GEOSPATIAL ANALYSIS OF HOUSEHOLD RADON LEVELS, BEDROCK, AND SURFICIAL GEOLOGY IN MICHIGAN

Radon is a naturally occurring colorless, tasteless, and odorless carcinogenic gas that comes from the decay of uranium-238 found in rocks and sediments. Radon is the second leading cause of lung cancer in the United States and results in 20,000 deaths annually. The U.S. Environmental Protection Agency (EPA) classifies household radon hazards into three zones. Zone 3 has radon levels between 0-2.5 pCi/L and represents a low risk zone. Zone 2 has radon levels between 2.5-4.0 pCi/L is considered a cautionary level. Zone 3 has radon levels greater than 4.0 pCi/L, and is the threshold above which mitigation is recommended. Michigan Department of Environmental Quality (MDEQ) provides testing and compiles data for Michigan. Zip Code level data was obtained from MDEQ and included more than 150,000 radon measurements taken between 1993 and 2014.

Radon data for Michigan zip codes were mapped and analyzed using ArcMap 10.3 and Excel to create a new zip code level radon hazard map. Based on the preliminary analysis, 24.4% of households and 16.9% of zip codes (N=1034) have radon above the EPA action level (Zone 3). Zip code level mapping revealed significant detail not present in published county-level radon maps. Polygons created by combining bedrock and surface geology revealed three regions of high radon risk (Zone 3).

Areas of elevated radon risk broadly correlate with Igneous/Metamorphic rocks, iron formations, and shale; overlain by stratified sediments, unstratified till, and discontinuous till. Areas that were once covered by glacial lakes and dune sand generally showed lower radon levels. Reduced levels in these areas are thought to be a result of thick sand units which attenuate the radon gas, and clay-rich lacustrine units which prevent gas movement. Several areas mapped as zone 2, using the EPA county-level mapping, were mapped as zone 3 using geology and zip-code level radon data. Zip-code level mapping, combined with geologic data represents a promising method for creating more accurate and informative maps so that homeowners can evaluate and mitigate radon hazards.