The Relationship Between Soil Radon Concentrations and the Huntingdon Valley Shear Zone, Southeastern Pennsylvania

Abstract

The Huntingdon Valley shear zone is a zone of high-strain ductile deformation which strikes N70E between the Schuylkill River and the Delaware River. In the study area, there are seven lithologic units within the shear zone: Wissahickon Formation (oligoclase-mica schist), Chickies Formation ( quartzite), felsic gneiss (pyroxene-bearing rock), mafic gneiss (hornblende-bearing rock), and three carbonate units. Strain is heterogeneous across these units. High soil-radon (222Rn) concentration correlates both with lithology and the magnitude of strain of the bedrock. High concentrations of radon gas {222Rn) were found in soils derived from highly deformed rocks of the Huntingdon Valley shear zone and two other zones to the south. For this study, the Huntingdon Valley shear zone (HVSZ) was verified as being a structure associated with high radon emissions regardless of lithology. Within one lithology, Wissahickon Formation, radon levels outside the zones of shear range from 170 to 2,639 pCi/L, whereas inside the zone, soil radon was recorded as high as 6,380 pCi/L. The Chickies Formation averages 1,931 pCi/L inside the shear zone and 634 pCi/L outside the shear zone. The felsic gneiss has low radon emissions throughout, while the Conestoga shows differences. These differences could be the result of unexposed shear zones.