U.S. Geological Survey

Radon in Sheared Rocks

Sheared fault zones in the Appalachian region of the Eastern United States have the potential for creating anomalously high amounts of indoor radon. These fault zones, many of which have known uranium occurrences, are usually characterized by high gamma radioactivity. Factors controlling the radon concentrations at these locations are bedrock uranium concentration, high permeability, and high radon emanation. These factors may be directly attributed to the deformation process of ductile shear, known as mylonitization of the rock. During mylonitization, the uranium concentration is increased by: (1) the introduction of uranium-bearing fluids into the shear zone, or (2) volume loss, which leaves the rock relatively enriched in uranium. Grain-size reduction of uranium-bearing accessory minerals common to metamorphic and igneous rocks, such as titanite, zircon, monazite, and apatite, makes uranium available for redistribution into the foliation. This process increases the radon emanation from rock dramatically. The texture imparted to the rock during shear also increases its permeability. Oxidation of iron during deformation and subsequent weathering results in the distinctive iron "staining" characteristic of many shear zones. Iron oxides and other metal oxides scavenge uranium and radium available through the weathering processes, increase the radon emanation from the rocks and soils, and make radon readily available to local ground waters. Shear zones in Pennsylvania, Virginia, New Jersey, and Maryland show anomalously high radioactivity and uranium, indoor radon, and soil radon concentrations that set them apart statistically from their unsheared host rocks.

• Introduction and Methods

• Boyertown, Pennsylvania

• Brookneal Zone, Virginia

• Glen Gardner, New Jersey

• Montgomery County, Maryland

• Discussion and Conclusions

• References Cited