222Rn activity in groundwater of the St. Lawrence Lowlands, Quebec, eastern Canada: relation with local geology and health hazard

One hundred ninety-eight groundwater wells were sampled to measure the 222Rn activity in the region between Montreal and Quebec City, eastern Canada. The aim of this study was to relate the spatial distribution of 222Rn activity to the geology and the hydrogeology of the study area and to estimate the potential health risks associated with 222Rn in the most populated area of the Province of Quebec. Most of the groundwater samples show low 222Rn activities with a median value of 8.6 Bq/L. Ninety percent of samples show 222Rn activity lower than 100 Bq/L, the exposure limit in groundwater recommended by the World Health Organization. A few higher 222Rn activities (up to 310 Bq/L) have been measured in wells from the Appalachian Mountains and from the magmatic intrusion of Mont-Saint-Hilaire, known for its high level of indoor radon. The spatial distribution of 222Rn activity seems to be related mainly to lithology differences between U-richer metasediments of the Appalachian Mountains and magmatic intrusions and the carbonaceous silty shales of the St. Lawrence Platform. Radon is slightly enriched in sodium-chlorine waters that evolved at contact with clay-rich formations. 226Ra, the parent element of 222Rn could be easily adsorbed on clays, creating a favorable environment for the production and release of 222Rn into groundwater. The contribution of groundwater radon to indoor radon or by ingestion is minimal except for specific areas near Mont-Saint-Hilaire or in the Appalachian Mountains where this contribution could reach 45% of the total radioactive annual dose.