U.S. Geological Survey

Radon in Sheared Rocks


MONTGOMERY COUNTY, MARYLAND

In a recent study of Montgomery County, the radon potential of Piedmont metamorphic rocks was found to be substantially lower than their sheared counterparts. Figure 10 shows a generalized version of the radon potential map developed for the county (Gundersen and others, 1988b). The map units are ranked 1, 2, or 3 in order of increasing radon potential. A map unit ranked "1" indicates that less than 40 percent of the homes in this rating area will have indoor radon screening levels greater than 4 pCi/L. All indoor radon screening levels will be in the range of 1-20 pCi/L. A map unit ranked "2" indicates that about 50 percent of the homes will have indoor radon screening levels greater than 4 pCi/L and as many as 10 percent of these will be over 20 pCi/L. A radon potential rating of "3" indicates that 60 percent or more of the homes in this area will have indoor radon levels greater than 4 pCi/L and as many as 30 percent of the homes will be greater than 20 pCi/L. These ratings are based on 567 measurements of soil radon, 278 measurements of surface equivalent uranium, and 319 charcoal canister measurements of indoor radon. These data were then compared with available geologic maps by Cloos (1964) and Froelich (1975) and permeability data from the Soil Conservation Service for Montgomery County.

The high-potential area in the western part of the county (3p) is underlain by phyllite that is characterized by high uranium concentrations not caused by shearing of the rock. Uranium in the phyllite is associated with graphite and is probably of depositional origin. However, the remaining areas rated "3" have high indoor radon and soil radon levels caused by mylonitic shear zones. The shear zones in Montgomery County yielded the highest average soil radon (2,300 pCi/L) and the highest average indoor radon (24 pCi/L) when compared with the unsheared rocks. The three rock types in which shears are developed are described below.

Schist, Map Units 2s, 3s, and 3p/s
This rock unit underlies a significant portion of the central and eastern part of the county. It is composed of quartz, feldspar, and biotite with varying amounts of muscovite, chlorite, magnetite, staurolite, and garnet. Allanite, apatite, titanite, monazite, and zircon commonly occur as accessories and are the source of uranium in the rock. Interlayers of quartzite, metagraywacke, and phyllite are common. Equivalent uranium measurements average 2.7 ppm and have a maximum of 6.7 ppm where the schist is sheared. Permeability of soils developed from this rock type is moderate. Most of the schist is rated "2", but where it is sheared it is rated "3". In general, soil radon ranges between 500 and 1,500 pCi/L in unsheared schist. Where the schist is sheared, the range is between 2,000 and 3,000 pCi/L. Indoor radon ranges between 5 and 30 pCi/L in the sheared schist and between 1 and 10 pCi/L in the unsheared schist.

Gneiss, Map Units 2g and 3g
Numerous bodies of gneiss occur within the schist. They are composed of quartz, potassium feldspar, sodium feldspar, and variable amounts of hornblende and biotite. Granodiorite and pegmatite are common. Equivalent uranium measurements average 2.4 ppm and have a maximum of 3.7 ppm in sheared gneiss. Permeability of soils is moderate. In the southern part of the county, the gneiss is characterized by a well-defined mylonitic texture. The northern outcrops of gneiss have a few scattered, narrow (1-10 m) mylonite zones. High indoor radon and soil radon concentrations are found in all mylonite developed in gneiss. In general, soil radon concentrations range between 2,000 and 3,000 pCi/L in the sheared gneiss and in the unsheared gneiss range between 250 to 1,500 pCi/L. Indoor radon concentrations range between 5 and 30 pCi/L in sheared gneiss and in the unsheared gneiss range between 1 and 10 pCi/L.

Mafic and Ultramafic Rocks, Map Units 1m, 2m, and 3m/s
Mafic and ultramafic bodies commonly occur within the schist and are typically bordered by shear zones. The rocks vary and include greenschist, quartz diorite, tonolite, amphibolite, metagabbro, serpentine, and chlorite-actinolite-talc schist. Permeability is slow in soils developed on this rock type. The igneous mafic rocks typically have a low uranium concentration, with equivalent uranium averaging 2.1 ppm and having a maximum of 4 ppm in the sheared mafic rocks. The source of uranium is probably from the adjacent sheared schist, and in a few rare cases, gneiss. In general, soil radon concentrations in the unsheared mafic rocks range between 200 and 1,000 pCi/L and in the deformed mafic rocks range between 2,000 and 3,000 pCi/L. Indoor radon measurements of 4000 pCi/L have been found in the map unit 3m/s and, in general, indoor radon measurements range between 10 and 30 pCi/L in the sheared mafic rocks.

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13 October 1995