Radon production and migration in karstic environment: experimental data and numerical modelling (Fourbanne site, French Jura Mountains) Mansouri N. 1, Gréau C. 1, Ielsch G. 1, Saâdi Z. 2, Bertrand C. 3 1. Institut de Radioprotection et de Sûreté Nucléaire, PSE-ENV/SEREN/BERAD, BP17, 92262 Fontenay aux Roses Cedex, France 2. Institut de Radioprotection et de Sûreté Nucléaire, PSE-ENV/SEDRE/UEMIS, BP17, 92262 Fontenay aux Roses Cedex, France 3. Laboratoire Chrono-Environnement, UMR 6249 CNRS-UBFC, La Bouloie UFR Sciences et Techniques, 16 Route de Gray, 25030 Besançon Cedex, France GARRM 2018, 18-21 September 2018, Prague, Czech Republic
Contents Context Objectives Experimental study and data analysis Numerical modelling Conclusions Perspectives 2/18
Context Mapping of geogenic radon potential for France (IRSN, 2010): to determine the ability of geological formation to produce and transport radon towards surface Higher radon potential are generally observed in areas characterized by Hercynian granitic or metamorphic rocks, Permian sandstones/pelites, coal-rich sedimentary rocks, acid volcanites (trachytes ) for examples, and/or by major faults and mines. Karsts limestone plateau Karsts plain and chalky plateau Karsts folded Jura Area without karst outcrop Karst formation outcrop (BAILLY-COMTE, 2008) Jura Mountains, at the border with Switzerland High indoor radon levels (>300Bq/m 3 ) are observed in one karst region (Franche-Comté) Karstic areas are represented with low geogenic radon potential (limestone) 3/18
Objectives To evaluate the potential influence of karstic area on the radon levels in soils To evaluate the impact of karst on the geogenic radon potential Diagram of karst features : Disappearing surface stream Sinkhole Cave entrance Fractured rock Fractures Underground river Shale Limestone (From S. Higgins, 2008) Cave Strong heterogeneity of karsts typology over French territory (fractures, cave,..) Complexity of the geological structures of karst environments. Air exchange between the cave and the outside atmosphere 4/18
Experimental study and data analysis Jura Mountains, at the border with Switzerland In situ measurements in the cave In situ measurements in the soil In situ measurements across faults 5/20 5/18
Measurements in the cave Explored karstic network Continuous hourly measurement over one year (Rn-222, pressure, temperature) with BMC2 Ra-226 in rocks and sediments Hydrological and hydro-chemical measurements: water turbidity, water conductivity, water height,.. En Versenne cave 226 Ra (limestone) = 10-20 Bq/kg 226 Ra (sand) = 60-120 Bq/kg No significant spatial variations of Rn signal in the cave 6/20 6/18
Av Rn (Bq/m3) Température ( C) Rn concentrations in the cave Important seasonal fluctuation : higher Rn activity concentrations in summer Rn EV-1 Rn EV-2 Temp. ext. Temp. cavité 20000 15000 10000 5000 25,00 15,00 5,00-5,00-15,00 0-25,00 28/10 12/12 26/1 11/3 25/4 10/6 25/7 8/9 23/10 7/12 Radon content increase when temperature outdoor > temperature indoor Less natural ventilation in summer Several Rn anomalies in cave are correlated with pressure drops and heavy rains period 7/20 7/18
Measurements in soil air Location of probes FO-09 FO-13 226 Ra = 74 Bq/kg 222 Rn avg = 71 546 Bq/m 3 Hourly measurement of 222 Rn level in soil-air during 1 year (depth 1 m) 226 Ra = 83 Bq/kg 222 Rn avg = 72 906 Bq/m 3 226 Ra = 79 Bq/kg 222 Rn avg = 50 081 Bq/m 3 FO-07 226 Ra = 96 Bq/kg 222 Rn avg = 98 891 Bq/m 3 FO-14 FO-15 226 Ra = 54 Bq/kg 222 Rn avg = 66 875 Bq/m 3 Very significant fluctuations of radon concentrations The radon concentrations can be very important 8/18
Flux d'exhalation de Rn-222 (mbq/m2/s) Rn-222 dans in soil l'air air du sol (Bq/m3) Flux d'exhalation de Rn-222 (mbq/m2/s) Rn concentrations in soil air 00 50 00 50 00 50 0 Comparison of measurement results with other data previously acquired by IRSN in France salte Calcaire Granite Grès Orthogneiss Schistes Secteur Fourbanne de Fourbanne area Radon levels consistent 300 with measured soil radium-226 content Soil radium-226 contents 150 similar to those observed in granitic soils Basalte Calcaire Limestone Granite Grès Basalte Calcaire Granite Sandstone Grès Orthogneiss Schistes Sect 400 300 250 200 350 250 300 B 200 250 C 200 150 G G 150 100 O S 100 100 50 S 50 0 Existence of 50a relative 50 enrichment 100 150 0 in radium in the Ra-226 soil du of sol this (Bq/kg) 0 karstic 0 50 0 0 50 100 150 area 0 50 Rn-226 100Ra-226 in du soil sol (Bq/kg) Ra-226 150 du sol (Bq Ra-226 du sol (Bq/kg) 9/18
Rn in soil-air (kbq/m3) Elevation (m) Rn in soil-air (kbq/m3) Elevation (m) Measurements across faults o o Single-time measurement of 222 Rn level in soil-air (depth 50 cm) 226 Ra of soil measured in different part of the profiles 226-Ra (Bq/kg) 160 140 120 100 80 60 40 20 66 79 flat area 360 358 356 354 352 350 348 346 344 342 7 Rn measurements made in a flat area Total length of profile = 100 m Space between measurements = 15 m Larger Rn fluctuations Radon content in soil air increases near the fault no topographic effect 0 340 226-Ra (Bq/kg) 140 120 100 40 57 67 sinkhole 360 355 7 Rn measurements made in Fontenotte sinkhole (FO-13) Total length of profile = 70 m Space between measurements = 12 m 80 60 40 20 0 350 345 340 Significant Rn fluctuations over a short distance Radon content increases in soil air near the fault corresponding to the bottom of sinkhole 10/18
Numerical modelling Numerical tool: T2Rn Mathematical model Periods of interest Results Atmosphere Soil? Rock Cave 11/20 11/18
Numerical tool: T2Rn T2Rn: Rn flow and transport in soil and rock with matrix-fracture interaction (IRSN, Saadi et al. 2016) TOUGH2: transport of unsaturated groundwater and heat ( Lawrence Berkeley National Laboratory (LBNL), Pruess et al. 1999) EOS7R: module for radionuclide transport Multidimensional Two-phase Three-components Non-isothermal Flow and transport Fractured porous media 1D, 2D, 3D liquid, gas water, radon, air Heat transfer Darcy and Fick laws dual-, dual-k, MINC, ECM Radon physical properties: Diffusion; Dissolution; Adsorption; Emanation 12/20 12/18
Mathematical model Initial condition: position of water table Physical properties: porosity + soil permeability Geometrical characteristics of the domain: Layer thichness, Width, Boundary conditions: Rn concentration in the cave, Cave air Pressure, Atmospheric air pressure, Rainfal ATMOSPHERE Rn 20 Bq/m 3 SOIL Rn : 10 000-150 000 Bq/m 3 ROCK CAVE Rn : 500-15 000 Bq/m 3 Soil radon source and transport properties: Emanation coef., Diffusion coef., Henry low coef., Radium-226 activity mass content Hydrodynamic properties: Capillary pressure, relative permeability, What are the factors influencing radon levels in the soil? Is there a transfer between the cave and the ground? What is the impact of sinkholes and fractures? 13/18
Periods of interest Analysis of variations in the soil (FO.09) Selection of periods of interest : April May 2016 Location of probes FO-09 FO-13 FO-07 FO-14 FO-15 Radon peaks in soil also correlated with pressure drops and rain events Effect of pressure or rainfall? 14/18
1st model configuration Quantification of soil impact Temporal variations: air pressure, rainfall, evapotranspiration Doline Influence of surface temporal variations Not taking into account the physical parameters of the underlying rock With and without rainfall 2 m of soil : Ra = 75-91 Bq/kg Average levels of radon result from radium-226 emanation from the soil The sole consideration of the soil does not explain the fluctuations observed 15/18
2nd model configuration Quantification of the impact of the cave Influence of surface temporal variations Influence of the depression created by the cave 2 m of soil : Ra = 75-91 Bq/kg 16 m of rock : Ra = 17-19 Bq/kg 0.5 m of transition zone 16 m of cave Amplitudes of Rn fluctuations related to soil pressure gradients caused by pressure fluctuations in the cave Minimal cave impact on Rn levels: confirms that average radon levels result from soil radium 16/18
Conclusions This study confirmed that karstic environments could be the source of locally high radon contents in the soil Observation of a strong radon variability both spatial and temporal Lessons from the data analysis: Measured radon-222 levels in soil air are consistent with soil radium-226 levels In the cave, the seasonal variations are related to the difference in temperature with the surface which causes a flow of air and a dilution of the activities of volume of radon in winter Lessons from numerical modelling: The average levels of radon activity in the soil are essentially the result of radium- 226 emanation from the soil. Fluctuations in radon volume activity observed in the soil appear to be related to the existence of pressure gradients from the cave to the ground generated by pressure variations in the cave. 17/20 17/18
Perspectives: new data Conduct a measurement campaign in the habitat Distribution of radon measurement kits in the study area and at the Swiss border Complete the knowledge on the relationships between the radon activities in soil air and the radium-226 levels of soils in calcareous areas Measurements in other karstic areas (Franche Comté, Causse, Provence...) + measurements in non karstic areas Refine the results obtained by numerical modelling Geophysical measurements: to know the structure of a sinkhole Measurement of effective radium-226 concentrations (ECRa) of soil samples Measurements of the physical properties of limestone (porosity and permeability) Make the model of radon transport in the study area over a second period 18/18