Radon in soil

The second approach is to investigate both experimentally and theoretically the influence of soils at a localized level, on the scale of an individual house, for example. Production and transport of radon in soils and migration of radon into houses depend upon local soil conditions, and results of these studies will improve our basic understanding of the physical processes involved. This in turn will aid in the systematic development of more effective entry identification and mitigation techniques to prevent or reduce radon entry into dwellings. 

Factors Influencing Production and Migration of Radon in Soils... 

Figure 1. Schematic illustration of factors influencing the production and migration of radon in soils and into buildings. Geochemical processes affect the radium concentration in the soil. The emanating fraction is principally dependent upon soil moisture (1 0) and the size distribution of the soil grains (d). Diffusion of radon through the soil is affected primarily by soil porosity ( ) and moisture content, while convective flow of radon-bearing soil gas depends mainly upon the air permeability (k) of the soil and the pressure gradient (VP) established by the building.

Factors influencing the production and migration of radon in soils have been examined, and various sources of geographic data have been discussed. Two significant soil characteristics include air permeability and, less importantly, radium concentration. While there are, at present, few opportunities to compare the larger-scale data with on-site field measurements, those comparisons that have been made for both surface radium concentrations and air permeability of soils show a reasonable correspondence. Further comparisons between the aerial radiometric data and surface measurements are needed. Additional work and experience with SCS information on soils will improve the confidence in the permeability estimates, as will comparisons between the estimated permeabilities and actual air permeability measurements performed in the field. 

Measures to reduce radon concentrations have been studied in an old house in which the radon decay-product concentration initially exceeded 0.3 Working Level (WL). Some of the measures were only partially successful. Installation of a concrete floor, designed to prevent ingress of radon in soil gas, reduced the radon decay-product concentration below 0.1 WL, but radon continued to enter the house through pores in an internal wall of primitive construction that descended to the foundations. Radon flow was driven by the small pressure difference between indoor air and soil gas. An under-floor suction system effected a satisfactory remedy and maintained the concentration of radon decay products below 0.03 WL. 

The subsoil is the principal source of radon in this house. Both the activity concentration of radium-226 in subsoil and of radon in soil gas are above levels for building ground that might result in significant indoor radon concentrations. The radon decay-product concentration in the dwelling before remedial measures were taken was substantially higher than the reference value of 120 mWL. 

Radon in soil gas is present everywhere throughout the province in all geologic terrains. Soil gas Rn concentrations range from a low of 0.1 kBq/m3 to a high of 207.0 kBq/m3 with a mean of 25.3 kBq/m3 (Fig. 3). The highest Rn soil gas concentrations are also associated with Middle-to-Late Devonian leucomonzogranite of the South... 

Near the surface, radon diffuses upwards. If (z) is the concentration at depth z and De the effective diffusion coefficient of radon in soil ... 

In some cases the source of soil contamination is the soil itself. For example, soils rich in toxic elements such as arsenic, lead, mercury, and cadmium provide their own source of contamination. In addition, soils rich in uranium and its radioactive decay product radium provide continuous long-term sources of the radioactive gas radon in soil. The radon can diffuse from soil into the air of buildings or into groundwater, with resulting radiation exposures to human and animal populations. Other possible sources of contamination internal to soil itself are biological organisms, which are either themselves health threatening or which produce toxic chemicals. 

Measurements of radon in soil are expressed in terms of levels in soil-gas. However, these measurements do not directly relate to rates of radon released to the atmosphere. Factors which affect radon soil-gas levels include radium content, soil porosity, moisture content, and density. Technically, measurement of soil-gas is difficult and there are few studies which report such data. 

There has been little attempt to standardize a method for measuring radon in soil. However, a method which utilizes liquid scintillation counting for determining concentration is given by Wadach and Hess (1985). A description of this method may be found in Table 6-2. 

The use of methods for the detection of radon in soil and water is covered under Geochemical techniques. Other geophysical techniques applicable to the standard situations in which problems involve structure and lithology are beyond the scope of this paper. 

A good description of the theory and method for determining radon was given by Dyck, who was a pioneer in the field. According to Morse, In areas of shallow overburden, radon in soil gas can extend evaluation to depths beyond reach of the scintillometer.. . Day-to-day variations of radon content in soil gas are confusing, but seldom obscure trends and anomalies. ... 

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