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 Rn-222 activity in the region between Montreal and Quebec City, eastern Canada. The aim of this study was to relate the spatial distribution of Rn-222 activity to the geology and the hydrogeology of the study area and to estimate the potential health risks associated with Rn-222 in the most populated area of the Province of Quebec. Most of the groundwater samples show low Rn-222 activities with a median value of 8.6 Bq/L. Ninety percent of samples show Rn-222 activity lower than 100 Bq/L, the exposure limit in groundwater recommended by the World Health Organization. A few higher Rn-222 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 Rn-222 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. Ra-226, the parent element of Rn-222 could be easily adsorbed on clays, creating a favorable environment for the production and release of Rn-222 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. (C) 2014 Elsevier Ltd. All rights reserved.