U.S. Geological Survey

Radon Potential of the Upper Midwest


The scheme used to evaluate areas for radon potential is divided into two basic parts, a Radon Index (RI), used to rank the general radon potential of the area based on the geologic factors described above, and a Confidence Index (CI), used to express the level of confidence in the prediction based on the quantity and quality of the data used to make the determination. This scheme works best if the areas to be evaluated are delineated by geologically-based boundaries (geologic provinces) rather than political ones (State/county boundaries) in which the geology may be varied across the area. A more detailed discussion of the RI and CI is given in (1).

Table 1 presents the Radon Index (RI) matrix. Five main categories are evaluated and a point value of 1, 2, or 3 is assigned to each category. These categories were selected because the factors they represent are considered to be of primary importance in controlling radon potential and because at least some data for these factors are consistently available for every geologic province. Because each of these factors encompasses a wide variety of complex and variable components, the subjective professional judgment and experience of the geologists performing the evaluation are heavily relied upon in assigning point values to each category. The total of the points assigned to each of the five factors determines the radon potential of the area. A radon potential of low, in which the average indoor radon levels for the area are expected to be less than 2 pCi/L; moderate, in which the average indoor radon levels for the area are expected to fall between 2 and 4 pCi/L; or high, in which the average indoor radon levels for the area are expected to be greater than 4 pCi/L; is assigned based on the total RI score.

To add additional weight to the geologic factor in cases where additional reinforcing or contradictory geologic evidence is available, Geologic Field Evidence (GFE) points are added or subtracted from an area's score. Relevant geologic field studies are important in enhancing our understanding of how geologic processes affect radon distribution. In some cases, geologic models and supporting field data reinforce an already strong (high or low) score; in others, they can provide important contradictory data. For example, areas of the Dakotas, Minnesota, and Iowa that are covered with Wisconsin-age glacial deposits exhibit a low aerial radiometric signature and would score only one point in that category. However, data from geologic field studies in North Dakota and Minnesota (14) suggest that eU is a poor predictor of geologic radon potential in this area because radionuclides have been leached from the upper soil layers, but are present and possibly even concentrated in deeper soil horizons, generating significant soil-gas radon. This positive supporting field evidence adds two GFE points to the score which help to counteract the invalid conclusion suggested by the radiometric data. No GFE points are awarded if there are no documented field studies for the area.

Except for architecture type, the same factors are used to establish a Confidence Index (CI) for the radon potential prediction for each area (Table 2). Architecture type is not included in the confidence index because house construction data are readily and reliably available through surveys taken by agencies and industry groups including the Bureau of the Census, National Association of Home Builders, and the Federal Housing Administration. The remaining factors are scored on the basis of the quality and quantity of data used in the RI matrix.

For the geologic data factor, a high confidence score is given to an area where a proven geologic model for radon generation and mobility can be applied. Rocks for which the processes are less well known or for which data are contradictory are regarded as "variable", and those about which little is known or for which no apparent correlations have been found are deemed "questionable".

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13 October 1995