A statistical study of an indoor radon data set of about 1700 short-term measurements shows a striking relationship between indoor radon concentration and the geological factors, such as stratigraphic unit and rock type

Charlet, J. M.

Tondeur, F.

Zhu, H.-C.

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IL NUOVO CIMENTO Maggio-Agosto 1999VOL. 22 C, N. 3-4Indoor radon levels in relation to geology in southern Belgium (*)(**)H.-C. ZHU (1), J. M. CHARLET (1) and F. TONDEUR (2)(1) Faculté, Polytechnique de Mons, Service GEFA - 9 Rue de Houdain, 7000 Mons, Belgium(2) Institut Supérieur Industriel de Bruxelles - 150 Rue Royale, B1000 Brussels, Belgium(ricevuto il 9 Giugno 1998; approvato il 18 Novembre 1998)Summary. — A statistical study of an indoor radon data set of about 1700 short-termmeasurements shows a striking relationship between indoor radon concentration andthe geological factors, such as stratigraphic unit and rock type.PACS 91.25.Ey – Interactions between exterior sources and interior properties.PACS 91.65.Dt – Isotopic compositionOchemistry.PACS 92.60.Sz – Air quality and air pollution.PACS 01.30.Cc – Conference proceedings.1. – IntroductionA potential radon risk was found in the Walloon region in previous studies(Vanmarcke et al., 1988). More detailed radon maps for the Walloon region haveconfirmed the significant radon risk in large parts of the region (Poffijn et al., 1992;Tondeur et al., 1994, 1995; Zhu et al., 1994, 1996).As no particular radon problem associated to building materials has been noticedtill now in Belgium, we expect the subsoil to be the main source of radon in the studyregion. If this is true, the geological environment, which includes the radon source andmigration pathway, should largely determine the radon potential. Comparisons ofindoor Rn measurements and geological information could be used to identify areaswhich have a high probability of generating elevated indoor radon levels. In this work,we use an indoor radon data set of about 1700 short-term measurements to study thestatistical relationship between indoor radon and the geological environment. On thebasis of our results, it will be possible to use geological information to identify areaswith an expected high radon potential.(*) Paper presented at the “Fourth International Conference on Rare Gas Geochemistry”,Rome, October 8-10, 1997.(**) The authors of this paper have agreed to not receive the proofs for correction.G Società Italiana di Fisica 353H.-C. ZHU, J. M. CHARLET and F. TONDEUR3542. – The dataThe data set used in this study was collected by the Institut Supérieur Industriel deBruxelles (ISIB) in 1680 homes of southern Belgium from June 1988 to February 1995(Tondeur et al., 1994, 1995), of which 1339 short-term (3 to 4 days) measurements werecarried out on the ground floors. The measured result was entered into the databasealong with the geographic coordinates and geological information such as rock types,geologic formations and metamorphism. The geological information which is obtainedmainly from geological maps on a scale of 1/40 000 is coded for the reason of dataprocessing.Stratigraphy is divided into 43 stages from Cambrian to Oligocene (cf. fig. 2).Superficial formations are grouped into 3 categories, Quaternary sediments (q1, q2,q3), alluviums (ale) of slopes and modern alluvion (alm) of valleys. Lithologies areclassed with 16 rock types (cf. fig. 3).The geological structure of the area under study is characterized by a foldedPaleozoic basement and a Mesocenozoic subhorizontal cover modified by brittlestructures. The Paleozoic basement includes the folded lower Paleozoic, characterizedby phyllites and quartzites, and the Upper Paleozoic characterized by pelitic formationand limestones.In analysing the data, two options are considered for the superficial covers. In thefirst approach, superficial formations are not considered. That is, if a house isconstructed on a geological formation with one or more superficial formations, Rn datais attributed to the older formation underneath. In the second approach, superficialformations are considered as a distinct stratigraphic unit, namely q1-q3, ale and alm,respectively (fig. 2). This implies that the other stages then only include the houseswithout superficial formations.Fig. 1. – Location of declustered houses.INDOOR RADON LEVELS IN RELATION TO GEOLOGY IN SOUTHERN BELGIUM 355Fig. 2. – Distribution of homes over different Rn levels in relation with geological stages.3. – Declustering of the dataBecause the sampling was voluntary, a higher density of observations appears inpopulated areas and in the areas where a radon problem was known previously.Over-sampled areas are therefore present in low-concentration areas (for example, thecity of Brussels), and in elevated Rn areas such as the Dyle valley and severalcommunes in the Ardenne massif. If we make use of this clustered data set forcalculations of statistical correlations between indoor Rn concentration and thegeological environment of homes, such as geological stage or series, lithologic type andmetamorphism etc., there will be a risk of biasing the results. To reduce the effect ofover-representation in the over-sampled areas, a data selection has to be performedusing a declustering technique. The idea is simply to superimpose a regular grid on thestudy area and to pick up within each 1.53 1.5 km grid cell a single sample closest tothe centre of the cell. About 30% of sample values are removed in this way. The newdata set (fig. 1) has almost the same statistical distribution as the original one.H.-C. ZHU, J. M. CHARLET and F. TONDEUR356Fig. 3. – Distribution of homes over different Rn levels in relation with lithologic types.4. – Results4.1. Geological formations and indoor radon. – From this section onwards, we shallonly refer to declustered data for ground floors (949 houses).As stated before, we expect that the geological environment should largelydetermine the radon potential in the region where we study.The relation between indoor radon concentrations and geology will be discussed interms of the percentage of homes observed to be above certain given levels: 200, 400and 800 Bq m23.The proportion of dwellings with a Rn concentration above the chosen referencelevels and in relation with each individual geological stage is calculated. As indicatedINDOOR RADON LEVELS IN RELATION TO GEOLOGY IN SOUTHERN BELGIUM 357before, the calculation has first been done without considering the superficial cover as adistinct unit. The results are shown in fig. 2 as white bars. Figure 2a indicates the totalnumber of houses for each of the stages, while fig. 2b, 2c and 2d show the percentage ofhomes in relation with geological stages. Figures 2b and 2c show a similar pattern withtwo peaks. One peak is present between the Gedinian and Siegenian (G2 to S3), theother in the lower Visean (V1). For Rn levels of 800 Bq m23 or more, the percentage ofhomes reaches the highest value in G2, with a general decrease from G2 to the youngerformations (fig. 2d).G1 is a special case. It seems that no home has more than 400 Bq m23, but it isexpected to have a higher proportion of homes over the higher Rn levels based on ourknowledge of groundwater radon levels for the same region (Zhu et al., 1995; Charletand Zhu, 1997). The reason here is that observations are extremely rare for the stageG1 (fig. 2a). In general, however, areas where the stages from G1 to E1 are presenthave high radon potential.From Co to H1, only Co, Gv, Fa1, Fa2, V1 and H1 have houses above 800 Bq m23.Thus, the stages from Co to H1, mainly characterized by carbonates and shales, have atleast moderate potential, of which Co, Gv, Fa1 and V1 are likely to have high Rnpotential.Dv has moderate potential, while Rv and Sl have high potential.Stages of the post-Carboniferous (including H2) have low to very low indoor Rnpotential although Cp is likely to pose a radon risk, while B and On have sporadicmoderate Rn potential.In the second approach, superficial formations are considered as distinct stages,namely q1-q3, ale and alm. Similar calculations are performed. The results are shownas hatched bars in fig. 2. Comparing the results of the two approaches in fig. 2b, 2c and2d, it can be seen that houses constructed on a given stage without superficial depositshave usually higher Rn concentrations than those on the same stage with superficialsediments. So this proves again that superficial deposits can significantly decreaseindoor Rn concentrations.Among the three types of superficial formations (q1-3, ale and alm), alluvions (alm)have the strongest effect on indoor Rn decreases. Sediments of q1-3 also effectivelydecrease indoor Rn concentrations, but the decrease is not significant for the houseswhere alluviums (ale) are present.4.2. Lithology and indoor radon. – Regional lithology is also an important factor inthe control of indoor Rn concentrations. Similar methods used for geologicalformations are also applied to lithology. Figure 3 shows the results of the twoapproaches. It is noted that rock types of quartz-phyllite and grauwacke havedominantly the highest percentages of homes over the given Rn levels (fig. 3b, 3c and3d). So these rock types are thought to be related to high radon potential. Quartzite,psammite and chalk are also very important rock types in the control of indoor radon,while phyllite, mica-sandstone, sandstone, limestone, calc-shale and conglomerate arerelatively important. These rocks can have high to moderate radon potential. Sand,clay, marl and others are likely to have low radon potential.Comparing the results of the two approaches in fig. 3b, 3c and 3d, a significantradon decrease related to superficial formations is also observed, except on sand.H.-C. ZHU, J. M. CHARLET and F. TONDEUR3585. – ConclusionsThe statistical analysis shows that the stages from G1 to E1 have the highest radonpotential, while the stages Dv, Rv, and Co to H1 have moderate to high radon potential,of which Rv, Co, Gv, Fa1 and V1 are likely to have high radon potential. The stagesfrom the Westphalian (H2) to the Oligocene (On) have low or very low indoor Rnpotential. The results for lithology show that quartz-phyllite, grauwacke, quartzite,psammite and chalk are very important for radon accumulation, whereas calc-shale,phyllite, sandstone and limestone are important, but to a lesser degree.R E F E R E N C E SCHARLET J. M. and ZHU H.-C., Radon in the underground waters, relation with geological risksand environmental pollution, the case of Belgium (northwest Europe), in Rare GasGeochemistry, Applications in Earth & Environmental Sciences, edited by H. S. VIRK (GuruNanak Dev University, Amritsar, India) 1997, pp. 270-282.POFFIJN A., UYTTENHOVE J., DROUGUET B. and TONDEUR F., The radon problem in schools andpublic buildings in Belgium, Rad. Prot. Dos., 45 (Suppl.) (1992) 499-501.TONDEUR F., GERARDY I., LICOUR C. and DUBOIS N., Répartition géographique et géologique durisque “radon” en Belgique francophone, Ann. Associat. belge Radioprotect., 19 (1994) 205-226.TONDEUR F. and GERARDY I., Répartition géographique et géologique du risque “radon” enBelgique Francophone, in Gas Geochemistry, edited by C. DUBOIS (Science Reviews,Northwood) 1995, pp. 467-479.VANMARCKE H., POFFIJN A., RAES F., EGGERMONT G., UYTTENHOVE J., BERKVENS P.,VANDINGENEN R., BOURGOIGNIE R. and JACOBS R., Radon in het leefmilieu, Ann. BelgianAssociat. Radiat. Protect., 13 (1988) 33-56.ZHU H.-C. and CHARLET J. M., Geostatistical analysis of radon data in groundwaters in WalloonRegion, Ann. Associat. belge Radioprotect., 19 (1994) 161-174.ZHU H.-C., CHARLET J. M., DOREMUS P., FLEMAL J. M. and HALLEZ S., Some geological factorsrelative to concentration of radon in underground water in Walloon region, in GasGeochemistry, edited by C. DUBOIS (Science Reviews, Northwood, UK) 1995, pp. 119-134.ZHU H.-C., CHARLET J. M. and TONDEUR F., Mapping indoor radon using geostatisticalapproach, in I. BARNET and M. NEZNAL (Editors), Proceedings of the Radon Investigationsin the Czech Republic VI and 3rd International Workshop on Geological Aspects of RadonRisk Mapping (Czech Geological Survey, Prague) 1996, pp. 51-61.

Indoor radon levels in relation to geology in southern Belgium

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Indoor radon levels in relation to geology in southern Belgium

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