From a very high radon concentration in a dwelling of the village of Porcheresse, one discusses on of the significance of the numerous radon indoor anomalies detected in the southern part of Belgium

Charlet, J.M.

Poffijn, A.

Zhu, H. C.

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IL NUOVO CIMENTO Maggio-Agosto 1999VOL. 22 C, N. 3-4The radon anomaly of Porcheresse (Ardennes, Belgium). A case study (*)J. M. CHARLET (1), H. C. ZHU (1) and A. POFFIJN (2)(1) Faculté Polytechnique de Mons - Belgium(2) Rijkuniversiteit te Gent - Belgium(ricevuto il 9 Giugno 1998; approvato il 18 Dicembre 1998)Summary. — From a very high radon concentration in a dwelling of the village ofPorcheresse, one discusses on of the significance of the numerous radon indooranomalies detected in the southern part of Belgium.PACS 29.40 – Radiation detectors.PACS 89.60 – Environmental studies.PACS 92.60.Sz – Air quality and air pollution.PACS 01.30.Cc – Conference proceedings.1. – IntroductionRadon concentration in dwellings is higher in the southern part (mean value: 85Bq/m3 ) than in the northern part of Belgium (mean value: 48 Bq/m3 ). This difference isobviously linked to the geological features: Paleozoic basement to the south, sands andclays of the Cenozoic covering the infra-Paleozoic basement to the north (fig. 1).In the Ardennes Massif constituted by the formations of the Lower Devonian(phyllites and quartzites), the radon concentrations in the underground waters anddwellings are the most important (see fig. 2, for example). These values appear veryhigh in a region where there is no crystalline basement or granites.In 1983 radiometric anomalies were discovered by a carborne survey (see fig. 1).From such spot anomaly a more detailed survey in the western part of the Ardenneanticlinal lead to the discovery of uraniferous mineralisations (see fig. 4, Oizy zone).These ocurrrences are linked to fractures filled with ferric oxyhydroxide. Theapplication of radon in springs and soils with in addition, ground gamma-ray surveyhave permitted the discovery of about ten U-anomalies and occurrences in an area of 3km2 (Charlet et al., 1995). They have been attributed to ferralitic paleo-alterationduring the continental post-Paleozoic evolution of the Ardennes Massif (Charlet et al.,1987). Besides a radon survey in dwellings of the area shows a great percentage ofhouses and cottages with strong anomalies (Doremus et al., 1992).(*) Paper presented at the “Fourth International Conference on Rare Gas Geochemistry”,Rome, October 8-10, 1997.G Società Italiana di Fisica 491J. M. CHARLET, H. C. ZHU and A. POFFIJN492Fig. 1. – Simplified geological map. V. Visé, D: Davendisse, M: Monceau, Ch: Chiny,1. Merocenozoic, 2. Silesian and limestone of Dinantian, 3. Middle and Upper Devonian, 4. LowerDevonian, 5. Cambrian-Ordovician and Silurian, 6. Limit of the methamorphic zones, 7. U-anomalies (after Charlet et al., 1983).Fig. 2. – Radon in spring waters, Kriged contour map (Zhu et al., 1996).THE RADON ANOMALY OF PORCHERESSE (ARDENNES, BELGIUM). ETC. 493We think that other radon anomalies in the dwellings of the Ardennes Massif are inrelation to similar geological context.2. – The anomaly of PorcheresseAs part of the national measuring campaign on radon in Belgium, a house with alevel of more than 4000 Bq/m3 in the living room was detected in the village ofPorcheresse (fig. 3). Detailed investigation performed afterwards did confirm thisextreme value. Excepting this no such high levels were found in the neighbourhood.Following this discovery, a field survey was undertaken. It has allowed to find someradiometric occurrences related to old iron mines (fig. 3, points 1, 2, 3 and 4). Thealignment of the mining works (pits, trenchs, sotckpiled and wastes...) marks out a veinof hematite under the dwellings where radon anomalies had been detected. The ironvein is hosted by the Lower Devonian formations (phyllites, quartzites) of the paleozoicbasement of the Ardenne Massif (5766). A radiometric survey of the stockpiled of ironores allowed to find samples with a high grade of uranium (5757) with lowconcentration in Th and K.Sample no. eU (ppm) Th (ppm) K20 (%)57575766113.54.309.70.052.45Note: about 6 km far from the Porcheresse iron mine an uraniferous occurrence had been discovered in 1982 by the MonsLaboratory (Dejonghe et al., 1982). Various U-minerals have been found in a fractured zone with hematite (uraninite-Uphosphates,...).Fig. 3. – Porcheresse environment.J. M. CHARLET, H. C. ZHU and A. POFFIJN494Fig. 4. – Geological context of the regional radiometric study (Ardennes massif).3. – Radiometric anomalies on a regional scale (fig. 4)On a regional scale, filling fractures have been studied in some quarries. They showan enrichment in U and sometimes in Th compared with the quartz phyllites of theGedinnian (see for example 5780).Sample no. eU (ppm) Th (ppm) K20 (%)57805782a5782bBras 13.2528.213.016.29.3515.69.79341.761.881.85—5782a: filling of fractures in the quarry of the Hatrival station (pt H see fig. 4 with ferric oxides and hydroxides and5782b: hosted rocks of the fractures with strong alteration (note the decreasing of the U and Th enrichment);Bras 1: carpeting of ferric oxide in fracture of the Bras quarry (very strong enrichment in Th).4. – Geochemical investigations of the filling of the fractures (figs. 5, 6)To understand a process of radioelement concentration in the fractured zones ageochemical study of REE (rare-earth elements) and trace elements has been performed.All the REE and trace elements were determined by ICP-MS (Inductively CoupledPlasma-Mass Spectrometry) at the Royal Museum for Central Africa (Tervuren,Belgium). They have been normalized to the Post-Archean Australian Shales(PAAS).THE RADON ANOMALY OF PORCHERESSE (ARDENNES, BELGIUM). ETC. 495Fig. 5. – Traces elements distribution.The iron ore 5757 shows a strong enrichment in U and Pb (Pb radiogenic) and adepletion in the other trace elements compared with the host rock (5766). The ferricoxide of the Bras Quarry shows an enrichment in U, Th and low REE.Both these two behaviours are typical characteristics of the evolution of a ferraliticenvironment (residual phases with Th-REE and reconcentration of U trapped onhigh-surfaced iron phases possibly a gossan).Fig. 6. – REE distribution.J. M. CHARLET, H. C. ZHU and A. POFFIJN4965. – ConclusionThe geological history of the Ardennes Massif leads to a favourable context for thedevelopment of radon concentrations in underground waters and dwellings:– A detrital sedimentation during the Paleozoic (phyllites, quartzites) withsometimes stratiform U-anomalies linked to the local black shales formations of theSiegenian (see fig. 1 Ch-Chiny).– During the hercynian movement the Paleozoic series are folded and faulted:opened fractures appeared in the competent formations (quartzites, quartz phyllites,metamorphised rocks).– During the post-Hercynian period the geomorphological and climate evolution ofthe Ardenne Massif leads to the development of ferralitic alteration. The tropical andsubtropical climates lead to supergene processes for which the intensity isdemonstrated by the great thickness of kaolin deposits or other alterites. In certainzones, these can actually reach several decameters of thickness and it may be assumedthat they have covered all the Ardenne massif.The fillings of opened fractures or microfractures by a loose material of highpermeability constituted by ferric-oxyhydroxide, quartz and clays lead to the trappingof uranium by the high specific surface of these mineral phases.So the local source of radon could be explained by– the structural evolution of the hercynian massif– the paleoclimatological evolution of post-Hercynian.* * *This research has been funded by the Health Ministery (Belgium) SPRI (Service deprotection des radiations ionisantes).R E F E R E N C E SCHARLET J. M, DEJONGHE L., DE WITTE S. M, DRUMEL B., HENRY J., HERBOSCH A., LEFIN J. Pand MARTIN H., Reconnaissance survey for uranium in the Belgian Paleozoic. Prof. Pas1983/1, 196 (1983).CHARLET J. M., DOREMUS P. and QUINIF Y., Concentration mechanisms of uranium inmineralized fractures of the lower Devonian of the Belgian Ardenne. The case of the Oizyarea, Uranium, 3 (1987) 387-405.CHARLET J. M., DOREMUS P. and QUINIF Y., Radon methods used to discover uraniummineralizations in the lower Devonian of the Ardenne Massif (Belgium), in GasGeochemistry, Science Reviews, edited by C. DUBOIS (Northwood, UK) 1995, pp. 1-18.DEJONGHE L., CHARLET J. M. and DUPUIS CH., L’anomalie radiométrique de Daverdisse et lesminéraux uranifères associès, Bull. Soc. Belge Géol., Pal., Hydrol., 105 (1982) 177-193.DOREMUS P., KOTZMANN-ROUTIER V., QUINIF Y., CHARLET J.M. and FLEMAL J. M., La pollutiondomestique par le radon. Etudes de cas: région de Mons, région de Monceau en Ardenne(Belgique), Mémoire du Service géologique, 32 (1992) 33-45.ZHU H. C, CHARLET J. M. and DOREMUS P., Kriging radon concentrations of groundwaters inwestern Ardennes, Environmetrics, 7 (1996) 513-529.

The radon anomaly of Porcheresse (Ardennes, Belgium). A case study

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The radon anomaly of Porcheresse (Ardennes, Belgium). A case study

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