Vol. 48, 01, 05ok.qxd 93 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words radon – indoor-bedrock relationship – granitoids 1. Introduction In the Czech Republic indoor radon measure- ments have been performed by the National Ra- diation Protection Institute (NRPI) since 1990. Up to now more than 130 000 track-etch detec- tors (Kodak LR 115) have been distributed with- in the whole country. The primary random distri- bution of detectors based on the demands of dis- trict and regional authorities was soon changed for goal – directed distribution according to geo- logical radon risk maps in various scales (Mikšová and Barnet, 2002). The soil gas radon database has been built up since 1990. According to the uniform methodics used in the Czech Republic the soil gas radon data produced by the Czech Geological Survey and companies joined in the Association Radon Risk are comparable and reproducible and serve as a basis both for radon risk mapping and sta- tistical characterization of geological units and rock types. The content of the database is de- scribed in detail in Mikšová and Barnet (2002). Up to now the database comprises the soil gas radon data from more than 8900 test sites (15 measurement each). Since 1998 the Czech Geo- logical Survey has vectorised all 214 map sheets of geological maps on a scale 1 : 50 000. Region- al radon data are also available in Austria (Fried- mann, 2000), United Kingdom (Appleton and Miles, 2002), Germany (Siehl et al., 2000), Slovenia (Popit and Vaupotic, 2002), Luxem- bourg (Kies et al., 1996) and other countries. The aim of the present study is to find a re- lationship between indoor and bedrock radon in the Central Bohemian Plutonic Complex (CBPC), Czech Republic. 2. Experimental part 2.1. Geological setting The Central Bohemian Plutonic Complex (CBPC) forms the elongated body of 3200 km2 between Klatovy (SW) and Říčany near Prague (NE). According to the subdivision of the Variscan orogenetic belt in the Bohemian Massif Mailing address: Dr. Ivan Barnet, Czech Geological Survey, Prague 1, Klárov 3, 118 21 Czech Republic; e-mail: barnet@cgu.cz Indoor – soil gas radon relationship in the Central Bohemian Plutonic Complex Ivan Barnet (1), Jitka Mikšová (1) and Ivana Fojtíková (2) (1) Czech Geological Survey, Prague 1, Czech Republic (2) State Office for Nuclear Safety, Prague 1, Czech Republic Abstract The relationship of indoor radon measurements and radon in bedrock was studied in the granitoid Central Bo- hemian Plutonic Complex (CBPC). The indoor data were linked to vectorised geological and radon risk maps using the coordinates of particular dwellings. For each geological unit and rock type it was possible to calculate the statistical characteristics of indoor radon measurements. A clear relationship between indoor radon values and radon in bedrock was confirmed in all 7 districts situated on CBPC, where the study was performed. 94 Ivan Barnet, Jitka Mikšová and Ivana Fojtíková (Matte et al., 1990) the CBPC is emplaced be- tween two terranes – the weakly metamorphosed Barrandian on the NW (or Teplá-Barrandian unit) in the sense of other authors – e.g., Vrána and Štědrá (1997), Scheuvens (1999), Scheuvens and Zulauf (2000), Dörr et al. (2002), Zulauf et al. (2002) and medium to high grade metamor- phosed Moldanubian assemblage of Precambrian and Proterozoic rocks on the SE. The tectonother- mal history of both mentioned major units has differed since the Proterozoic (fig. 1). The different P-T conditions are marked by the presence of largescale shear zones separating both units (West Bohemian shear Zone and Hoher Bo- gen shear zone on SW border and Central Bo- hemian shear zone on the SE border). The CBPC was emplaced along the Central Bohemian shear zone during late Devonian – early Carboniferous. The data from Pb-Pb zircon evaporation ages, from U-Pb zircon dating and 40Ar/39Ar biotite cooling ages presented by Janoušek et al. (2000a) cover the interval between 336 and 351 Myr. The relatively long time of magmatic activi- ty imprints the petrogenetic variety of granitoid types and as a consequence has led to newly used terminology – the plutonic complex. A long Fig. 1. The major granitoid types and the probable position of the Central Bohemian shear zone (modified af- ter Chlupáč et al., 2002; Zulauf et al., 2002). Codes of districts: PH – Praha-East, BN – Benešov, PB – Příbram, PI – Písek, ST – Strakonice, PJ – Plzeň-South, KT – Klatovy. 95 Indoor – soil gas radon relationship in the Central Bohemian Plutonic Complex period of petrogenetic research of the CBPC re- sulted in distinguishing more than 20 granitoid types. On the contrary, data coming from petro- genetic, geochemical and geochronological sim- ilarities simplify the variety of granitoids into four suites (Janoušek et al., 2000a,b). The northern part of CBPC is formed by Sázava suite comprising the amphibole – biotite diorite, granodiorite and tonalite. Říčany intru- sion crops out in the northeastern part of CBPC and is characterized by presence of porphyritic K-rich biotite granite. The Čertovo břemeno on the south eastern border with Moldanubian unit is formed predominantly by porphyritic mela- granite and melasyenite (durbachite). The ma- jor radiometric difference of Čertovo břemeno rocks from other granitoid types is caused by the relatively high concentration of uranium (15-20 ppm U are the frequent values) and pres- ence of accessory minerals, namely zircon (up to 520 ppm – reported by Holub, 1997). The geochemical and mineralogical composition of Čertovo břemeno suite also causes the major indoor radon problems within all geological units on a countrywide scale. The Blatná suite comprised mostly of amphibole – biotite gran- odiorites forms the central and southwestern part of CBPC. The petrochemical differences of the four distinguished granitoid suites can be observed from AFM and K2O/SiO2 graphs pre- sented by Janoušek et al. (2000a). A long time range of CBPC emplacement, the petrochemic and petrogenetic variety, and trends in tectonic and structural development implicate the importance of the Central Bo- hemian shear zone. The southwestern part of the shear zone was thoroughly studied (Pitra et al., 1999; Zulauf et al., 2002). The advantage of the southwestern part of the shear zone is the clear- ly observable tectonics separating CBPC from Moldanubian and relatively narrow extent of emplaced granitoids. On the other hand, the lat- eral extent of CBPC is about 40-50 km without distinct patterns or relics of shear zone orienta- tion in the central part. The processes of multi- ple intrusive activities during the Variscan orogeny have covered the surface evidences of deep tectonics but regional geological studies (Matte et al., 1990) and seismic profiling help to recover the paleosituation (Tomek et al., 1997). The indoor data from 7 former districts ex- tend by their position the range of CBPC’s granitoids. On the SE, S and SW of CBPC the bedrock is formed by medium to high grade metamorphosed Moldanubian paragneisses, migmatites and orthogneisses. The border with NW Tepla – Barrandian unit comprises the un- metamorphosed folded Palaeozoic sediments (Cambrian to Devonian shales, limestones and quartzites) and Lower Proterozoic slightly metamorphosed shales locally with silicites. 2.2. Radon measurements After one year exposure time the indoor radon detectors (two for each dwelling) were processed in the National Radon Reference Laboratory in Kamenná near Příbram. The re- sults of indoor geometric mean radon values for each dwelling were given in Equilibrium Equivalent Concentration (EEC). The Central Bohemian Plutonic Complex belongs to fo- cused areas (Barnet et al., 2002) and the densi- ty of indoor measurements exceeds the general average in other areas. The indoor measure- ments (together 16 145) were selected from 7 former districts Praha-East (444), Benešov (2985), Příbram (6341), Písek (2164), Strakon- ice (1779), Plzeň-South (1596) and Klatovy (836) covering the areal extent of CBPC. For the purpose of this study the coordinates of family houses (digitized by the Czech Statisti- cal Office) from 7 above mentioned districts were selected due to the direct contact of hous- es with bedrock. The soil gas radon measurements were per- formed using the Scintrex RD 200 radonmeter. The soil gas radon was sampled using the lost- tip method from 15 probes and the third quar- tile of the data set was used as a classification parameter together with the permeability of the test site, given in three categories (low-medi- um-high). This method is fully described in Mikšová and Barnet (2002) and serves as a uni- form method for building site assessment in the Czech Republic. The mean soil gas radon val- ues for particular rock types were calculated from the radon database of the Czech Geologi- cal Survey. 96 Ivan Barnet, Jitka Mikšová and Ivana Fojtíková 3. Method of Geographical Information System (GIS) analysis The first phase of data processing was link- ing the database of centroids of dwellings (Czech Statistical Office) with the database of indoor radon measurements (National Radia- tion Protection Institute). This part of data pro- cessing was performed using the FoxPro 3.0 programme and identificators of particular dwellings and municipalities. The resulting database from the area cover- ing the territory of former 7 districts comprised 16 145 dwellings with defined indoor radon mean and maximum value (EEC) and x, y coor- dinates. In the second phase of data processing this database was put into GIS based on MGE 7.1, Oracle and Microstation 95 programmes and transformed into the ArcView 3.2. pro- gramme. The vectorised polygons of geological units and rock types were selected for the area covered by indoor measurements (16 map sheets of bedrock geological maps and radon risk maps on a scale 1 : 50 000). The areal analy- sis performed using the MGE Analyst and Mapfinisher programmes linked the geological unit and rock type from the uniform legend for each dwelling. As each geological map comprises about 40-90 different rock types according to petrog- raphy and mineralogic composition, these rock types were grouped with respect to prevailing radon risk calculated from the soil gas radon database for each rock type. Consequently each dwelling was characterised by indoor radon da- ta (EEC) and corresponding mean soil gas radon concentration in the underlying rock type and category of radon risk from bedrock. The database of NRPI does not contain data on the technical characteristics of the dwellings (namely the quality of sealing) but a sufficient number of indoor radon measurements enables this load to be neglected. 4. Results The mean and maximum indoor Rn concen- trations in granitoid rock types of the CBPC are given in table I. Within each district the mean indoor radon concentration in dwellings situated on the par- ticular rock type was plotted against mean soil gas radon concentration. Figure 2 illustrates the indoor – soil gas radon relationship in six dis- tricts. Mean indoor values are given in EEC. For Praha – east district the dwellings were situated only on Quaternary sediments and gran- ites (insufficient for graphical presentation). From fig. 2 it is obvious that increasing soil gas radon concentration in rock types generally also causes an increase in indoor radon values. In all studied districts the highest indoor and soil gas radon concentrations are observed in durbachites, granites and granodiorites. The other rock types with enhanced indoor and soil gas radon concen- trations are Silurian black shales and Proterozoic metasilicites (both concentrating uranium). The high indoor and soil gas radon concen- tration observed in Ordovician sediments of Strakonice district are bound to one village sit- uated on the tectonic zone communicating with underlying granites. Figure 3 illustrates the position of dwellings from 7 districts with indoor radon levels exceed- ing 1000 Bqm–3 (EEC). Most of the houses are situated on the NW border of durbachites of Čer- Table I. Mean and maximum indoor Rn concentra- tions in granitoids of CBPC (GR – granite, GD – gra- nodiorite). Type of granitoid Mean Rn Max Rn (Bqm–3) EEC (Bqm–3) EEC GD Sedlčany 488.2 572.4 GR Čertovo 397.2 463.4 břemeno GD Kozárovice 350 400.8 GD Benešov 311.5 388.3 GR Marginal NW 310.2 377.4 GD Blatná- 270.5 340.8 Zvíkov GD Červenský 219.8 285.6 GR Říčany 167.9 211.4 GD Kozlovice- 157.9 191.7 -Maršovice GD Pošáry 78.9 91.2 Fig. 2. The relationship of indoor and soil gas radon in the rock types of 6 Bohemian districts. Explanation of rock types: SS – Silurian Sediments, DR – Durbachites (syenites), GR – Granites, GD – Granodiorites, KR – Moldanubian paragneisses, POR – Palaeozoic volcanites, MS – Quartzites, erlanes, GN – Moldanubian or- thogneisses, DS – Devonian Sediments, PT – Proterozoic metasediments, SPR – Loess, A – Amphibolites, Q – Quaternary, GA – gabbros, OS – Ordovician Sediments, KAS – Cambrian sediments, Qgravel – Quaternary gravel, N – Neogene sediments, AL – Alluvial sediments, PTs – Proterozoic silicites. 97 Indoor – soil gas radon relationship in the Central Bohemian Plutonic Complex 98 Ivan Barnet, Jitka Mikšová and Ivana Fojtíková tovo břemeno and Sedlčany granodiorite but do not follow the contact of either rock body with other types of granitoids of CBPC. The position of high indoor radon dwellings resembles the orientation of the Central Bohemian shear zone, especially in the southeastern region of Čertovo břemeno syenite and Sedlčany granodiorite ar- eas. This feature is not influenced by the density of indoor radon measurements and technical state of dwellings – in all 7 districts both factors are comparable. Therefore we suggest that the quasi-linear course of the high indoor radon dwellings position corresponds to the paleocon- tact of two major terranes – Teplá-Barrandian and Moldanubian. 5. Conclusions A large-scale indoor Rn and bedrock Rn comparison was performed on the area of the Central Bohemian Plutonic Complex. The GIS analysis based on the coordinates of particular dwellings proved its efficiency in comparing the indoor radon data and corresponding de- tailed geology. In all seven former districts the distinct trend of increasing indoor radon with increasing soil gas radon was proven in all rock types from Pre- cambrian to Quaternary in spite of missing data on the technical state of dwellings. This fact supports the prognostic ability of radon risk maps based on the geology and soil gas radon measurements. Even if these maps are not used for deriving the radon risk on particular building sites (which is influenced by minor local geo- logical features) they can serve as a useful tool for distributing the track-etch detectors on a wide country scale for setting the priorities of indoor Rn measurements. The linear position of extremely high indoor Rn dwellings within the CBPC does not follow the local contacts of granitoid rocks or present tectonic network, but corresponds to the sup- posed course of the Central Bohemian shear zone. The palaeosuture dividing two Variscan terranes was effaced after emplacing the CBPC. The weakened zone of the upper crust in the border area between Teplá-Barrandian and Moldanubian terranes could prepare suitable a environment for radon migration along the deep- seated shear palaeozones. The basement patterns Fig. 3. The position of dwellings exceeding 1000 Bqm–3 (EEC) within the CBPC. 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