Radon Mapping in Southern California: Recent California Geological Survey Radon Activities 2005 International Radon Symposium, San Diego, CA September 26, 2005 Ronald Churchill, Ph.D. Senior Geologist California Geological Survey Major CGS Radon Projects for the DHS Radon Program Since 1990 ƒ Geologic Controls on the Distribution of Radon in California (1991) ƒ Geology, Soils and Indoor Radon at Elementary Schools (1993) ƒ Santa Barbara County and Ventura County Radon Potential Maps (1995) ƒ South Los Angeles County Radon Potential Map (2005) http://www.consrv.ca.gov/CGS/minerals/hazardous_minerals/radon/ California Geological Survey South LA County Radon Mapping Project ƒ 9.02 million inhabitants (94.7% of county population) ƒ 1989 sq. mi. (southern ½ of the county) ƒ DHS short-term Rn data from 1729 sites (1990-2004) ƒ Range: < 0.3 pCi/l to 159.6 pCi/l ƒ 145 sites ≥ 4 pCi/l (8.4% incidence rate) 220 Geologic Map Units (69 units have ≥ 1 Rn test) 298 Zip Code Zones (233 zones have ≥ 1 Rn test) California Geological Survey The South LA County Radon Potential Map Goals ƒ Define and show areas of high, moderate and low potential for ≥ 4 pCi/l indoor-radon levels ƒ 1:100,000-scale map (1 inch = 1.58 miles) Data Utilized ƒ DHS short-term test data (1990-2004) ƒ NURE airborne radiometric data (1970s) ƒ Existing geologic maps California Geological Survey DHS Short-Term Rn Test Data California Geological Survey NURE Airborne Radiometric Data for eU California Geological Survey Mapping Procedure ƒ Examine relationships between indoor-Rn data, NURE data and geologic units using GIS methods ƒ Identify geologic units with increased Rn potential ƒ Develop definitions for “High,” “Moderate,” and “Low radon potential geologic units ƒ Develop Rn-zone boundaries based on the distribution of high and moderate radon potential geologic units ƒ Confirm that final Rn zones are statistically distinct California Geological Survey Geologic Units with Relatively High Radon Potential in Southern LA County From Indoor-Radon Test Data ƒ Miocene Marine Siliceous Shale ƒ Older or Recent Alluvium likely derived from Miocene Marine Shale From NURE Airborne Radiometric Data ƒ Miocene Marine Siliceous Shale ƒ Quartz Diorite ƒ Gneiss California Geological Survey Resulting Southern LA County Radon Potential Map California Geological Survey Results and Findings Radon Potential Zones Zone Area (%) Rate ≥ 4 pCi/l (%) High (pCi/L) High 1.46 28.3 85.8 High-Qa 2.06 20.6 104.2 Moderate 11.75 9.7 36.6 Low 84.73 2.4 159.6 Population Exposure Estimates Est. Population Exposed to ≥ 4 pCi/l Indoor Air High + Mod. Zones = 165,000 Low Zone = 188,000 Est. South LA County ≥ 4 pCi/l Incidence = 3.9 % California Geological Survey Geologic Unit Based vs. Zip Code Zone Based Radon Maps California Geological Survey Radon Mapping—Future Considerations Geologic based mapping will be more common It is the best regional screening approach and points to areas needing additional study and possible regulation More Indoor-Test Data are Needed! Existing Rn mapping needs periodic review and revision, irrespective of origin Additional types of data will be increasingly important for site-specific studies and detailed mapping Ground-level radiometric data, soil gas data, soil porosity/permeability data, building information California Geological Survey