INDOOR RADON ON LOESS DEPOSITS IN WALLONIA

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1 INDOOR RADON ON LOESS DEPOSITS IN WALLONIA François TONDEUR, Isabelle GERARDY, Nathalie GERARDY Institut supérieur industriel de Bruxelles, 150 rue Royale, B1000 BRUSSELS, BELGIUM ABSTRACT Pleistocene loess deposits and other low-permeability grounds have often been considered as radon-safe because they are supposed to form an efficient barrier against radon migration. Moderate but significant indoor radon problems being found on loess deposits in Wallonia, we suggest that soil gas radon could come into houses through a permeable dry loess column under the house. 1. INTRODUCTION The determination of the affected areas where significant indoor concentrations of radon are found, due to the infiltration of radon from the subsoil, is essential in developing a policy regarding the radon risk. The ICRP (1) proposes that areas where 1% or more of the buildings show an indoor radon concentration higher than ten times the national average should be considered as radon-prone areas, whereas the NRPB follows a somewhat different approach (2), considering that an area is radon-affected if 1% or more of the buildings exceed the national action level. The usual method for defining these areas is thus to organise a systematic screening campaign and to evaluate the percentage of affected buildings in each area. Although it is possible to take geological data into account when defining the limits of the various zones, it is difficult by this method to establish the hyperfine structure of radon risk mapping, related to the local structure and composition of the rocks (inhomogeneities, fractures,...) or to the movement of underground waters. Methods have thus been proposed to evaluate the radon potential at the local scale. They rely mostly on measurements of the relevant properties of the ground: radium and radon concentrations, emanation factor, permeability, porosity,.grain size distribution. The Czech Republic, for example, has established a classification of building sites based on soil radon concentration and on the fine grain fraction of the soil (3). Rybach et al. propose the use (4) of a radon availability index which is the product of the radon concentration in soil gas by the permeability. Soil radon concentration and soil permeability should be preferentially measured in situ. This approach is however difficult to apply on low-permeability formations like clay or loess. In the temperate oceanic climate of Belgium, these formations remain usually close to water saturation at 1 meter depth, even during the summer. Thus the permeability is extremely low and difficult to measure in situ. Often it is also impossible to extract enough soil air to perform radon measurements, and to make meaningful in situ passive radon measurements (5).

2 It has sometimes be argued that these low-permeability formations should completely block the migration of radon in the soil, thus avoiding any significant radon risk(6). However, Scott has pointed out significant radon problems on clay soils of US prairie states (7). We also have shown (8) that moderate but significant indoor radon problems are sometimes found in houses built on loess deposits. One hypothesis that could explain these problems is that a column of dry loess is established under the house. As dry loess normally retracts, macroscopic cracks are expected to appear, which would enhance its permeability by several orders of magnitude. Instead of the standard air flow pattern through the soil that is expected to develop on permeable grounds (Figure 1.a), and that passes only to a small depth (<2-3 meter) under the walls, air could be obliged by the wet impermeable loess cover to come from the underlying more permeable formation, possibly through several meters of dry, permeable loess (Figure 1.b). A similar hypothesis has been discussed by Scott (7). Figure 1 : In the standard case (a), the pressure gradient induced either by wind or by the stack effect creates an air flow from the atmosphere into the house through the soil. The hypothesis, in the case of loess (b), is that the stack effect produces an air flow through the dry loess column from the underlying formation, the direct flow from the atmosphere being completely blocked by wet loess around the building. If this hypothesis is valid, the radon source could be either loess itself, or the underlying formation. In both cases, the strength of the air flow should be governed mostly by the permeability of the underlying formation. Our laboratory has started a project that aims at the understanding of the indoor radon problems in houses built on loess deposits in Wallonia, the frenchspeaking part of Belgium. The present report reviews a few data already available on indoor radon concentrations. More results will be presented at the workshop, according to the progress of the project.

3 2. LOESS DEPOSITS IN BELGIUM As shown on the map (Figure 2), pleistocene loess is deposited along an eastwest band across the median part of Belgium. The frontier between Flanders in the north and Wallonia in the south divides the loess area in two approximately equal parts. Our study is limited to the Walloon part of this area, i.e. the southern part. This part will again be divided into three geographic zones according to the limits of the provinces: the province of Hainaut in the west, the provinces of Namur and Walloon Brabant in the middle, and the province of Liège in the east. Figure 2: Limits (thick curves) of the loess deposits in Belgium. The hatched areas are the three zones considered in the present study. Loess is not deposited continuously in this area, being typically found with a thickness up to 18 meter on the many low-altitude plateaus between the river valleys. Because this area is a densely populated one,, it may be of great importance to evaluate accurately the radon problems on loess. 3. INDOOR RADON MEASUREMENTS The laboratory of ISIB is involved for several years in the measurement of indoor radon in the walloon region and in the region of Brussels. Most of our measurements are short-term measurements (3 to 4 days) with the charcoal canister method (9). We usually measure radon in a ground-floor room, in semi-confined conditions (no opening of the windows). A total of approximately 2000 buildings have been examined. For each of them, the geographical coordinates has been determined (usually with 50 m accuracy). The underlying geological stage has been obtained from the belgian geological map. This 1/40000 map is unfortunately one century old, and is not always very accurate.

4 In particular, the limits of loess deposits are not drawn on this map. Consequently, there are many cases where the presence or the absence of loess cannot be deduced from the map. Complementary information has sometimes been obtained from the owner, or from in situ sampling in a few cases, but the conclusion remains uncertain in several cases. These uncertain cases will not be considered, and we only keep here 125 ground floor measurements in houses almost certainly built on loess deposits in the area defined previously. A preliminary analysis performed in 1993 (10) with the incomplete database available at that time has shown that the geometrical mean (GM) of indoor radon concentrations measured on loess is somewhat higher than the value obtained for other cenozoic grounds (loess: GM=57 Bq/m 3, other cenozoic: 42 Bq/m 3 ). The geometrical standard deviation (GSD) is also somewhat higher on loess. Moreover, a significant number of moderately elevated indoor radon concentrations is observed on loess, situated in the range Bq/m 3 for ground floor measurements, and up to 1200 Bq/m 3 for basement measurements. It is not yet clear whether loess deposits should be considered as an affected area or not, in the usual sense of an area where more than 1% of the buildings bypass the reference level, provisionally taken as 400 Bq/m 3 in Belgium. Therefore it is important to improve our knowledge of the mechanisms by which loess may become the source of a moderate but significant radon risk. Basically, two hypotheses can be tested : either loess itself can be a significant source of radon, in which case the underlying formation could influence indoor radon by its ability to provide an air flow, i.e. its permeability, or radon comes from the underlying formation with a relatively high concentration because it comes from a depth that is larger than in the usual cases. 4. REGIONAL VARIABILITY OF INDOOR RADON ON LOESS Table 1 shows a finer analysis of our data: the GM and GSD are shown, together with the number of cases, for 8 subgroups of houses build on loess, according to the provincial zones and to the geological formation present under loess. Table 1 : Geometrical mean in Bq/m 3, number of cases and geometrical standard deviation (in the format GM ( N ) GSD) for ground floor indoor radon measurements in houses built on loess deposits in Wallonia, according to the province ( East = Liège, Centre = Namur + walloon Brabant, West = Hainaut) and to the era to which belongs the geological formation found under the loess deposit. West Centre East Global Cenozoic 36 (14) (21) (16) (51) 2.6 Mesozoic - 56 (2) (25) (27) 1.9 Paleozoic 34 (10) (12) (25) (47) 2.7 Global 35 (24) (35) (66) (125) 2.5

5 Table 1 shows a significantly higher global GM in the east zone than in the west zone (t=3.6), the central zone giving an intermediate value. This result is also observed for all underlying formation groups. Table 1 also suggests a smaller GM when loess is deposited on the paleozoic basement. This result is only weakly significant because the global GM given for the mesosoic underlying formations is biased by the fact that these formations are nearly absent except in the east zone, where the GM s are higher. We note however that paleozoic formations give the lowest GM in all three regions. We note also that, when loess is present, indoor radon problems are amplified if the underlying stage belongs to the mesozoic or cenozoic era, whereas indoor radon seems to be inhibited by loess on paleozoic stages, as shown in table 2. Table 2 : geometrical mean indoor radon concentration (Bq/m 3 ) without (9) or with loess cover according to the age of the underlying formation. no loess cover with loess cover Cenozoic Mesozoic Paleozoic The results of table 2 strongly suggest that the source of radon is loess itself. Indeed, if the GM enhancement on Mesozoic or Cenozoic formations was due to the fact that soil air is extracted at a larger depth when loess is present, the same enhancement would be expected for paleozoic formations, instead of the observed reduction. Presumably, the lower GM observed on paleozoic grounds with loess cover should be explained by their lower permeability, but this still has to be confirmed. The hypothesis that radon is produced in loess is reinforced by measurements of radon concentrations in soil gas that have been made in loess by Kies (11) in Luxemburg, thus not far from the region we consider. Values in the range kbq/m 3 have been found. Were such values also found in Wallonia, they would be high enough to produce indoor radon problems, if the pathway allowing for radon migration exists. Our database also shows a significantly higher GM indoor radon concentration in the house where no basement or crawl space is present under the room where the measurement has been made (Table 3). Table 3 : geometrical mean indoor radon concentration (Bq/m 3 ) according to the existence of a basement or crawl space under the measurement room. West Centre East Global with basement or c.sp. 23 (10) 39 (16) 62 (43) 48 (69) no basement or c.sp. 47 (10) 84 (10) 98 (16) 77 (36)

6 5. PRESENT AND FUTURE OF THE PROJECT If the hypothesis that loess itself is the source of radon is confirmed, the variability of the GM indoor radon concentration in Wallonia should be associated with variations of the properties of loess from one province to the other: average thickness of the loess cover, radium concentration, soil radon concentration, radon emanation factor, porosity, grain size distribution,... The study of these properties is still going on. We expect to be able to present more data at the workshop. We have also initiated a collaboration with the University Centre of Luxemburg, to follow the evolution of loess on a ground that will be gradually dried on a 100 m 2 surface, simulating the effect of a house. The follow-up will include initial measurements of density, moisture, porosity, grain size distribution, radium content, emanation factor, permeability, radon in soil gas (if possible), and the periodic measurement of moisture, permeability and radon concentration during several years. ACKNOWLEDGEMENTS We wish to thank Professor Kies (C.U., Luxemburg) for useful comments and suggestions. REFERENCES 1. ICRP, Publication 65, Pergamon Press, J. Miles, Proc. 17th IRPA regional congress on radiological protection, Nuclear Technology Publishing, 1994, p285 3.I. Barnet, Radon Investigations in the Czech Republic V, Czech Geological Survey, Prague, 1994, p18 4. L. Rybach, F. Medici, H. Sturbeck, Radon et gaz rares dans les sciences de la terre et de l environnement (CIGG1), Service géologique de Belgique, Mémoire 32, 1990, p A.B. Tanner, Nucl. Geophys. 5 (1991) J.M. Charlet, Ann.Ass.Bel.Radiop. 19 (1994) A.G.Scott, Health Phys. 62 (1992) F.Tondeur, H. Zhu, J.M.Charlet, I. Gerardy, R. Perreaux, in The natural radiation Environment, Sci. Tot. Env., to be published (1996) 9. B.L. Cohen, R. Nason, Health Phys. 50 (1986) F. Tondeur, I. Gerardy, see ref. 2, p A. Kies, private communication (1996)