Soil radium

Geometric Means for Soil Gas Radon-222, Soil Radium-226, Permeability, RIN, and Indoor Radon-222... 

In U.S. EPA Office of Radiation Program s New House Evaluation Program (NEWHEP), two builders in the Denver area, two in Colorado Springs, and one in Southfield, Michigan, installed various radon-resistant features in houses during construction. A sampling of subsequent measurements of indoor radon, adjacent soil gas radon, and soil radium content is summarized in Table 31.6.36... 

The presence of radon gas has become a much publicized environmental and health issue because of the radiation danger it poses. Radioactive isotopes such as radium-226 are naturally present in many types of rocks and soils. Radium-226 emits an alpha particle and is converted into radon gas, which diffuses out of the rocks and sou. 

Twenty isotopes are known. Radon-22, from radium, has a half-life of 3.823 days and is an alpha emitter Radon-220, emanating naturally from thorium and called thoron, has a half-life of 55.6 s and is also an alpha emitter. Radon-219 emanates from actinium and is called actinon. It has a half-life of 3.96 s and is also an alpha emitter. It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in tiny amounts into the atmosphere. Radon is present in some spring waters, such as those at Hot Springs, Arkansas. 

Rona E, Urry WD (1952) Radium and uraninm content of ocean and river waters. Am J Sci 250 241-262 Rosholt J, Doe B, Tatsnmoto M (1966) Evolntion of the isotopic composition of nraninm and thorinm in soil profiles. Geol Soc Am Bull 77 987-1004... 

Radon is a naturally occurring, chemically inert, radioactive gas. It is colorless, odorless, and tasteless. It is part of the uranium-238 decay series, the direct decay product of radium-226. Radon moves to the earth s surface through tiny openings and cracks in soil and rocks. High concentrations of radon can be found in soils derived from uranium-bearing rocks, such as pitchblende and some... 

Radon gas is the result of the radioactive decay of radium-226, an element that can be found in varying concentrations throughout many soils and bedrock. Figure 31.1 shows the series of elements that begins with uranium-238, and, after undergoing a series of radioactive decays, leads eventually to lead-210. At the time radium decays to become radon gas, energy is released.9 Of all the elements... 

Several studies have been attempted to make simple correlations between radon or radium concentrations in the soil and indoor radon concentrations.4344 No significant correlations were made between these variables. 

Indoor Radon in Basement Soil Gas Radon Radium-226 in S... 

As shown in Table I, radium content of surface soils not associated with U mining or milling varies by slightly more than an... 

Soil is a mixture of solid materials, air and, usually, water and organic matter. The radium content of soil often reflects that of the rocks from which the solid materials are derived by physical and chemical activity. The observed ranges are from 0 to 20 Bq kg for ultrabasic rocks (dunite) to 1 to 1835 Bq kg for igneous metamorphic rocks (gneiss) (Wollenberg, 1984). While these ranges are broader than those for measured for soils, the mean values for rocks, excluding alkali rocks, is consistent with the means observed for 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.

As noted in Table I, average surface radium concentrations appear to vary by about a factor of 20. This can also be seen from the distributions from the NARR data. Soil permeabilities, on the other hand, have much larger variations, and thus, in principle, may have a greater influence on the spatial variations in average indoor radon concentrations that have been observed. As with the case of surface radium concentrations, the spatial variability of air permeabilities of soils is an important element in developing a predictive capability. 

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. 

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 indoor air arises primarily from radium in the soil. The radon in the soil gas flows under a pressure gradient from the soil into the building. In some cases building practices can lead to high radon levels in the living areas of the house. Radon is chemically quite inert and does not pose a significant radiation health hazard in itself because the retained fraction in the body is so low (Mays et al., 1958). It is, however, an excellent vehicle for the dispersion of its short-lived radioactive decay products. 

Although not part of soil, lichens, by virtue of their solubilising action on rocks, contribute to the elemental enrichment of soil. Several studies have identified lichen acids as complexing agents for the iron and aluminium of rocks (95, 96). An examination of the various structures indicates that the basic structure responsible for the chelation is the carboxylic acid group with an orthophenolic group. Grodzinskii (97) has found lichens to be intense accumulators of elements in the uranium-radium, actinouranium and thorium orders. 

Radon s source is a step in the transmutation of several elements uranium —> thorium — radium —> radon —> polonium —> lead. (There are a number of intermediate decay products and steps involved in this process.) Radon-222 forms and collects just a few inches below the surface of the ground and is often found in trapped pockets of air. It escapes through porous soils and crevices. 

---