LIVING WHERE RADON LEVELS CAN BE 25 TIMES HIGHER THAN SAFE LIMITS: A CASE STUDY OF THE GEOLOGIC CONTROLS ON RADON RATES, BLACK HILLS, SD

The Black Hills region is a unique geologic location where remnants of Trans-Hudson orogenic belt is exposed. Archean to Early Proterozoic (>2.5 to 1.7 b.y.) basement rocks constitute the core of Black Hills uplift and also source of radioactive, uranium-bearing minerals and carcinogen Radon gas. The core of Black Hills is flanked by Phanerozoic sedimentary rocks of which provenance was also the Precambrian basement rocks. Our recent indoor radon measurements indicate that there are some localities dangerously exceeding EPA safe radon limits of 4 pCi/L and reaches up to 300-400 pCi/L. In this study, locations with indoor radon measurements higher than 4 pCi/L are determined as high-risk radon zones (HRZ). Considering that exposure of living forms to excessive radon for a long time can cause irreversible and serious health problems such as lung cancer, Black Hills region including densely-populated Rapid City area requires greater attention and detailed investigation of geologic factors controlling unsafe radon levels. Initial correlation of geologic maps with HRZ indicates that Precambrian rock formations are the main source of this public health hazard. It is also known that shale units can have higher radioactivity due to presence of radioactive elements in composition. Outside the core of Black Hills area, some HRZ coincide with Cretaceous age shale units. However, some HRZ appear on rock units (i.e. Spearfish formation) known as much less radioactive than basement rocks. These rock formations are usually highly-deformed, fractured and faulted. It appears that fractures within less-radioactive rock formations serve as migration/transportation pathway for radon sourced from deeper, older shaley and crystalline rocks. The correlation between rock radioactivity and high indoor radon measurements indicates that radon levels get higher within densely-fractured locations even though rock unit forming the foundation for household is one of the less-radioactive rock formations. It is clear that high radon levels in the Black Hills and Rapid City areas are mainly controlled by the rock formations but it is not the solely reason of HRZ occurrences. Deformational structures (i.e. faults, fractures) can also play a significant role on increasing the chances for exposure to unsafe radon levels in a region.