STATISTICAL-ANALYSIS OF THE RN-222 POTENTIAL OF ROCKS IN VIRGINIA, USA

More than 3,200 indoor radon-222 (Rn-222) measurements were made seasonally in an area of about 1,000 square kilometers of the Coastal Plain and Piedmont physiographic provinces in Virginia, U.S.A. Results of these measurements indicate that some geological units are associated, on the average, with twice as much indoor Rn-222 as other geological units, and that indoor Rn-222 varies seasonally. The Kruskal-Wallis test was used to test whether indoor Rn-222 concentrations for data gathered over the winter and summer seasons differ significantly by rock unit. The tests concluded that indoor Rn-222 concentrations for different rock units were not equal at the 5-percent significance level. The rocks associated with the highest median indoor Rn-222 concentration are specific rocks in the Mesozoic Culpeper basin, including shale and siltstone units with Jurassic diabase intrusives, and mica schists in the Piedmont physiographic province. The pre-Triassic Peters Creek Schist has the highest ranking in terms of indoor Rn-222 concentration. The rocks associated with the lowest indoor Rn-222 concentrations include coastal plain sediments, the Occoquan Granite, Falls Church Tonalite, Piney Branch Mafic and Ultramafic complex, and unnamed mafic and ultramafic inclusions, respectively. The rocks have been ranked according to observed Rn-222 concentration by transforming the average rank of indoor Rn-222 concentrations to z scores.