Bedrock structure control on soil-gas radon-222 anomalies in the Toronto area, Ontario, Canada
Imshun Je, and Nicholas Eyles
University of Toronto at Scarborough, Environmental Earth Sciences, Scarborough, ON, Canada
Soil-gas radon is a newly recognized environmental hazard that has received much interest in the United States, but few studies have been conducted in Canada, particularly in urban areas. This paper describes the results of a 2-yr study of near-surface in situ radon soil-gas levels in the eastern end of the Greater Toronto Area (GTA: population 5.5 million) of southern Ontario, Canada. Transects up to 550 m in length, cross surface lineaments (stream valleys) associated with a prominent but deeply buried mid-Proterozoic shear zone (Central Metasedimentary Belt Boundary Zone: CMBBZ) that underlies the eastern GTA. Early Paleozoic and Pleistocene glacial strata that overlie the CMBBZ are extensively fractured. Regional background levels of radon identified in this study (100-200 pCi/L) are consistent with results of previous work in southern Ontario but repeat measurements also identified anomalously high radon levels (up to 1000 pCi/L). Anomalies occurred at or closely adjacent to the same locations along each transect over the duration of the study. Strata are extensively fractured and it is likely that fractures provide pathways for radon transport to the surface by carrier gases such as CO 2 , H 2 and CH 4 released from black shales. This transport mechanism has been recognized elsewhere in southwestern Ontario in regard to radon migration along faults and used to define hydrocarbon reservoirs. In light of these findings, hazardous or near-hazardous levels of radon may occur elsewhere above fractured strata that underlie the GTA. Mapping of fractures and related structures would appear to provide a useful means of delineating potentially hazardous radon levels across this heavily urbanized area.
This record provided courtesy of AGI/GeoRef.
