Vol. 48, 01, 05ok.qxd 109 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words radon-indoor-bedrock relationship – radon mapping 1. Introduction The Czech Geological Survey (CGS) has been participating in solving the problems of the inhabitants’ irradiation caused by natural ra- dionuclides since 1990 (Barnet, 1996). The Radon Programme of the Czech Re- public (CR) started within the scope of the Governmental Decision No. 538 in 1999. Pre- sent research on radionuclides in dwellings has revealed that the CR is one of the countries with the highest level of average 222Rn concentration in buildings. Therefore great attention is paid to indoor Rn measurements. A number of houses above the guidance lev- el of 200 Bqm–3 of equilibrium equivalent con- centration being detected in the last five years is given in table I. The main target of research work was to process all data and information from available databases and approve the existing relationship between increased indoor radon values and rock types in bedrock considering that a signif- icant part of the territory of the CR is formed by the Bohemian Massif, which belongs to the European Variscan belt represented by the Pro- terozoic and pre-Variscan Paleozoic crystalline basement. (Mísař et al., 1983). Similar studies were performed by Kies et al. (1996), Kemski et al. (2000), Popit and Vaupotic (2002). 2. Data sources Geologically based radon data originates from CGS measurements and also from more than 100 private companies of the Association Mailing address: Dr. Jitka Mikšová, Czech Geologi- cal Survey, Prague 1, Klárov 3, 118 21 Czech Republic; e- mail: miksova@cgu.cz Radon data processing and outputs for the needs of the State Office for Nuclear Safety (according to the Czech Radon Programme) Jitka Mikšová and Ivan Barnet Czech Geological Survey, Prague 1, Czech Republic Abstract Much of the population living in the Czech Republic is exposed to radiation from natural sources, especially to the radon effect. The aim of geological research defined by the State Office for Nuclear Safety (SONS) was to detect areas with estimated high radon concentration in soil gas. A uniform method of measurements and uniform method- ology of radon risk category assessment of geological units and a centralized radon database was established. Radon risk classification was based on statistical evaluation of soil gas radon concentration and permeability in investigat- ed geological units. Prognostic radon risk maps in various scales were the main outputs of this research. With the help of GIS tools spatial analyses were found a correlation between soil gas radon values in selected geological units and indoor measurements in dwellings. After verification of the efficiency of track etch detectors placed in dwellings with the help of prognostic maps 75% reliability of these maps was proven. This reliability of analyses induced the SONS to widely use radon risk maps to determine areas with predicted high radon risk category. 110 Jitka Mikšová and Ivan Barnet Radon Risk. Data sources used for GIS appli- cations come mostly from state organizations. – Czech Geological Survey (CGS): Soil gas radon database (about 9000 test sites in different rock units), vectorized geological maps (214 map sheets 1 : 50 000). – Czech Office for Surveying, Mapping and Cadastre: Raster topography. – Ministry for Regional Development of the Czech Republic: Database – UIR (special data register of regional identification), contours of cadastres, database of residential units and other details of measured dwellings. – National Radiation Protection Institute: In- door radon database (indoor radon measurement – 130 000 points, geometric mean in cadastre – 6299 cadastres). – Czech Statistical Office: Database of geo- graphic position (of measured dwellings) and character of dwellings. The field method of soil gas radon measure- ments and methodology of radon risk category assessments are standardized. A uniform method for soil gas radon measurements is used by all or- ganization dealing with building site assessment. The soil gas sample is taken from a depth of 0.8 m using rods with a «lost tip». At least 15 measurements are performed at each test site. The 75% quantile of radon data set serves as an input parameter for radon risk classification. The per- meability is derived from grain-size analysis or from permeametric measurements in situ. All subjects dealing with soil gas radon measurements pass the calibration of devices in radon chamber and field-testing at the reference sites to ensure the comparability and reliability of results. In the Czech Geological Survey the portable radonmeter RDA 200 (Scintrex) with exchangeable Lucas cells is used for soil gas radon measurements. 3. Data processing All data are placed in a centralized database administrated by CGS. A sufficient number of measurements (nearly 9000 measured test site with 15 measurements and 29 items concerning localization, radon, geological and classifica- tion data for every test site) makes a statistical- ly reliable data set to determine radon risk from bedrock in particular geological units and rock types (Mikšová and Barnet, 2002). The construction of predictive radon risk maps is based on contours of geological units. The division of rock types into prevailing radon risk categories is done with the help of statistical methods using soil gas radon con- centration and permeability. The predictive radon risk maps are not based on the uranium data from the territory of the Czech Republic but solely on the soil gas radon and perme- ability measurements (Mikšová and Barnet, 2002). The rock units are classified after pre- vailing radon risk category: low risk (mostly younger sediment formations from Creta- ceous to Neogene), interstage risk (mostly in- homogeneous Quaternary sediments), medi- um risk (mostly Paleozoic sediments and crystalline gneisses), high risk (granitoids). Table I. Number of indoor Rn measurements in the Czech Republic in 1998-2002. Year Number of measured dwellings Number of dwellings with range of measured EEC (Bqm–3) > 200 200-299 300-600 >600 1998 5634 2014 925 773 316 1999 5257 1171 533 455 183 2000 6760 1570 668 684 218 2001 11546 2150 1107 802 178 2002 10841 1749 850 722 177 111 Radon data processing and outputs for the needs of the State Office for Nuclear Safety (according to the Czech Radon Programme) Fig. 1. The selection of cadastres performed by simple intersection of cadastres and contours of durbachite bodies (bold outlined). The building projects of newly built houses are modified after the radon risk category measured on the building ground (Czech Technical Norm 730601, 1996). The spatial analysis was made to find sig- nificant information on the relationship be- tween soil gas radon concentration and indoor measurements. Three different approaches for spatial comparison of cadastre’s polygons and contours of the geological units were tested. The way combining the demand for geographi- cal preciseness and the demand for statistical reliability for further analyses was chosen. This method comprised the selection of cadastres with centroidal points situated inside the con- tour of rock unit, but the border of the cadastre was partially intersecting the contour of the ge- ological unit (Barnet et al., 2002). An example of resulting selection performed by three meth- ods is given for cadastres situated in the durba- chite bodies of Třebíč massif in Moravia (figs. 1, 2 and 3). The indoor radon means calculated for dif- ferent rock types on the basis of spatial analy- sis are given in table II. The «indoor intersec- tion» values represent the results of the first method of analysis where polygons of cadas- tres are intersecting the polygons of geological units with no respect to ratio of aerial intersec- tion. The «indoor inside» values are calculated for polygons of cadastres, which are fully situ- ated inside the contours of the geological unit. The «indoor centroid» values correspond to ge- ographical centroids of cadastres, which lie in- side the contour of the geological unit, but the polygon of cadastre can intersect partially the contour of the geological unit. The values placed in shaded columns in table II approve the relationship of mean soil gas radon values in rock types and mean indoor radon values. 112 Jitka Mikšová and Ivan Barnet Fig. 3. The selection of cadastres based on placing the centroids of cadastres inside the rock contours plus intersect- ing the contours of durbachite bodies (bold outlined). This selection represents the shape of rock body most precisely. Fig. 2. The selection of cadastres based on placing the polygons of cadastres inside contours of durbachite bod- ies (bold outlined) without intersecting. Table II. The values of indoor radon geometric means in major rock types calculated by three different methods. Major rock types Indoor Rn (means Bqm–3) Soil gas Rn (means kBqm–3) Indoor No. Indoor No. Indoor No. Mean No. intersection int inside ins centroid cent rock rock Moldanubian paragneisses 99.24 570 116.96 79 100.53 275 28.08 528 – Monotonous series Moldanubian crystalline 104.2 464 91.27 42 95.19 236 31.25 465 rocks – Variagated series Proterozoic – phyllites, 95.81 1062 101.76 54 94.69 470 26.61 345 metamorphosed shales Orthogneisses, granulites, 106.01 583 109.49 23 110.62 181 32.05 383 migmatites – Moldanubian Ultrabasic rocks 96.03 51 0 0 139.93 3 17.2 4 – Moldanubian Granitoids – Cadomian 89.89 192 80.6 3 95.74 59 17.2 155 Palaeozoic 91.1 201 103.75 2 96.18 87 39.25 154 – metamorphosed Palaeozoic 93.7 552 102.65 121 99.51 320 27.92 402 – unmetamorphosed Volcanites – Proterozoic, 91.12 606 64.4 2 97.13 99 37.11 45 Palaeozoic Granites – Variscan 104.32 547 128.64 36 105.98 209 58.8 424 Granodiorites – Variscan 152.38 390 203.09 39 177.86 197 66.3 307 Durbachites, syenites 205.36 165 334.91 39 272.3 77 56.78 180 Diorites, gabbros – Cadomian 107.67 133 0 0 87.23 20 22.63 55 Mesozoic – Alpine folding 54.39 128 47.53 4 52.74 53 17.35 1788 Tertiary – Alpine folding 65.22 429 71.76 53 67.15 273 20.28 759 Permocarboniferous 74.46 515 72.36 20 75.29 234 33.72 326 Mesozoic – sediments 62.23 1224 60.46 67 60.72 590 17.53 1788 Tertiary – sediments 78.35 820 66.99 9 72.57 192 20.28 759 Neovolcanites – Tertiary 65.47 306 69.2 3 60.64 52 21.55 35 Quaternary 73.06 2274 73.25 199 68.57 1003 25.37 525 113 Radon data processing and outputs for the needs of the State Office for Nuclear Safety (according to the Czech Radon Programme) 4. Software platform The source soil radon gas database was based on Visual FoxPro, for predictive radon risk map production converted to Oracle 8i. The data model for geological maps was originally created in ArcInfo (ESRI Corp.). Later this model was converted into MicroStation (Bent- ley Systems Corp.) – MGE (Intergraph Corp.) – Oracle (Oracle Corp.). This model was also used for radon risk maps formation. The spatial analyses were done in ESRI software Arc GIS. 5. Outputs In 1998 the CGS issued the Digital Atlas GEOCR500 – the geological, radiometric and radon risk maps on the scale 1 : 500 000 to- gether with eight maps with geoscientific topic. This radon risk map was based on the vector- ized contours of geological units and on the re- sults of gamma dose rate measurements. This Atlas was published on CD-ROM in GIS project for ArcView 3.0 (Barnet and Mikšová, 2001). 114 Jitka Mikšová and Ivan Barnet In 1998 the CGS finished the vectorization of geological maps at the scale 1 : 50 000. These maps covered the whole state territory. This fact allowed the formation of more de- tailed radon risk maps based on geological maps. Since 1999 about 154 map sheets of Radon risk maps at the scale 1:50 000 (from to- tal number of 214 sheets) have been published up to 2003 (fig. 4). These maps are available in digital and printed form. The digital outputs are published on CD-ROM. The selection from the mosaic map enables to open the full view of the map and detailed view of four single quadrants. The points of the test sites are linked to the selected items from the radon database in a separate window. 6. Conclusions By comparing the values of radon concen- tration in dwellings and the measured values on the test sites in geological units, the close cor- relation between the radon concentration and geological bedrock was established, especially in the areas where igneous and metamorphosed rocks were found. The data processing based on vectorized geo- logical maps was proven as a highly efficient and relevant tool for determination areas where in- creased indoor radon values caused by radon ex- halation from bedrock can be detected. After verification of the efficiency of track etch detectors placed in dwellings with the help of prog- nostic radon risk maps at the scale of 1:50 000, 75% reliability of these maps was proven. The outputs of geological research are used by regional centers of SONS and munic- ipal authorities to set the priority for distribut- ing the track-etch detectors in dwellings. In the case of randomly distributed track-etch de- tectors only 2% of measured dwellings ex- ceeded the guidance level 200 Bq m–3 EEC. Using radon risk maps on a scale 1:50 000 for setting of detectors into areas with assumed medium or high radon risk category in bedrock the number of affected dwellings in- creased to 20%. Fig. 4. Czech Republic – completed radon risk map sheets 1:50 000 (bold framed). The priority of map sheets pro- cessing was oriented to cover dominantly the extent of granitoid bodies within the Czech Republic. 115 Radon data processing and outputs for the needs of the State Office for Nuclear Safety (according to the Czech Radon Programme) Predictive radon risk maps were issued by CGS in the form of printed maps or on interactive CD-ROM in GIS projects. Presentation of radon research work, especial- ly radon risk mapping, are accessible to the wider public on the portal of CGS – www.geology.cz – radon mapping – in Czech and English versions. REFERENCES BARNET, I. 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