U.S. Geological Survey

Radon Potential of the Upper Midwest


DISCUSSION

Glaciated areas present special problems for radon-potential assessment because bedrock material is commonly transported hundreds of km from its source in the central United States. In New England, glacial deposits are typically more locally derived. Glaciers are quite effective in redistributing uranium-rich rocks; for example, in Ohio, uranium-bearing black shales have been disseminated over much of western Ohio and eastern Indiana, now covering a much larger area than their original outcrop pattern (Fig. 1, 43kb), and display a prominent radiometric high on the radioactivity map of the United States (8). The physical, chemical, and drainage characteristics of soils formed from glacial deposits vary according to source bedrock type and the glacial features on which they are formed. For example, soils formed from ground moraine deposits tend to be more poorly drained and contain more fine-grained material than soils formed on kames, moraines, or eskers, which are generally coarser and well-drained. In general, soils developed from coarser-grained tills are poorly structured, poorly sorted, and poorly developed, but are generally moderately to highly permeable and are rapidly weathered, because the action of physical crushing and grinding of the rocks to form tills enhances and speeds up soil weathering processes (16).

Clayey tills, such as those underlying most of North Dakota, eastern South Dakota, parts of western and southern Minnesota, and northern Iowa, have high emanation coefficients (17) and usually have low to moderate permeability, depending on the degree to which the clays are mixed with coarser sediments. Tills consisting of mostly coarse material tend to emanate less radon because larger grains have lower surface area-to-volume ratios, but because these soils have generally high permeabilities, radon transport distances are generally longer, and structures built in these materials are able to draw soil air from a larger source volume, so moderately to highly elevated indoor radon concentrations may be achieved from comparatively lower-radioactivity soils (14, 18). In till soils with extremely high permeability, atmospheric dilution may become significant, and if the soils have low to moderate radium contents, elevated indoor radon levels would be less likely to occur. Soil moisture has a significant effect on radon generation and transport and high levels of soil moisture generally lower the radon potential of an area. The main effect of soil moisture is its tendency to occlude soil pores and thus inhibit soil-gas transport. Soils in wetter climates from northern Minnesota to northern Michigan generally have lower radon potential than soils derived from similar tills in the southern parts of those states and in Indiana and Illinois, in part because of higher soil moisture conditions to the north.

SUMMARY

Two end-member classes of glacial soils can be identified in the upper Midwest: 1) clay-rich soils with lower permeability and higher emanation coefficients, and 2) coarser-grained soils with lower emanation coefficients and higher permeability. A large group of soils fall between these two extremes due to the mixing of clayey and sandy or gravelly source components in tills from different bedrock sources. Identification of the bedrock source components of the glacial deposits, and the climatic controls on soil formation in each area, are of primary importance in understanding the radon generation and transport characteristics of the resulting glacial soils. Three main factors are important to consider when predicting radon potential in glaciated terranes: 1) Glaciers transport and redistribute bedrock, so a bedrock geologic map may not accurately reflect the parent material lithology of glacially-derived soils, but knowledge of source rock lithologies will aid in determining the radon emanation and transport characteristics of the derivative tills. 2) Grinding of the rocks by glaciers reduces grain size and therefore increases grain surface area, enhancing radon emanation by exposing more radium at grain surfaces than in coarser-grained soils. Glacial mixing and crushing also speed up the weathering process, so radionuclides may be leached from shallow horizons of till soils more rapidly than in soils developed on untransported bedrock, thus giving a surface gamma radioactivity reading that may underestimate the uranium and radium concentrations at depth. 3) Soil moisture has a significant effect on radon generation and transport and high levels of soil moisture generally lower the radon potential of an area.

[Back][Home] [Next]


URL: http://energy.cr.usgs.gov/radon/midwest5.html
Server information: webman@energy.cr.usgs.gov
13 October 1995