Radon concentrations in soil gas and their relation to geology were studied in Colebrookdale Township on the west side of Boyertown, Pa. The Boyertown occurrence was one of the first well-publicized discoveries of severe levels of indoor radon in the United States. The Proterozoic geology of Boyertown is described in detail by Gundersen and others (1987) and is only summarized here. Three major rock types underlie the area of interest shown on the maps in Figure 3 and Figure 4. In the rock descriptions below, minerals are listed in order of decreasing abundance.
The highest indoor radon and soil radon measurements are found over the mylonite developed in the QFB. Uranium in the QFB is located in titanite, monazite, zircon, and allanite. These minerals have low radon emanation; much of the radon produced does not escape the mineral or surrounding grains. When the QFB was mylonitized, the uraniferous minerals were broken down and uranium was made available for reaction with hot, oxidizing fluids present in the shear zone. During deformation, the uranium was redistributed into the developing foliation along with hematite. Uranium is a much more effective producer of mobile radon when it is in the foliation. The mylonite weathers primarily along the foliation, exposing the uraniferous surfaces. Chemical analyses of the Boyertown rocks show uranium and radium to be in equilibrium (Agard and Gundersen, 1991). Which is due in part to radium being scavenged by iron-oxides as it is produced by uranium in the foliation. Radon emanation also is increased by this process. The Boyertown mylonite zone is structurally complex and the source of the uranium enrichment in the zone is equivocal. The uranium/thorium ratio is highly variable in fresh rock samples of the Boyertown mylonite, and because veinlets of quartz are common in small, sheared fractures throughout the zone, it is suggested that fluid has been introduced into the zone, perhaps bearing uranium.