U.S. Geological Survey

Radon in Sheared Rocks


INTRODUCTION

Many of the major shear zones in the eastern United States have aerial radioactivity anomalies or have published uranium occurrences (Grauch and Zarinski, 1976; Carpenter, 1981; Baillieul and Daddazio, 1982; Baillieul and Dexter, 1982; Sargent and others, 1982; Madson and others, 1983). Shear zones have been identified as the cause of some of the highest indoor radon concentrations ever recorded in the United States. Two of the most notable of these occur at Boyertown, Pa. (Gundersen and others, 1987; Smith and others, 1987; Gundersen and others, 1988a), and Clinton, N.J. (Henry and others, 1991). This paper documents the Boyertown radon occurrence, as well as three other case studies, to illustrate the geologic processes responsible for making shear zones a source of high radon concentrations.

An introduction to the geologic processes that form mylonites and the resulting textural and mineralogical characteristics is essential to understanding the radionuclidic and geologic data presented in this paper. Mylonite is a rock type that develops as a result of ductile shear in which temperatures are generally above 250 degrees C. During ductile shear, the rock behaves plastically, rather than by fracturing, which is considered brittle shear. The development of a mylonite involves changes in the microstructure, porosity, permeability, and chemical composition of the parent rock. These changes can influence the mobility and concentration of uranium and may promote the emanation of radon (radon available to pore space). Grain size reduction of minerals is characteristic of mylonites and is achieved by plastic processes (the crystals undergo ductile deformation, breaking down and becoming smaller in size). Granulation and fracturing play a minor role, especially with more resistant minerals such as uraniferous titanite and zircon. The grain-size reduction of these minerals may free uranium and make it available for chemical reaction and redistribution. Mylonitic rocks have a strong anisotropy, or foliation, due to the stretching and alignment of minerals during progressive deformation. This anisotropy may channelize fluids, which dissolve or replace minerals and thus change both the volume and composition of the deformed rock. Fluids also may interact with elements freed during grain size reduction. As a result, the bulk composition of a mylonite often differs from that of its parent rock.

Figure 1 is a schematic of a homogeneous granite before and after deformation, showing the foliation that develops and the mineral changes that take place. The change in chemistry and mineralogy of a rock through mylonitization is documented by Beach and Fyfe (1976), Beach (1980), Sinha and others (1986), and Sinha and Glover (1986). In the mylonite zones described below, there is evidence of redistribution of uranium during deformation by oxidizing fluids into the developing foliation of the deforming rock. This redistribution and the development of foliation increases the radon emanation from the rock, its uranium concentration, and its permeability. Figure 2 is a general location map of the mylonite zones described here.

METHODS

In each study area, techniques to assess the radionuclide content of the soil and rocks as well as their physical and chemical geologic characteristics were used. Radon in soil gas was measured by use of a technique developed by Reimer (1991). A carbon steel probe is inserted into the soil to a depth of 75 to 100 cm and 10 to 20 cc of gas is withdrawn with a syringe. Samples are then measured in an alpha-sensitive scintillometer adapted with a Lucas cell and counted for two minutes. Three to five counts are made and then averaged. In the Eastern United States this sampling depth corresponds approximately to the lowermost B and uppermost C soil horizons. All soils sampled in this study are residual, or saprolitic, soils derived from the underlying bedrock. Sampling was conducted in the summer during periods of dry, stable weather. In general, soils were dry to moderately moist. Radon concentrations are expressed in picocuries per liter (pCi/L).

Permeability measurements are from the Soil Conservation Service County Reports and indicate water permeability. Slow permeability is defined as less than 0.6 inches per hour (in/h), moderate permeability is 0.6 to 2 in/h, and moderately rapid permeability is 2 to 6 in/h.

Indoor radon measurements were supplied by State agencies and homeowners. Data from Boyertown, Pa. are 3-month alpha-track, winter measurements. The remaining data are 2-3-day charcoal canister winter measurements. All data are from homes that have basements and are proprietary information.

Uranium concentration was determined by fluorimetry. Thorium and radium concentrations were determined by gamma spectroscopy. Equivalent uranium was measured at the soil surface by use of a portable gamma spectrometer.

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13 October 1995