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.     

Use of radon isotopes,gamma radiation and dye tracers to study water interactions in a small stream in Brazil

In this study, natural (²²²Rn) and fluorescent (uranin) tracers were used to investigate the interactions between surface and subsurface waters in a small hydrographical basin located in the southeast region of Brazil. Levels of ²²²Rn were measured in 117 water samples with the use of an alpha solid-state detector. After the identification of the probable discharge sections along the stream, a measurement of the natural flows, upstream and downstream of these sections, was done with the use of a fluorimeter and fluorescent tracers. Also, scanning was done to verify a correlation between the natural gamma radiation and the ²²²Rn in the areas where its activity was higher. The results showed some sections where the ²²²Rn activity is more significant and contributed to the growth of the flows along the stream. It was possible to confirm a correlation between the discharge sections and the natural gamma radiation, what can be used as a preliminary approach to finding these sections in scenarios similar to the one studied here.     

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.     

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.     

Uranium, radon, helium and other trace elements and gases in well waters of parts of the St. Lawrence Lowlands, (Ottawa region) Canada

To test the usefulness of groundwater for U prospecting, 130 domestic wells were sampled from a 22,000 km² rectangular area extending south and west from Ottawa, Ontario. The waters were analyzed for twenty variables including ten gases and five trace elements.The dissolved gases give information on subsurface chemical and geological environments. Carbon dioxide highs occur at the contact of granites and limestones; He highs are along major faults; CH₄ and H₂S reveal strongly reducing environments, and O₂ in wells indicates shallow wells or active water turnover. Rn reveals U mineralization and granitic rocks.The South March U-Cu surface occurrence gives coincident U, Cu, Zn, Pb and Rn highs. A weak U-Rn-Cu high in the Richmond-North Gower area suggests more radioactive subcropping March formation there.A strong regional He anomaly coincident with the outcropping Rockcliffe Formation and controlled by the Hazeldean Fault is believed to be due to deep-seated He, but its coincidence with a weak regional Cu anomaly, a Rn anomaly in the Bells Corners area, its linear northeast trend, the presence of Helikian U-rich source rocks to the southwest, and northeast-dipping Paleozoic sandstones provide both indications of, and the right environment for epigenetic type U-Cu occurrences.     

Spatial Distribution of Dissolved Radon in the Choptank River and Its Tributaries: Implications for Groundwater Discharge and Nitrate Inputs

The Choptank River, Chesapeake Bay’s largest eastern-shore tributary, is experiencing increasing nutrient loading and eutrophication. Productivity in the Choptank is predominantly nitrogen-limited, and most nitrogen inputs occur via discharge of high-nitrate groundwater into the river system’s surface waters. However, spatial patterns in the magnitude and quality of groundwater discharge are not well understood. In this study, we surveyed the activity of ²²²Rn, a natural groundwater tracer, in the Choptank’s main tidal channel, the large tidal tributary Tuckahoe Creek, smaller tidal and non-tidal tributaries around the basin, and groundwater discharging into those tributaries, measuring nitrate and salinity concurrently. ²²²Rn activities were <100 Bq m^?3 in the main tidal channel and 100–700 Bq m^?3 in the upper Choptank River and Tuckahoe Creek, while the median Rn activities of fresh tributaries and discharging groundwater were 1,000 and 7,000 Bq m^?3, respectively. Nitrate-N concentrations were <0.01 mg L^?1 throughout most of the tidal channel, 1.5–3 mg L^?1 in the upper reaches, up to 13 mg L^?1 in tributary samples, and up to 19.6 mg L^?1 in groundwater. Nitrate concentrations in tributary surface water were correlated with Rn activity in three of five sub-watersheds, indicating a groundwater nitrate source. ²²²Rn and salinity mass balances indicated that Rn-enriched groundwater discharges directly into the Choptank’s tidal waters and suggested that it consists of a mixture of fresh groundwater and brackish re-circulated estuarine water. Further sampling is necessary to constrain the Rn activity and nitrate concentration of discharging groundwater and quantify direct discharge and associated nitrogen inputs.     

Some limitations in using <Superscript>222</Superscript>Rn to assess river–groundwater interactions: the case of Castel di Sangro alluvial plain (central Italy)

²²²Rn was used to assess river–groundwater interactions within Castel di Sangro alluvial aquifer (Italy). The effectiveness of results obtained through this indicator was verified by also analyzing δ¹⁸O, major ions and temperature in both surface and groundwater, and carrying out piezometric head monitoring and discharge measurements. Hydrogeological investigations suggested that the river infiltrates into the aquifer in the south-eastern aquifer portion, while groundwater discharges into the river in the north-eastern portion. The latter phenomenon is supported by ²²²Rn data. Nevertheless, flow-through conditions cause the modelled discharge along this river reach, estimated by ²²²Rn data in a degassing-corrected two-component mixing model, to be greater than the measured discharge. Concerning river infiltration into the aquifer, δ¹⁸O, major ions and temperature data show that the river contribution is negligible in terms of aquifer recharge. Thus, the observed increase in ²²²Rn concentration in that portion of the aquifer is due to the enrichment process caused by infiltration of rainwater (²²²Rn free) which flows from the local divide area. Hence, in the study site, the use of only ²²²Rn to predict river–groundwater interactions causes some estimation inaccuracies and it must be coupled with other hydrochemical and hydrogeological parameters to gain a thorough understanding of such interactions.     

Anomalous change in atmospheric radon concentration sourced from broad crustal deformation: A case study of the 1995 Kobe earthquake

Before the Kobe earthquake, an anomalous increase in atmospheric Rn concentration was observed. By separating the measured concentration of atmospheric Rn into three components according to the distance from the monitoring station, the variation of Rn exhalation rate can be estimated for the respective area using the daily minimum and maximum concentrations. The mean rate of Rn exhalation gradually increased in an area of 20 km around the monitoring station, becoming five times higher than normal in the period between October 1994 and the date of the earthquake. This area had a large co-seismic displacement of up to 30 cm, which roughly corresponds to the crustal strain of 10⁻⁶-order, and it is considered the main source for the atmospheric Rn prior to the Kobe earthquake. Analyses revealed that the pre-seismic change in the atmospheric Rn concentration exhibited an anomalous pattern which would yield information on the spatial distribution of the mechanical response of the ground.     

Flux of4He from Carnmenellis granite: modelling of an HDR geothermal reservoir

The diffusion coefficient (D) of He in the Carnmenellis granite, recovered from the Rosemanowes hot dry rock (HDR) geothermal reservoir, is experimentally determined in the laboratory in a temperature range of 100–300°C. Temperature variation of D fits an Arrhenius plot, but yields an activation energy for He release from rock significantly lower than the value for feldspar or quartz, suggesting that most of the He in the reservoir granite resides within grain boundaries and jointing cement and may be easily released.The reservoir surface area (S) is estimated based on the laboratory determined value of D from granite and the measured⁴He contents of circulation fluids from RH 15 well. In the first year of reservoir circulation,⁴He-based reservoir surface area was about twice that based on²²²Rn. It increased and stabilized at about 6 times after 2 a. The excess He release from newly opened up fracture surfaces within the reservoir during its expansion is shown to be responsible for the temporal increase in the He-based reservoir surface area over that of Rn.     

Precursors of instability in a natural slope due to rainfall: a full-scale experiment

A full-scale landslide-triggering experiment was conducted on a natural sandy slope subjected to an artificial rainfall event, which resulted in mobilisation of 130 m³ of soil mass. Novel slope deformation sensors (SDSs) were applied to monitor the subsurface pre-failure movements and the precursors of the artificially triggered landslide. These fully automated sensors are more flexible than the conventional inclinometers by several orders of magnitude and therefore are able to detect fine movements (<?1 mm) of the soil mass reliably. Data from high-frequency measurements of the external bending work, indicating the transmitted energy from the surrounding soil to these sensors, pore water pressure at various depths, horizontal soil pressure and advanced surface monitoring techniques, contributed to an integrated analysis of the processes that led to triggering of the landslide. Precursors of movements were detected before the failure using the horizontal earth pressure measurements, as well as surface and subsurface movement records. The measurements showed accelerating increases of the horizontal earth pressure in the compression zone of the unstable area and external bending work applied to the slope deformation sensors. These data are compared to the pore water pressure and volumetric water content changes leading to failure.     

Tracing of residual multiple DNAPL sources in the subsurface using 222Rn as a natural tracer at an industrial complex in Wonju, Korea

Depth-discrete tracing of residual dense non-aqueous phase liquid (DNAPL) sources in the subsurface is of great importance in making decisions related to contaminated groundwater remediation. Temporal variations in the natural tracer ²²²Rn and contaminant concentrations in groundwater contaminated with multiple chlorinated contaminants, such as trichloroethene, carbon tetrachloride, and chloroform, were examined to trace residual multiple DNAPL contaminants at an industrial complex in Wonju, Korea. The ²²²Rn activities and multiple DNAPL concentrations in the groundwater fluctuated irregularly according to the groundwater recharge. The natural tracer ²²²Rn in groundwater present in the soil layer, originating from the underlying crystalline biotite granite, had a wide range from 29,000 to 179,000 Bq/m³, and total concentrations of chlorinated solvents ranged from 0.06 to 17.77 mg/l, indicating the ambiguous results of ²²²Rn for tracing the residual DNAPL sources. In this paper, a method is presented to locate zones with a high probability of containing depth-discrete residual multiple DNAPL sources using ²²²Rn and considering relative contaminant concentrations. The results demonstrate that the combination of the ²²²Rn activities as a natural tracer and the relative contaminant concentrations is able to be used as a useful tool for tracing residual DNAPLs.     

Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China

The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H₂, CO₂, CH₄, O₂, N₂, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H₂ anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H₂ (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO₂, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H₂ concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H₂, CO₂, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity.     

Groundwater fluxes into a submerged sinkhole area,Central Italy,using radon and water chemistry

The groundwater contribution into Green Lake and Black Lake (Vescovo Lakes Group), two cover collapse sinkholes in Pontina Plain (Central Italy), was estimated using water chemistry and a ²²²Rn budget. These data can constrain the interactions between sinkholes and deep seated fluid circulation, with a special focus on the possibility of the bedrock karst aquifer feeding the lake. The Rn budget accounted for all quantifiable surface and subsurface input and output fluxes including the flux across the sediment–water interface. The total value of groundwater discharge into Green Lake and Black Lake (∼540 ± 160 L s⁻¹) obtained from the Rn budget is lower than, but comparable with historical data on the springs group discharge estimated in the same period of the year (800 ± 90 L s⁻¹). Besides being an indirect test for the reliability of the Rn-budget “tool”, it confirms that both Green and Black Lake are effectively springs and not simply “water filled” sinkholes. New data on the water chemistry and the groundwater fluxes into the sinkhole area of Vescovo Lakes allows the assessment of the mechanism responsible for sinkhole formation in Pontina Plain and suggests the necessity of monitoring the changes of physical and chemical parameters of groundwater below the plain in order to mitigate the associated risk.     

The implications of laboratoryRn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of²²²Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm² g⁻¹ , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factork (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.     

Collection of radon decay products — a uranium exploration technique

The spontaneous deposition of short-lived radon decay products onto solid surfaces (“collectors”) provides the basis for a simple and efficient way of prospecting for uranium. The alpha activity of two of the decay products, ²¹⁸Po and ²¹⁴Po, can be measured by conventional counting techniques following the exposure of a collector to a radon source. Laboratory studies have shown: (a) radon decay products can be collected on a wide variety of materials; (b) the number of radon decay products increases with the collector surface area; (c) a negative charge applied to the collectors enhances the number of decay products collected; (d) the shape of the collectors is relatively unimportant; and (e) reproducibility is about ± 5% of the measured value.Field tests on known uranium anomalies involved suspending collectors for an overnight exposure, in either covered holes or inverted containers buried in soil. Subsequent removal of the collectors was immediately followed by counting the alpha activity. Anomalies were readily detected at three different test sites. Replicate measurements at selected sites both on an hour-to-hour and on a day-to-day basis showed remarkably good agreement. Some anomalies were also outlined using activity measurements from collectors suspended above soil aliquots in sealed containers.Because the half-lives of two of the thoron (²²⁰Rn) decay products are much longer than those of the radon (²²²Rn) decay products, a thoron correction can be applied to the original activity measured from the collectors, thus discriminating against thoron in thorium-rich terrains.The counting of radon decay products deposited onto collectors is an effective method for delineating radon anomalies in uranium exploration. The field techniques are simple to apply, and results are available after very short exposure times (about 18 hours) and after short counting intervals (5 minutes).     

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.     

Radon in groundwater as a tracer to assess flow velocities: two test cases from Israel

 Two test cases from Israel are presented herein employing the decay rate of radon along the flow path to assess groundwater flow velocities. Groundwater flow reaching the fault zone emerges in several places along the rift fault zone as thermal springs because of deep water confinement. The high water temperature of the surface is indicative of high vertical flow velocities, which maintains the original high temperatures. Knowing the Rn content at a source point and at a given down-gradient, and assuming no Rn addition from the water itself or along the flow path, one can calculate the flow velocity based on the Rn half-life time. The decay of Rn in western Galilee was found to be ∼570–150 pCi/l, and in the Dead Sea area from 5000–2000 pCi/l along a respective flow path of 1000 and 200 m, Based on the above, the calculated flow velocities were compared with those obtained from pumping tests in the study area. The method is applicable, because of the short Rn half-life, to cases of high Rn contents, short distances and high flow velocities. Received: 18 January 2000 · Accepted: 21 March 2000     

Mapping radon-prone areas - a geophysical approach

 Radon-prone areas in Israel were mapped on the basis of direct measurements of radon (²²²Rn) in the soil/rock gas of all exposed geological units, supported by the accumulated knowledge of local stratigraphy and sub-surface geology. Measurements were carried out by a modified alpha-track detection system, resulting in high radon levels mainly in rocks of the Senonian-Paleocene-aged Mount Scopus Group, comprised of chert-bearing marly chalks, rich in phosphorite which acts as the major uranium source. Issues of source depth, seasonal variations and comparison with indoor radon levels are addressed as well. This approach could be applied to other similar terrains, especially the Mediterranean Phosphate Belt.     

Analysis of the exchange of groundwater and river water by using Radon-222 in the middle Heihe Basin of northwestern China

In arid regions of western China, water resources come from mountain watersheds and disappear in the desert plain. The exchange of surface water and groundwater takes place two or three times in a basin. It is essential to analyze the interaction of groundwater with surface water to use water resources effectively and predict the change in the water environment. The conventional method of analysis, however, measures only the flow of a stream and cannot determine groundwater seepage accurately. As the concentration of Radon-222 (²²²Rn) in groundwater is much higher than in surface water, the use of ²²²Rn was examined as an indicator for the analysis of the interaction between surface water and groundwater. Measurement of the ²²²Rn concentration in surface water was conducted to detect groundwater seepage into a stream in the middle Heihe Basin of northwestern China. Furthermore, the simultaneous groundwater flow into and out of a stream from the aquifers was quantified by solving the ²²²Rn mass balance equation, in which the losses of gas exchange and radioactive decay of ²²²Rn are considered. Meanwhile, river runoff was gauged to determine the exchange rates between surface water and groundwater. The result shows that ²²²Rn isotope can be used as a good environmental tracer with high sensitivity for the interaction between surface water and groundwater, especially in the fractured aquifer system, karst aquifer system and discharge basins.