U.S. Geological Survey

Radon in Sheared Rocks


DISCUSSION AND CONCLUSIONS

The results of this study indicate that mylonites generate anomalously high indoor radon and soil radon concentrations. From the rock descriptions and data presented, it is evident that each rock type has a distinctive radionuclidic signature. A plot of the average indoor radon versus the average soil radon concentrations for the mylonite zones and associated rock types discussed in this paper is shown in Figure 11. Data from studies in Easton, Pa., have also been included (Gundersen and others, 1988a). Note that the rocks cluster in three catagories. These catagories are controlled by: (1) the rock composition, (2) the radionuclide concentration, and (3) the permeability. Rocks composed chiefly of dark-colored minerals such as hornblende, pyroxene, and biotite (commonly called mafic rocks) seldom have uranium concentrations of more than 5 ppm. Permeability is slow in mafic rocks because the dark minerals commonly weather to clay. The mafic rocks in all of the case studies generally have soil radon values less than 500 pCi/L and indoor radon less than or equal to 4 pCi/L, except where they are sheared.

Uranium concentrations are higher in rocks of granitic composition, such as the QFB, schist, gneiss, and hornblende granite, because of the occurrence of minerals such as allanite, monazite, zircon, and titanite. These uraniferous minerals are common in metamorphosed sedimentary rocks and granites. Soils derived from them are sandier and more permeable because of the abundance of quartz. Most of the rocks have moderate amounts of uranium (5 to 25 ppm) and moderate amounts of soil radon (500 to 2000 pCi/L).

Mylonites formed in the various rock types are consistently moderate to high in radon and uranium concentrations. Several factors contribute to the high radon values in mylonites. First, uranium increases in concentration relative to the parent rock. Uranium is removed from its crystalline source in minerals such as titanite and reacts with oxidizing fluids present in the developing mylonite. As shear strain increases, grain-size reduction (the breaking down of the minerals) increases, and more uranium is mobilized and concentrated in the foliation. Concentrations increase by two processes: (1) new uranium is brought into the zone by fluids, and (2) uranium is left in the zone as other elements leave the system, and thus cause volume loss and relative enrichment of uranium.

Second, emanation of radon from uranium increases in the foliation. Uranium is removed from a site of low radon emanation (the crystal) to a site of high radon emanation (the foliation). Weathering of mylonites occurs mostly along foliations and exposes the source of uranium even further.

Third, the foliation developed during mylonitization increases the permeability of the rock dramatically. This pervasive foliation is also mimicked in the soil profile and provides paths for water and air movement.

In conclusion, shear zones can be local as well as regional areas of high radon concentrations, especially in mountain belts such as the Appalachians. Shear zones are not the only cause of high indoor radon levels and some shear zones will not create radon problems. Shears developed in rocks having less than 1 ppm uranium may not produce sufficient radon to cause a problem unless uranium is introduced from another source during deformation. However, shear zones developed in rocks having higher uranium concentration, such as granitic rocks, have a high probability of causing an indoor radon problem.

High radon levels in water accompany high soil radon in Boyertown, PA (Wanty and Gundersen, 1988). Preliminary water data from two other shear zones, the Hylas fault in Virginia and a brittle shear fault in the Silver Plume Quartz Monzanite near Conifer, Co., indicate that many domestic wells have radon concentrations greater than 10,000 pCi/L . High radon emanation, especially along fracture surfaces, contributes significantly to radon concentrations in water. Mylonites tend to be aquifers because of the increased permeability caused by the shear bands formed during deformation, whereas the same rock, when not sheared, may not be an aquifer.

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13 October 1995