Radon Transport Research Articles

Characterising the Source of Radon Indoors

Average indoor radon concentrations range over more than two orders of magnitude, largely because of variability in the rate at which radon enters from building materials, soil, and water supplies. Determining the indoor source magnitude requires knowledge of the generation of radon in source materials, its movement within materials by diffusion and convection, and the means of its entry into buildings. This paper reviews the state of understanding of indoor radon sources and transport. Our understanding of generation rates in and movement through building materials is relatively complete and indicates that, except for materials with unusually high radionuclide contents, these sources can account for observed indoor radon concentrations only at the low end of the range observed. Our understanding of how radon enters buildings from surrounding soil is poorer; however, recent experimental and theoretical studies suggest that soil may be the predominant source in many cases where the indoor radon concentration is high.

Understanding Radon Transport into Houses

Understanding Radon Transport into Houses

Understanding Radon Transport into Houses

Understanding Radon Transport into Houses

Benthic fronts and global excess radon distribution

Abstract GEOSECS measurements of excess radon 222 as a function of depth above the sediments in the Atlantic and Pacific are sufficient to resolve the median profile in each ocean, but not the mean profile. The transport process maintaining the oceanic means is highly intermittent, and is not consistent with mixing by small scale turbulence generated at the ocean bottom. We suggest that transport of radon above the benthic boundary layer is driven by isopycnal motions associated with baroclinic instabilities on benthic fronts. A method for generating benthic fronts together with the subsequent instabilities is demonstrated in the laboratory.

Exhalation of radon and thoron: the question of the effect of thermal gradients in soil

Experimental measurements show no evidence for diurnal variation in exhalation of radon and thoron from soil due to convection induced by thermal gradients in the top few decimeters of soil as suggested by several authors. Estimates based on convective calculations indicate that even in the unlikely event of either vertical temperature gradients large enough to cause vertical instability, or horizontal gradients sufficient to cause significant convection, any effect would be too small to be detected. These same calculations suggest that it is difficult to conceive of cases involving typical thermal gradients in unfractured porous media such as soil where thermally induced convection would play an important role in transport of radon or thoron.

Transport of radon in flowing boreholes at Stripa, Sweden

Granitic rock in an underground experimental waste storage site at Stripa, Sweden, is unusually high in natural radioelements (∼40 ppm uranium), with higher concentrations occurring locally in thin chloritic zones and fractures. Consequently, groundwater seeping through fractures into open boreholes is highly anomalous in its radon content, with activity as high as 1 μCi/1. When total count gamma‐ray logs are run in boreholes where groundwater inflow is appreciable, the result is quite unusual: the radon daughter activity in the water adds considerably to the gamma contribution from the rock, and in fact often dominates the log. The total gamma activity increases where radon‐charged groundwater enters a borehole and decays as the water flows along the hole in response to the hydraulic gradient. As a consequence the gamma log serves as a flow profile, locating zones of water entry (or loss) by an increase (or decrease) in the total gamma activity. If mixing within the borehole does not occur, the activity decreases exponentially along the hole away from the entry point because of the steady decay of radon and its daughter products as they migrate with the flow in the water column.This spatial decay rate can be converted to a linear flow rate since the 3.8‐day half‐life of radon governs the response time. For example, if the volumetric flow rate in a 76‐mm hole falls within the range 0.5–50 liters per day and if observations are available from a 10‐m length of hole, then the flow rate can be measured quantitatively. Proportionately higher rates can be measured if longer hole lengths are available for observation. A model for flow through a thin crack emanating radon at a rate E shows that the radon concentration of water entering a hole is E/λh, where λ is the radon decay rate and h the crack aperture, assuming that the flow rate and crack source area are such that an element of water resides within the source area for several radon half‐lives or more. Using this simple relationship, independent measurements of emanation and concentration produce reasonable estimates of fracture aperture. Although uranium concentration values at Stripa are unusually high, neither the emanation coefficients nor the fracture properties appear to be unusual for granitic rock. It therefore seems likely that many granitic sites must exist where the radon content in groundwater is higher than in other geological terranes, although perhaps not as high as the microcurie per liter concentrations found at the Stripa site.

Transport of radon through cracks in a concrete slab.

A model involving the use of line sources is developed to describe the transport of radon through the cracks or gaps which appear in concrete slabs used in building foundations. The strength of these sources is determined from the results of the diffusion model proposed by Landman in a previous work. Once the strength of the source is known, additional transport mechanisms can be treated in a simple manner. Pressure differences across the slab and in the underlying soil are discussed. The rate of exhalation through a portion of the cracked slab is determined and compared to the rate of exhalation from the same surface area of bare soil. In typical cases, their ratios vary from 0.25 to 0.50. Therefore, these transport mechanisms account for a larger portion of the levels of radon found in many houses than do previous models.

Evidence for nondiffusive transport of 86Rn in the ground and a new physical model for the transport

Concentration of radon has been measured in the soil near the ground surface with solid‐state, nuclear track detectors with the inverted cup technique. Measurements were made in the overburden at depth intervals 0.1–0.7 m, at 0.1–6 m, and at a constant depth of 0.2 m, in a narrow rectangular matrix. The results disagree with the hypothesis that radon concentration only depends upon local production and migration by diffusion with a diffusion length of about 1 m. A transport length of 0.1–0.2 m is observed near the ground surface and the transport is dominated by a flow component. Radon measurements in the ground surface over the Laisvall lead mine have given evidence of radon transport through rock exceeding a distance of 100 m, which is possible only if the migration is a flow transport with a characteristic transport length larger than about 10 m/day. To explain the radon transport in the overburden and through the rock with a common transport system, the existence of a general upward flow of geo‐gas is proposed. This geo‐gas works as a carrier mechanism for radon. The physical conditions for the existence of a flow transport of radon are discussed.

Transport of radon from fractured rock

The transport of 222Rn from fractured rock has been studied in an abandoned mine. Pressure‐induced flow (both natural and artificial) is quite important and can easily cause an order of magnitude increase in the instantaneous transport of radon into the tunnel airspace compared to that due to flowfree diffusion. Permeability studies indicate that large‐scale cracks of surface density of the order of several cracks per square meter dominate flow. In first order, effective flux due to natural pressure variation follows an approximate dP/dt dependence. In higher order, there exists an enhancement of time‐averaged flux by typically a factor of 2 due to natural pressure variation. Mathematical modeling indicates that the relation between pressure and the strength and time dependence of the radon transport is difficult to explain solely with conventional models for semi‐infinite homogeneous porous media. A model of cul‐de‐sac chambers is proposed to account for some of this dependence.

Radon transport in the earth: A tool for uranium exploration and earthquake prediction

ABSTRACT The222Rn concentration in the ground near the surface of the earth provides a sensitive signal for recognizing subterrestrial flow of fluids. Such flows can indicate the existence of regions of enriched222Rn and hence the presence of subsurface uranium ore. If such flows are caused by stress buildup prior to earthquakes, they can give premonitory signals. Systematic variations in near-surface radon have been observed near ore deposits and in earthquake zones, and are ascribed to subterrestrial flow of fluids. Changes in radon concentration have been seen that are not ascribable to earthquake-related effects. Temperature, pressure, moisture, and prevailing winds have been considered but do not correlate with the radon changes. Earth tidal triggering of these events is the only known possibility that presently cannot be ruled out; it is being assessed. Earthquake related changes in radon concentration have been measured at considerable distances from the hypocenters of earthquakes. A simple dislocation model leads to estimates of distance that increase to hundreds of kilometers at earthquake magnitudes of five and above. The essential components of the theory are that the stress from a dislocation loop varies at large distances × as x−3, while the stress value increases with the magnitude of the earthquake represented. A threshold sensitivity is inferred from our radon measurements associated with earthquakes near Blue Mountain Lake, New York. A global 222Rn-monitoring network is desirable for earthquake prediction.

Changes in subsurface radon concentration associated with earthquakes

The concentration of radon at shallow subsurface depths (∼1 m) changes in response to the vertical flow of fluids past the measuring site. Thus locally produced radon serves as a tracer gas to detect the flow of subsurface fluids which may occur in association with earthquakes. Therefore it is not necessary to invoke either changes in the local rate of radon production or the long‐distance transport of radon to explain changes in the concentration of radon, although these phenomena may also occur. We have measured radon concentration at 60‐cm depth by using solid state nuclear track detectors to measure weekly values and electronic detectors to measure hourly values. An automated system records the outputs of the electronic detectors of radon, as well as the values of various atmospheric parameters. The radon variations we observe at Blue Mountain Lake, New York, are not controlled by changes in atmospheric parameters. From a 3‐year record of weekly radon readings we conclude that the largest seismic event in New York State during this period (M = 3.9), which occurred at the beginning of our record at a distance of 14 km from our site, may be responsible for the highest radon readings, which were also recorded at this time. The seismicity and the radon concentrations have decreased jointly since then. The automated system shows a tenfold increase in radon concentration in association with an unusual burst of seismic activity. Radon was also measured in a grid of 54 sites near Thoreau, New Mexico. The largest nearby earthquake (M = 4.6) occurred during the most anomalous monthly pattern of radon.

Radon emanation over an ore body: Search for long-distance transport of radon

A major hope for discovering subsurface uranium ore is that measurable concentrations of the radioactive gas 222Rn can be recognized near the surface of the earth. Integrated measurements, made over several weeks time, show promise of giving greater reproducibility than short-term measurements, which are more subject to meteorological variability. The use of improved methods of integrated radon measurements — free of 220Rn, of thermal track fading, and of moisture-condensation effects — allow readings to be made that generally are highly stable over time. At a site 16 km north of Thoreau, NM, readings at 60 cm depth taken over a 13-month interval for a set of 55 positions give different, but nearly constant monthly readings at each position, the typical standard deviation being 22%. Superimposed on that stable pattern have been three periods during which spatially grouped radon readings increased by a factor of two or more over their normal values. The simplest tenable description of the source of the increases are sporadic puffs of upflowing gas, originating at as yet unknown depths. The measurements are consistent with an upward velocity of flow of ∼ 10 −3 cm/s. If this velocity is maintained to depth it is still insufficient to transport detectable amounts of radon from the ore body which is at 90 m depth, but it would be sufficient to reveal ore at 50 m or less. Down-hole measurements of permeability yield values that generally are too low to allow signals to be delivered from the ore body by any of the mechanisms that have been modeled. Occasional localized regions of adequately high permeability have been found, but their orientation and extent have not been measured. Although it is improbable that this ore body was discovered by distant transport of radon, the possible existence of occasional gas flow in the earth gives encouragement that at some sites discovery will be aided by such flow.

Evaluation of radon systems at yeelirrie, Western Australia

An orientation radon soil-gas survey to evaluate the alpha scintillation counter and alpha-sensitive films was completed at Yeelirrie, where significant near-surface carnotite mineralization occurs in a semi-arid environment. Traverse and grid sampling totalling 200 sites included measurements of the radon and gamma flux and local uranium and thorium levels. Detailed meteorological records were also compiled. The two radon monitoring techniques provided essentially the same response which was keyed to local uranium distribution rather than to the overall disposition of the mineralized zones. Transport of radon over long distances is also examined with reference to the results obtained in this survey. The fluid convection model promises improved results from radon surveys carried out during appropriate insolation conditions.

Use of lead 210 as a tracer of transport processes in the stratosphere

Short-term changes in the distribution of lead 210 in the lower stratosphere result from variations in the rates of transport processes within and between the troposphere and the stratosphere. As a result, lead 210 should be of use as a tracer of such processes. The highest concentrations of lead 210 in the stratosphere are found within a layer several kilometers thick that lies immediately above the tropopause. Experiments with a simplified numerical model of atmospheric transport indicate that this distribution could result from an equilibrium between eddy diffusion and particle settling. Lead-210 concentrations within this layer decreased during the winter of 1964–1965 and increased during the summer of 1965, perhaps as a result of seasonal variations in the rates of vertical transport of radon in the troposphere. Lead-210 concentrations in the region immediately above this layer in the polar stratosphere also decreased during the winter of 1964–1965, probably as a result of general subsidence of air within sections of the polar stratosphere. These observations suggest that lead 210 may be of value in tracing the exchange of air between the troposphere and the stratosphere and in tracing vertical transport within the polar stratosphere.