Mapping the geologic radon potential of the United States: Lessons learned

The geologic radon (Rn) potential of the United States (USA) was released in 1993 by the U.S. Geological Survey and U.S. Environmental Protection Agency. It was designed to assist jurisdictions in targeting their resources and assist building code officials in deciding whether radon-resistant features were applicable in new construction. The geologic Rn potential was determined using 5 types of data: 1) lithologic, 2) aerial radiometric, 3) soil properties such as moisture and permeability, 4) indoor Rn, and 5) building architecture. A three tier Radon Index was used to rank Rn potential based on the above factors, and a Confidence Index was used to express uncertainty based on quantity and quality of data. The geologic factor is based on the type and distribution of lithologic units in the mapped area. The uranium (U) content of the lithologic unit, its permeability, and deformation were taken into consideration as well as field studies of Rn emanation. Rock types with naturally high U that may cause indoor Rn in the USA include: carbonaceous black shales, glauconitic sandstones, phosphatic sediment, chalk, bauxite, lignite, certain carbonate, glacial, fluvial, and coal deposits, U-bearing granites and pegmatites, granitic metamorphic rocks, felsic and alkalic volcanic rocks, syenites and carbonatites, and many sheared or faulted crystalline rocks.
Rock types least likely to cause Rn problems include marine quartz sands, non-carbonaceous shales and siltstones, some clays and fluvial sediments, and metamorphic, volcanic, and igneous rocks of mafic composition. Exceptions exist within these general lithologic groups. Carbonate contains little U and Rn, but the residual soils that form above them in karstic terrain can have very high U, radium (Ra) and Rn. Faults and shear zones in crystalline rocks have enhanced permeability and weathering that facilitates Rn emanation and are associated with the highest known indoor Rn levels in the USA. High Rn has been measured in coarse glacial deposits and gravel bars where high permeability drives high Rn.
Use of Rn potential maps by states, and the resulting state regulations, have varied widely with different results that have affected policy, regulation, construction, and economics. Are the geologic Rn potential maps still relevant today in light of new knowledge and techniques? How have the maps and methodology faired in terms of subsequent testing and validation? These questions were examined looking at data from how states utilized the maps and how readily the maps predicted subsequent indoor Rn. Attempts to map Rn potential using only radiometric data, only housing characteristics, statistical analyses of indoor Rn data, or combinations of the three have failed or been inadequate in describing the variability of Rn across the USA. Radon potential mapping must use a variety of geologic factors that describe the characteristics of a given rock or soil to emanate Rn.