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