Free phase radon in the Earth's subsurface: Occurrence, sources, transport pathways, and insights

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Radon is a radioactive element that occurs in dissolved and free phase gas forms. It is the 6th leading cause of cancer (and 2nd leading cause of lung cancer after smoking), but imperceptible to the human senses. Radon is ubiquitous and highly variable in indoor air, shallow soil, and groundwater, but its source(s) are not clear. Although radon in shallow soil and homes is most often attributed to shallow (less than 100 metres or so depth) uranium-rich soil and rocks, this has not been borne out in research studies. An understanding the major subsurface source(s) and transport pathways that are are responsible for radon's ubiquitous but spatially heterogeneous occurrence would help mitigate its significant public health impacts. Although we know very little about the main source(s) and transport pathways of subsurface radon, we have some important clues. Radon often co-occurs at relatively high concentrations (along with carbon dioxide) in free phase gas emissions from volcanoes, caves, underground mines, and some geologic faults. Its concentrations are higher in air over the continents than the oceans, consistent with a continental source.  This research will evaluate the possibility that Rn is widely produced at significant depths (i.e., > 1 km) in the earth's continental crust or mantle, and transported to the subsurface under buoyancy as free phase gas through a widespread pathway network. This notion is conceptually challenging because radioactive decay causes Rn concentrations to decrease by one-half every 3.82 days, so the transport from depth has to be rapid.  Although little field research has been conducted, free phase gas transport through fractures and pores has long been reported, with rates up to tens of kilometers per day, however, so this is possible.    I will work with ten trainees (five BSc, three MSc, and one PhD student and one PostDoc) to sample deep sources of gases and associated fluids (including gassy springs and ponds, wells, geysers, and geologic faults) where gas bubbles issue from the subsurface. Radon will be analysed alongside a suite of other parameters to provide insight into the source(s) and depth(s) from which radon and other fluids originated, and also relative subsurface residence times. This work will providing insight into radon sources and pathways to support governments and industry to mitigate  cancer risk from radon, especially in homes.  This research will also provide insight into geogenic carbon emissions to the atmosphere (which are poorly constrained globally, except for volcanic emissions).  Thirdly, this work will  provide insight into free phase gas transport pathways and velocities in the earth's subsurface about which we know relatively little, but is needed for appropriate monitoring and measurement around facilities (e.g. leaking petroleum wells, gas storage sites, and carbon capture and storage sites) so Canadians can greenhouse gas emissions effectively.