MAKING A DERIVATIVE RADON-RISK MAP OF WILLIAMSBURG, VIRGINIA

We have been testing homes for radon in Williamsburg, Virginia for many years and have paired results with each home’s geologic setting. The outcome identifies the Pliocene Yorktown Formation as a radon-producing geologic unit. Sediment cores from Coastal Plain units analyzed for ²³⁸U, ²²⁶Ra, and ²¹⁰Pb activities contained elevated concentrations of these radionuclides in the >2mm sediment fraction of the lower Yorktown. Sediments containing fossils and marine-mammal bones have up to 300 Bq ²²⁶Ra/kg, which is 10-times the crustal average, indicating that the lower Yorktown can be a source for radon. Results from radon tests show higher radon gas levels in homes built in and slightly above the Yorktown Formation than in other formations.

The availability of highly accurate topographic data (2-foot contour intervals), building footprints, and infrastructure GIS data from the City of Williamsburg provided a means to add geologic information and make a radon-risk map. We used field observations, borehole data, and the 1:24,000-scale geologic map of Williamsburg (Bick and Coch, 1969) to determine the top of the Yorktown Formation to be 58 ft above sea level throughout the city. Areas below 58 ft were mapped as high risk, and areas between 58 ft and 64 ft were mapped as moderate risk. Areas above 64 ft were mapped as low-risk because there is no evidence of overlying units producing radon. We created polygons using the contour data that delineated each of these risk areas. These polygons were clipped to the extent of the Williamsburg city limits. The final map is embedded in an interactive ArcGIS StoryMap that allows residents to input their address and identify the associated risk. The StoryMap also contains background information about radon and its health effects. Links to external information sources are also provided. To test the validity of the risk-map, another study is testing homes within each risk area.