Complex investigation of radon concentrations in soil gas and electrical tomography in Lithuanian Karst Region

The national radon survey in Lithuania started in 1995. Measurements have been performed in 400 randomly selected detached houses. Two separate used by inhabitants rooms used to be selected for measurements. E-PERM electrets by Rad Elec Inc were used for registration of indoor radon concentrations. The measurements were performed during heating season for 3 or more weeks. The results indicate dependenee of indoor radon concentrations on location of house. Five regions have been detected with average indoor radon concentrations exceeding the national level (55 Bq m") by almost two times. Houses with indoor radon concentrations higher than internationally recommended and nationally adopted levels of action (400 Bq m-3 for existing and 200 Bq m-3 for new houses) have been detected during the survey. One more region with elevated indoor radon concentrations has been identified in the karstic area near Birzai. For identification of this region the data available in the Lithuanian Geological Survey have been used because the measurements in randomly selected houses have been unable to disclose uniquity of this region. Analysis of results shows with small exceptions that the main souree of indoor radon in Lithuania is soiI. Since it is necessary to create the system of radon risk mapping selective measurements of radon concentrations in the soil air have been performed. Electrical tomography measurements were also included in order to get a possible correlation between distribution of radon concentration in soil gas and apparent resistivity anomalies of subsurface.