Radon content in groundwaters drawn from the metamorphic basement, eastern São Paulo State, Brazil

The 222Rn activity concentration was measured in groundwaters drawn from three wells drilled in different rocks of the metamorphic basement exposed at eastern São Paulo State, southeastern Brazil. The first well cuts a K-feldspar bearing granite, the second well cuts a granite–gneiss and the third well cuts a gneiss. The groundwater samples were collected between July 2005 and August 2006 with a time interval of about one month between sampling campaigns. The year-long mean activity concentrations were (76±7)Bq/dm3 for the first well, (26±3)Bq/dm3 for the second well and (39±4)Bq/dm3 for the third one. The activity concentrations show a time-dependent variability that is interpreted as a consequence of rainfall seasonal variations.