Radon Transport Research Articles

Effectiveness of a Passive Subslab Ventilation System in Reducing Radon Concentrations in a Home

The effectiveness of a passive subslab ventilation system in reducing radon concentrations in an occupied home was investigated by measuring radon concentrations and pressure differentials during a 1-year period when a passive subslab ventilation system was being cycled on and off. Radon concentrations in the house were 30% lower during periods when the stack was open to the atmosphere. The effect was most pronounced when the home was unoccupied and during the winter and spring months. Furnace use and wind speed were the best predictors of transient changes in basement radon concentrations, whether the stack was open or closed. Pressure differential measurements show that subslab depressurization occurs when the stack is open during the winter and spring months due to bouyancy-driven air flow up the stack, but not during the summer. Numerical simulations of gas flow and radon transport into the house from the surrounding soil were calibrated to observed pressure differentials and radon concentrations. The model predicts that peak radon concentrations caused by furnace use will be reduced by flow out of the stack. However, the model is unable to account for the reduction in average radon concentrations observed while the stack is open in the winter. 19 refs.,more » 16 figs., 1 tab.« less

Measurements of222Rn migration in soil

The concentration of radon (222Rn) was measured in the soil near the ground surface, using CR/39 solid state nuclear track detectors. The measurements were carried out in PVC tubes at 0.25 m intervals up to 1.25 m. The detectors were etched in 7N NaOH solutions at 80°C. The α-tracks from radon's decay were counted using a microscope. A microscope-camera-computer system developed for automatic counting was also used. The results provide evidence for the non-diffusive transport of radon in soils. A transport length of (46.9±3.2) cm was estimated for radon transport near ground surface. Also the variation of soil's radon concentration was correlate to humidity and atmospheric pressure.

Radon transport into dwellings: Considering groundwater as a source

A mathematical model is used to investigate whether radon degassing from groundwater may contribute to indoor radon levels. Specifically, the transport of radon in the soil‐gas phase from the groundwater‐soil gas interface to under‐pressurized dwellings is modelled. The question whether radon in groundwater may contribute to indoor radon levels arises from observed high radon concentrations in groundwater, and recent findings that advection in the gas phase may be an important transport mechanism for radon into slightly under‐pressurized dwellings. Most previous radon transport investigations did not consider groundwater as a potential source for contributing to indoor radon. The mathematical model includes a method to directly calculate indoor radon concentrations and an equivalent continuum approach to represent cracks in concrete foundations. The results of the simulations indicate that radon, which partitions from groundwater to the soil gas, may be advectively transported by the gas phase to slightly underpressurized dwellings in relatively permeable soils such that indoor radon concentrations may exceed 148 Bq/m³, which is the action limit imposed by EPA.

The influences of diffusion and advective flow on the distribution of radon activity within USEPA's soil chamber

The U.S. Environmental Protection Agency's (EPA's) soil chamber was designed to study the production and transport of radon and other potential indoor air pollutants originating in the soil. This paper presents an analysis of steady-state diffusion in the soil for two different conditions of moisture. The model accounts for multiphase emanation and transport. When the position dependence of the moisture profile was taken into account, the model and measurements agreed well (average deviations of 5% – 13%). Non-steady-state measurements are also discussed. Diffusion plays an important role in radon transport even for this relatively high permeability (3 × 10 −11 m 2) soil. The implication is that diffusion plays an important role in transporting radon to a point near the building shell even though the entry mechanism by which radon penetrates the shell may primarily be advective flow.

Radon transport in fractured porous media — Experimental study in caves

The spatial and temporal variation of 222Rn concentration in three horizontal caves and in one vertical cave was measured to study the influence of different morphological and meteorological parameters on the forming of airborne radon concentrations inside. In horizontal caves, the daily mean radon concentration as a function of the daily average surface temperature showed a step-function type dependence with low winter and high summer values reflecting the main direction of underground airflows. Restriction of airflows increased winter but decreased summer radon levels. The transition pattern between the low winter and high summer values gradually linearized as the number of vertical fractures communicating with the surface increased. Contrary to horizontal caves, in the vertical cave barometric pressure variations played the most important role in controlling subsurface radon concentrations. Decreasing pressure increased radon levels, and increased pressure decreased radon levels. In the pressure-radon correlation curve, there was a small hysteresis which indicated the nonlinearity of the process.

An investigation of factors affecting the entry of radon into structures on the island of Guam

Factors affecting the entry of 222Rn gas into structures on the Island of Guam were investigated during the summer of 1993. Research findings indicated that radon transport into buildings on Guam, and perhaps in other tropical areas, is driven by sub-grade soil pressure (positive with respect to atmospheric pressure) rather than interior buildings vacuums. Immediate and substantive increases in indoor radon concentrations were associated with environmental effects of wind and rain. Radon entry, and, hence, indoor radon concentrations, are significantly greater during the rainy season as opposed to the dry season. In the absence of mechanically induced interior vacuums in buildings, external environmental forces creating sub-slab pressures are the predominant factor in affecting radon entry in Guam. Indoor radon potentials can be correlated to the locations where the underlying geology is limestone. Furthermore, the radon source appears to be within the first few feet of the surface of these limestones rather than uniformly distributed throughout the limestone. The effects of seismic activity on radon entry are short-lived unless significant damage occurs to a structure. Radon entry during calm weather conditions may also be a function of the rising and falling of ocean tides.

Experimental evaluation of geometrical shape factors for short cylindrical probes used to measure soil permeability to air

Permeability of soil has become recognized as an important parameter in determining the rate of transport and entry of radon from the soil into indoor environments. This parameter is usually measured in the field by inserting a cylindrical tube with a short porous section into the soil and measuring the flow rates that result from a range of applied pressures. A variety of mathematical relationships have been used to analyze the resulting data. It is demonstrated that a commonly used mathematical approximation to describe flow through porous cylinders breaks down when the lengths of the cylinders approach their radii. It is also shown that this problem can be avoided by approximating short porous cylinders as ellipsoids. This study compared side-by-side measurements of soil permeability for a number of porous cylinders and spheres with different sizes and orientations. It is shown that all the data can be analyzed with a single curve when the appropriate shape factors (geometrical factor that describes the shape and size of the porous section of the probe) are used. This study also looked at the effects of moisture profile in the soil on the permeability obtained by different measurement methods. It is shown that the effective permeability (equivalent value for a uniform, isotropic medium) in the soil differs by two orders of magnitude in a 1-m deep soil column when the measurement locations differ by only 35 cm. The effective permeability was obtained by inverting the arithmetic average of the reciprocal values of the position-dependent permeability.

Geologic and climatic controls on the radon emanation coefficient

Geologic, pedologic, and climatic factors, including radium content, grain size, siting of radon parents within soil grains or on grain coatings, and soil moisture conditions, determine a soil's emanating power and radon transport characteristics. Data from field studies indicate that soils derived from similar parent rocks in different regions have significantly different emanation coefficients due to the effects of climate on these soil characteristics. An important tool for measuring radon source strength (i.e., radium content) is ground-based and aerial gamma radioactivity measurements. Regional correlations between soil radium content, determined by gamma spectrometry, and soil-gas or indoor radon concentrations can be traced to the influence of climatic and geologic factors on intrinsic permeability and radon emanation coefficients. Data on soil radium content, permeability, and moisture content, when combined with data on emanation coefficients, can form a framework for development of quantitative predictive models for radon generation in rocks and soils.

Modeling of radon transport in unsaturated soil

This study applies a recently developed model, LEACHV, to simulate transport of radon through unsaturated soil and compares calculated soil radon activities against field‐measured values. For volatile and gas phase transport, LEACHV is modified from LEACHP, a pesticide version of LEACHM, a well‐documented one‐dimensional model for water and chemical movement through unsaturated soil. LEACHV adds consideration of air temperature changes and air flow driven by barometric pressure change to the other soil variables currently used in LEACHP. It applies diurnal barometric pressure and air temperature changes to reflect more accurately the typical field conditions. Sensitivity analysis and simulated results have clearly demonstrated the relative importance of barometric pressure change, rainfall events, changes in water content, gas advection, and radon source term in radon transport process. Comparisons among simulated results illustrated that the importance of barometric pressure change and its pumping phenomenon produces both fluctuation in soil gas radon activities and an elevation of the long‐term average radon activity in the shallow soil. Barometric pressure pumping was found to produce an effect on radon activity in shallow soils of an equal magnitude to the distributed source parameter. Comparison between measured and simulated soil radon activities showed that LEACHV can provide realistic estimates of radon activity concentration in the soil profile.

“FRAC ure” — A simulation code for forced fluid flow and transport in fractured, porous rock

The 3D Finite Element Program FRACTure was developed with the specific aim of studying the coupling of interactive mechanisms in geoscience and in particular those relevant to the long term behaviour of a Hot Dry Rock reservoir. The flexible modular structure facilitates the addition of further processes and elements to the existing library and the handling of linear and non-linear constitutive laws and the calculation of their interactions. The Hot Dry Rock applications involve essentially forced fluid flow of cool fluid injected into a hot fractured rock matrix. A study of the relevant processes required finite element solutions for the hydraulic, thermal and elastic fields and especially their interactions. Particular attention has been paid to modelling the perturbations arising from poro-elastic and thermo-elastic effects in the rock matrix and of a non-linear, stress dependent joint closure law. The fracture network is designed in a way to represent nature as realistically as possible: fracture flow takes place along planes or in pipes (fracture intersections), porous matrix flow in 3D bodies. In devising the coupling scheme careful thought has been given to the different time constants for each physical process. Apart of simulating the coupled Hot Dry Rock behaviour, the code has been successfully practised in a variety of applications in geoscience: analysis of geoelectric measurements, radon transport, solute tracer tests, simulations of heat probe operations and hydrogeologic system modelling.

Evidence for radon transport by carrier gas through faulted clays in Italy

Extensive soil-gas surveys in sedimentary basins in Italy were performed to study the potential of some naturally occurring gases as indicators for concealed fracture zones, hydrocarbon and geothermal fluids. One conclusive result is a positive correlation between anomalously high values of radon and carbon dioxide in the soil-air over faults. The correlation coefficient for 1173 gas samples is 0.41. Statistically derived contourlines of Rn and CO2 anomalies show similar locations, shapes and directions. Fairly good Rn−CO2 coupling evidence appears even on a point-to-point analysis. Furthermore, it was recognized that the highest Rn values are in contrast to the low Ra content of the underlying clayey rocks and that conventional Rn transportation mechanisms seem to be inadequate for the clay sequences. All these facts strongly suggest that Rn is transported from the subsoil, through fault-linked pathways, by carrier gases of which CO2 could be one of the major components. The theory of geogas microbubbles is a possible explanation of the observed results. The carrier effect of ascending microbubbles can explain both the origin of soil-gas Rn anomaly and the Rn−CO2 coupling phenomenon.

Radon Levels Survey at the Underground Transport Metro System in Mexico City

The Metro underground transport system moves more than 8 million people daily across Mexico city. The Metro is the most popular mass transport system. The average time a traveller remains inside the underground stations and tunnels is more than two hours. The airborne radon levels were measured, during the three months of summer, at 20 different stations, including the more deeply located halls. Stations were in different zones of the city. The radon concentration in underground corridors and stations varied from 60 Bq.m-3 to 350 Bq.m-3. The environmental factors, such as outside temperature, humidity, and days of rain were compiled in order to understand radon entry and transport better. Travellers inside the metro system are, of course exposed to airborne contaminants from several other sources.

Measurement of effective diffusion coefficient of radon in porous media with etched track radon monitors

We have developed two methods to measure the effective diffusion coefficient of radon in porous media. We used etch track type radon monitors to measure the radon exposure on both the exposure and the measuring sides. We have used a computer code to model the transport of radon through porous media. We found the effective diffusion coefficient by changing its value in the code to find the best fit of model calculation to experimental data.

Radon transport equation in Earth's crust

A time-space continuum for radon transport in porous media with multiphase is presented which provides for simultaneous decay and production of radon involving loquid, gases and minerals. Analytical expression for the change in porosity and permability with time are obained for and assemblage of minerals reacting with liquid field under quasisteady state conditions. Fluid flow is described by Darcy's law combined with Stoke's law, employing a phenomenological expression relating permeability and porosity.

Radon transport in a drilled well studied by etched track type radon monitors

We have measured the radon concentration profile along a 20 meter deep drilled well with etched track type radon monitors. The well has a diameter of 12 cm. It has water at 11.6 meter below the surface and exhales nearly 100% carbon dioxide of geological origin at rate of about 300 liters/hour. We made radon measurements both underwater and in the gaseous atmosphere. Radon transport models (such as microbubble, macrobubble, eddy diffusion, etc.) were tested to find the best fit to experimental radon profiles. The measuring site is at a geologically active area, and the effect of a small earthquake on the exhalation rate of CO 2 from the well was also observed.

Measurements on, and Modelling of Diffusive and Advective Radon Transport in Soil

Abstract Results are presented of measurements on radon transport in soil under controlled conditions with a laboratory facility consisting of a stainless steel vessel (height and diameter 2 m) filled with a uniform column of sand. At several depths under the sand surface, probes are radically inserted into the vessel to measure the radon concentration in the soil gas. To study advective radon transport a perforated circular box is placed in the sand close to the bottom of the vessel. By pressurising this box, an air flow through the sand column is induced. Radon concentration profiles were measured without an air flow as a function of time, and for several values of the air flow, equilibrium radon concentration profiles were measured.

Soil Gas and Radon Entry into a Simple Test Structure: Comparison of Experimental and Modelling Results

A radon test structure has been established at a field site at Riso National Laboratory. Measurements have been made of soil gas entry rates, pressure couplings and radon depletion. The experimental results have been compared with results obtained from measured soil parameters and a two-dimensional steady-state numerical model of Darcy flow and combined diffusive and advective transport of radon. For most probe locations, the calculated values of the pressure couplings and the radon depletion agree well with the measured values, thus verifying important elements of the Darcy flow approximation, and the ability of the model to treat combined diffusive and advective transport of radon. However, the model gives an underestimation of the soil gas entry rate. Even if it is assumed that the soil has a permeability equal to the highest of the measured values, the model underestimates the soil gas entry rate by a factor of two.

Soil Gas and Radon Entry into a Simple Test Structure: Comparison of Experimental and Modelling Results

Abstract A radon test structure has been established at a field site at Riso National Laboratory. Measurements have been made of soil gas entry rates, pressure couplings and radon depletion. The experimental results have been compared with results obtained from measured soil parameters and a two-dimensional steady-state numerical model of Darcy flow and combined diffusive and advective transport of radon. For most probe locations, the calculated values of the pressure couplings and the radon depletion agree well with the measured values, thus verifying important elements of the Darcy flow approximation, and the ability of the model to treat combined diffusive and advective transport of radon. However, the model gives an underestimation of the soil gas entry rate. Even if it is assumed that the soil has a permeability equal to the highest of the measured values, the model underestimates the soil gas entry rate by a factor of two.

Measurements on, and Modelling of Diffusive and Advective Radon Transport in Soil

Measurements on, and Modelling of Diffusive and Advective Radon Transport in Soil

The RAETRAD model of radon generation and transport from soils into slab-on-grade houses.

Remediation planning and 222Rn-related construction zoning require knowledge of how close and strong 226Ra sources can be in different foundation soils under different groundwater conditions without excessively elevating indoor 222Rn levels. A two-dimensional numerical-analytical model was developed to simulate (a) 222Rn emanation, decay, and movement by diffusion and advection in soils around houses and in their understructures; and (b) 222Rn accumulation in a single-zone house. The model represents foundation soils and a house in elliptical-cylindrical geometry. 222Rn may diffuse through its floor slab or may enter via idealized cracks and openings. The model was validated with analytical calculations of two-dimensional air pressure fields and with one-dimensional calculations of 222Rn generation with diffusion and diffusion combined with advection. Agreement generally was within < 1% when finite-difference approximations were minimized. Benchmark comparisons with indoor 222Rn measurements in two test-cell structures under passive and depressurized conditions averaged within 11% of measured values, well within measurement uncertainty. The corresponding average bias was only 3%. Larger variations were observed when applying the model to 50 houses. In this application, a negative bias of nearly 50% was observed due to data gaps and to poorly-characterized floor slabs and crack distributions.