸Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [²³⁴U/²³⁸U] and high ²³⁸U concentrations. The flow model shows that it is not possible to obtain the high [²³⁴U/²³⁸U] and low [²³⁸U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.     

Controls on pore-fluid concentration of 4He and 222Rn and the calculation of 4He/222Rn ages

Controls governing the production of ⁴He and ²²²Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. ²²²Rn activity is used to quantify the uranium sources for ⁴He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GL_Rn//GL_He, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated ⁴He. It was found that /GL_Rn//GL_He can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for ²²²Rn decay, r_e.⁴He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of ⁴He/ ²²²Rn measurements to continental freshwater springs indicates that the weathering release of accumulated ⁴He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the ⁴He/²²²Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.     

Controls on pore-fluid concentration of He and Rn and the calculation of He/Rn ages

Controls governing the production of ⁴He and ²²²Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. ²²²Rn activity is used to quantify the uranium sources for ⁴He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GL_Rn//GL_He, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated ⁴He. It was found that /GL_Rn//GL_He can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for ²²²Rn decay, r_e.⁴He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of ⁴He/ ²²²Rn measurements to continental freshwater springs indicates that the weathering release of accumulated ⁴He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the ⁴He/²²²Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.     

Uranium prospecting with Rn in frozen terrain

Published reports show that ²²²Rn contents of soil gas can increase under snow-covered or frozen soils. By utilizing these observations and results of field tests described here, it can be stated that U prospecting with ²²²Rn in frozen terrain is practicable.Rn profiles in frozen and snow-covered soils over U mineralization in the Bancroft area of Ontario outlined known radioactive zones more clearly than did scintillometer profiles.Tests in Gatineau Park, Quebec, in the proximity of a radioactive pegmatite dike showed that lake ice acts as a restrictive barrier to Rn movement from lake waters beneath. Water samples, collected and allowed to freeze in plastic bottles, retained dissolved Rn quite effectively thus permitting sample collection and storage under the most severe winter conditions. Rn sampling of through-ice lake waters is therefore a feasible prospecting tool.Samples of spring run-off (snow-melt) waters and slushy snow collected from within a known weakly radioactive zone near South March, Ontario, were shown to contain less Rn than found in the same stream waters in the summer. No pronounced Rn degassing event of frozen soils was apparent early in the spring thaw in percolating run-off waters draining from the zone. The usefulness of spring thaw hydrogeochemistry using Rn is discussed.     

Natural Gas Hazard (CO2, 222 Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the Ciampino-Marino Area, Alban Hills Volcano, Italy

Groundwater surveys were performed by detailed(around 300 sites) grid-analysis of water temperature, pH, redox potential, electrical conductivity, ²²²Rn, alkalinity and by calculating the pCO₂, throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO₂ exhalation from soils during the last 20 years and earlier ashistorically recorded, the work aimed at assessing the Natural Gas Hazard (NGH) including the indoor-Rn hazard. The NGH was defined as the probability of an area to become a site of poisonous peri-volcanic gas exhalations from soils to the lower atmosphere (comprising buildings). CO₂ was found to be a ``carrier' for the other poisonous minor and in trace components (H<sub>s</sub>S, CH<sub>4</sub>, <sup>222</sup>Rn, etc.). This assessment was performed by extrapolating in the aquifer CO<sub>2</sub> and <sup>222</sup>Rn conditions, and discriminating sectors where future CO<sub>2</sub> flux in soils as well as indoor-Rn measurements have to be noted. A preliminary indoor-Rn survey was performed at about 200 sites. The highest values were found in the highest pCO<sub>2</sub> and high <sup>222</sup>Rn values in groundwater. This indicates convection and enhanced permeability in certain sectors of the main aquifer, i.e., along the bordering faults and inside the gas-trap of the Ciampino Horst., where ``continuous gas-phase micro-macro seepage mechanism' is invoked to explain the high peri-volcanic gases flux.     

Hydrogeological and geochemical control of the variations of Rn concentrations in a hard rock aquifer: Insights into the possible role of fracture-matrix exchanges

To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L⁻¹. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m⁻² s⁻¹ in granite and from 0.5 to 0.9 mBq m⁻² s⁻¹ in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10⁻⁵ to 10⁻³ m² s⁻¹. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm.     

Radon concentrations in soil gas,considering radioactive equilibrium conditions with application to estimating fault-zone geometry

A calculation method for determining the amount of Rn isotopes and daughter products at the start of measurement (CRAS) is proposed as a more accurate means of estimating the initial Rn concentration in soil gas. The CRAS utilizes the decay law between ²²²Rn and ²²⁰Rn isotopes and the daughter products ²¹⁸Po and ²¹⁶Po, and is applicable to α-scintillation counter measurements. As Rn is both inert and chemically stable, it is useful for fault investigation based on the soil gas geochemistry. However, the total number of α particles emitted by the decay of Rn has generally been considered to be proportional to the initial Rn concentration, without considering the gas condition with respect to radioactive equilibrium. The CRAS method is shown to be effective to derive Rn concentration for soil gases under both nonequilibrium conditions, in which the total number of decays increases with time, and equilibrium conditions, which are typical of normal soil under low gas flux. The CRAS method in conjunction with finite difference method simulation is applied to the analysis of two active fault areas in Japan, and it is demonstrated that this combination could detect the sharp rises in ²²²Rn concentrations associated with faults. The method also allows the determination of fault geometry near the surface based on the asymmetry variation of the Rn concentration distribution when coupled with a numerical simulation of ²²²Rn transport. The results for the new method as applied to the two case studies are consistent with the data collected from the geological survey. It implies that the CRAS method is suitable for investigating the fault system and interstitial gas mobility through fractures. The present analyses have also demonstrated that high Rn concentrations require the recent and repeated accumulation of ²²²Rn parents (²³⁰Th and ²²⁶Ra) in fault gouges through deep gas release during fault movement.     

High energy X-γ ray spectrometer on the Chandrayaan-1 mission to the Moon

The Chandrayaan-1 mission to the Moon scheduled for launch in late 2007 will include a high energy X-ray spectrometer (HEX) for detection of naturally occurring emissions from the lunar surface due to radioactive decay of the²³⁸U and²³²Th series nuclides in the energy region 20–250 keV. The primary science objective is to study the transport of volatiles on the lunar surface by detection of the 46.5 keV line from radioactive²¹⁰Pb, a decay product of the gaseous²²²Rn, both of which are members of the²³⁸U decay series. Mapping of U and Th concentration over the lunar surface, particularly in the polar and U-Th rich regions will also be attempted through detection of prominent lines from the U and Th decay series in the above energy range. The low signal strengths of these emissions require a detector with high sensitivity and good energy resolution. Pixelated Cadmium-Zinc-Telluride (CZT) array detectors having these characteristics will be used in this experiment. Here we describe the science considerations that led to this experiment, anticipated flux and background (lunar continuum), the choice of detectors, the proposed payload configuration and plans for its realization     

土壤氡析出率活性炭累积吸附法

为解决大面积多点快速土壤氡析出率测量问题, 设计开发并进行了土壤氡析出率活性炭累积吸附法的方法技术研究.在参考氡析出率装置和野外实验数据的基础上, 进行了测量装置及材料的选择、装置刻度、野外累积暴露时间、活性炭盒吸附氡有效衰变常数等研究.通过2002-2003年22次重复观测, 北京西北郊区粘土类土壤氡析出率平均值为20.15mBq.m-2.s-1, 最大值和最小值分别为26.75和13.49mBq.m-2.s-1, 标准偏差为3.55mBq.m-2.s-1.与仪器静电收集法比对结果表明, 两种方法的相对误差小于±10%.实验结果表明: 活性炭方法测量结果稳定可靠, 简便易行, 适用于全国大面积土壤氡析出率调查.      

Ra-226 and Rn-222 in saline water compartments of the Aral Sea region

The Aral Sea has been shrinking since 1963 due to extensive irrigation and the corresponding decline in the river water inflow. Understanding of the current hydrological situation demands an improved understanding of the surface water/groundwater dynamics in the region. ²²²Rn and ²²⁶Ra measurements can be used to trace groundwater discharge into surface waters. Data of these radiometric parameters were not previously available for the study region. We determined ²²²Rn activities after liquid phase extraction using Liquid Scintillation Counting (LSC) with peak-length discrimination and analyzed ²²⁶Ra concentrations in different water compartments of the Amu Darya Delta (surface waters, unconfined groundwater, artesian water, and water profiles from the closed Large Aral Sea (western basin).The water samples comprise a salinity range between 1 and 263 g/l. The seasonal dynamics of solid/water interaction under an arid climate regime force the hydrochemical evolution of the unconfined groundwater in the Amu Darya Delta to high-salinity Na(Mg)Cl(SO₄) water types. The dissolved radium concentrations in the waters were mostly very low due to mineral over-saturation, extensive co-precipitation of radium and adsorption of radium on coexisting solid substrates.The analysis of very low ²²⁶Ra concentrations (<10 ppq) at remote study sites is a challenge. We used the water samples to test and improve different analytical methods. In particular, we modified a procedure developed for the α-spectrometric determination of ²²⁶Ra after solid phase extraction of radium using 3M Empore™ High Performance Extraction Disks (Purkl, 2002) for the analysis of the radionuclide using an ICP sector field mass spectrometer. The ²²⁶Ra concentration of 17 unconfined groundwater samples ranged between 0.2 and 5 ppq, and that of 28 artesian waters between <0.2 and 13 ppq. The ICP-MS results conformed satisfactorily to analytical results based on γ-measurements of the ²²²Rn ingrowth after purging and trapping on super-cooled charcoal. The ²²⁶Ra concentrations were positively correlated with the salinity and the dissolved NaCl concentrations. The occurrence of unusually high ²²⁶Ra activities is explained by radium release from adsorption sites with increasing salinity. The inferred spatial variability of ²²²Rn in the Aral Sea and of ²²²Rn and ²²⁶Ra in the groundwater of the Amu Darya Delta is discussed in the context of our own previous hydrochemical studies in the study sites. Relatively low ²²²Rn activities in the unconfined GW (1–9.5 Bq/l) indicate the alluvial sediments hosting the GW to be a low-²³⁸U(²²⁶Ra) substrate. Positive correlations between U and ²²⁶Ra, and U and ²²²Rn are likely related to locally deposited Fe(Mn)OOH precipitates. The ²²²Rn activity of the GW, however, distinctly exceeds the ²²²Rn concentration in the Aral Sea (10 mBq/l), in principle, making ²²²Rn a sensitive tracer for the inflow of GW. The high water volume of the Large Aral Sea and wind induced mixing of its water body, however, hamper the detection of local groundwater inflow.     

Helium, radon and radiocarbon studies on a regional aquifer system of the North Gujarat-Cambay region, India

The study reports the age evolution of groundwater as it flows from the recharge area through a regional alluvial aquifer system in North Gujarat-Cambay region in western India. Radiocarbon (¹⁴C), ⁴He and ⁴He / ²²²Rn dating methods have been employed. Sediments from a drill core in the Cambay Basin were also analysed for uranium (U) and thorium (Th) concentrations and the measured values have been used to estimate the ⁴He and ²²²Rn production rate for groundwater age calculations. Additionally, factors controlling the distribution of ²²²Rn, ⁴He and temperature anomalies in groundwater, vis-à-vis their relation to the tectonic framework and lithology of the study area, have also been examined.The multi-isotope study indicated a reasonable correspondence in groundwater age estimates by the three methods employed. The groundwater ¹⁴C ages increased, progressively, in the groundwater flow direction: from the foothills of Aravalli Mountains in the east, and reached a value of ∼35 ka towards the region of lowest elevation, linking Little Rann of Kachchh (LRK)-Nalsarovar (NS)-Gulf of Khambhat (GK) in the western part of the study area. In this region, groundwater ages obtained for free flowing thermal wells and springs employing ⁴He and ⁴He / ²²²Rn systematics are in the order of million years. Such anomalous ages are possibly due to enhanced mobilisation and migration of ‘excess helium’ from hydrothermal circulation vents along deep-seated faults. Excluding such anomalous cases and considering all uncertainties, presently estimated ⁴He and ⁴He / ²²²Rn groundwater ages are in reasonable agreement with ¹⁴C age estimates in the Cambay Basin for helium release factor (Λ_He) value of 0.4 ± 0.3. The ⁴He method also indicated west-southwards progression of groundwater ages up to ∼100 ka beyond the Cambay Basin.Large ‘excess helium’ concentrations are also seen to be generally associated with anomalous groundwater temperatures (> 35 °C) and found to overlie some of the basement faults in the study area, particularly along the east and the west flanks of the Cambay Basin. Groundwater ²²²Rn activities in most of the study area are 800 ± 400 dpm/l. But, a thermal spring at Tuwa on the east flank of the Cambay Basin, having granitic basement at shallow depth, recorded the highest ²²²Rn activity (∼63,000 dpm/l).     

Rn solution by the circulating fluids in a “hot dry rock” geothermal reservoir

The mechanism of²²²Rn release into fracture fluids by direct alpha-recoil, lattice and grain boundary/micro-crack diffusion is discussed. Experimental measurements of²²²Rn release into surrounding air and water phases have been made for crystalline rock specimens with well defined surface areas. The²²²Rn flux from an infinite plane surface and hence the effective diffusion length of²²²Rn in the rock matrix has been estimated.The²²²Rn flux from plane crystalline rock surfaces has been used in conjunction with a simple hydrological model of the reservoir to calculate the²²²Rn content of the return fluids of a geothermal doublet circulation system. For given production rate and piezometric difference between the injection and production wells, the²²²Rn content of the return fluid is dependent upon the distribution of flow path lengths and fracture apertures in the reservoir. Matching of the calculated and experimental²²²Rn contents of the return fluids has been used to select appropriate parameters for the reservoir model and hence to estimate the extent of the heat-transfer surface. The model estimates the fracture width of the flow paths, total swept surface area and fracture volume within the reservoir.     

222Rn solution by the circulating fluids in a “hot dry rock” geothermal reservoir

The mechanism of²²²Rn release into fracture fluids by direct alpha-recoil, lattice and grain boundary/micro-crack diffusion is discussed. Experimental measurements of²²²Rn release into surrounding air and water phases have been made for crystalline rock specimens with well defined surface areas. The²²²Rn flux from an infinite plane surface and hence the effective diffusion length of²²²Rn in the rock matrix has been estimated.The²²²Rn flux from plane crystalline rock surfaces has been used in conjunction with a simple hydrological model of the reservoir to calculate the²²²Rn content of the return fluids of a geothermal doublet circulation system. For given production rate and piezometric difference between the injection and production wells, the²²²Rn content of the return fluid is dependent upon the distribution of flow path lengths and fracture apertures in the reservoir. Matching of the calculated and experimental²²²Rn contents of the return fluids has been used to select appropriate parameters for the reservoir model and hence to estimate the extent of the heat-transfer surface. The model estimates the fracture width of the flow paths, total swept surface area and fracture volume within the reservoir.