A preliminary assessment of geochemical and geologic controls on radon in Alberta groundwater

ABSTRACT: Exposure to 222Rn (radon) in air is the leading cause of lung cancer among non-smokers globally. Radon is produced naturally from some geologic formations by the alpha-decay of Radium-226 as part of the Uranium-238 radioactive decay chain. World-wide, some correlations have been identified between geologic formations and radon in indoor air and groundwater, but typically these associations are only evident between broad lithological groups (e.g., sedimentary versus metamorphic bedrock) and rely on few samples with anomalously high radon concentrations. While water well based groundwater monitoring surveys typically find low dissolved radon concentrations, free phase gas analyses and total dissolved gas pressure (PTDG) are rarely considered. The association of radon with earthquakes and faults suggests buoyant free phase gas transport may occur in the subsurface. A survey of indoor air in 9,507 Albertan homes found 14% with radon levels in indoor air above Health Canada guidelines of 200 Bq/m3 (www.EvictRadon.org). Radon's short half-life (3.8 days) suggests that a persistent radon source and exposure pathway exists. With the objective to identify potential exposure pathways of radon, this study combined radon and PTDG measurements with Alberta's ongoing groundwater monitoring program to evaluate geogenic sources of radon in Albertan homes. Radon's relationship to groundwater geochemistry, major dissolved gas species, PTDG, and geologic formations were evaluated. Preliminary results indicate that: dissolved radon and methane concentrations have a hyperbolic relationship and radon concentrations are inversely proportional to PTDG. No other strong correlations have been determined so far. The sample with the highest radon concentration (77 Bq/L) had a dissolved uranium concentration ~10x to 100x greater than other samples, but a strong radon to uranium correlation was not present when the radon-rich sample was removed from analysis. Data collection is actively ongoing and will be presented.