Using 222 Rn/ 220 Rn versus 226 Ra/ 232 Th activity ratio and CO 2 concentration in soil gas to trace advective fluxes

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1 Using Rn/ 22 Rn versus Ra/ 232 Th activity ratio and CO 2 concentration in soil gas to trace advective fluxes Carlo Lucchetti 1, Mauro Castelluccio 1, Gabriele De Simone 1, Paola Tuccimei 1 1 Università Roma Tre, Dipartimento di Scienze, Roma, Italy

2 RESEARCH FOCUS Soil radon transport along fault systems where deep fluids uprise DISCRIMINATION OF SOIL RADON TRANSPORT Measurement of soil gas concentrations of Rn and 22 Rn at 8 cm depth; Rn/ 22 Rn activity ratio (t 1/2 very different); Evaluation of seasonal soil Rn fluctuations; Ra and 232 Th contents in soil; Rn/ 22 Rn versus Ra/ 232 Th activity ratio; Measurement of soil CO 2 concentrations at 8 cm depth (main radon carrier gas); Determination of enrichment coefficient of radon;

3 SOIL RADON CONCENTRATION Geological subsurface nature (particle size, mineralogical composition, parent elements) Soil gas permeability Meteo-climatic parameters Presence of faults, fractures or deep fluid uprise (Fick s law) DIFFUSIVE SOIL RADON TRANSPORT (Darcy s law) ADVECTIVE f = diffusive flow intensity (cm 3 cm -2 s -1 ); D = molecular diffusion coefficient (cm 2 s -1 ); dc= gas concentration change in the system (m 3 /m 3 ) along a length dz (m). Radon source in the vicinity of the measurement point v = gas velocity (cm s -1 ); k = permeability (m 2 ); Δp = pressure variation along a vertical z (m); µ = gas dynamic viscosity (kg m s -1 ); γ g = gas density (kg m -3 ). Deep Radon source Rn/ 22 Rn << Ra/ 232 Th Rn/ 22 Rn > Ra/ 232 Th

4 GEOLOGICAL SETTING Terme della Ficoncella Civitavecchia (RM) Lazio (Italy) Fiumicino (RM) Vigna Fiorita Ciampino (RM) SITE Terme della Ficoncella Fiumicino Vigna Fiorita GEOLOGICAL BEDROCK Travertines and Flysch sediments Pleistocene-Holocene sediments (Tiber Delta plain) Lahar flows and ignimbrites of the Colli Albani Complex

5 INSTRUMENTS AND ANALYTICAL METHODS SOIL GAS CONCENTRATION OF Rn AND 22 Rn SOIL CO 2 CONCENTRATION CO 2 concentration (at 8 cm depth): infrared detector (Dräger X-am 7) Ra AND 232 Th CONTENT IN SOIL Radon and thoron activity concentrations (at 8 cm depth): hollow probe (1) (Radon v.o.s. corp.) attached (2) to a drying unit (3) and to the continuous radon monitor (4) (RAD7 Durridge Co.), connected in series. γ - spectrometer with Hyper Pure Germanium detector (HPGe)

6 RESULTS FROM TERME DELLA FICONCELLA (CIVITAVECCHIA) Travertine Ra/ 232 Th = 6.14 Rn kbq/m 3 ADVECTIVE Ra/ 232 Th 6.14 DIFFUSIVE Rn/ 22 Rn activity ratio (Terme della Ficoncella) TFF1 TFF2 Ra/232Th TFF1 and TFF2 permanent station Rich soil in Ra and poor in 232 Th; TFF1: mean CO vol.%; TFF2: mean CO vol.%; 2 1 TFF1 radon transport predominantly diffusive; 22 Rn kbq/m 3 TFF2 radon transport diffusive and advective;

7 SOIL GAS MEASUREMENTS ACROSS CIVITAVECCHIA FAULT Ra/ 232 Th Flysch = Fault FLYSCH 1 TRAVERTINE 3 1 Rn/ 22 Rn 6, 5, 4, 3, 2, 1, Ra/ 232 Th CO 2 (vol.%) Profile Terme della Ficoncella, Measures distance (m) 3 1,2 Ra/ 232 Th ,2 Ra/ 232 Th Rn/ 22 Rn 1,,8,6,4, CO 2 (vol.%) Rn/ 22 Rn 1,,8,6,4,2 3,5 3 2,5 2 1,5 1,5 CO 2 (vol.%), Measures distance (m), Measures distance (m)

8 RESULTS FROM VIGNA FIORITA (CIAMPINO) CIAMPINO Colli Albani Complex Degassing of this area occurs at faults of the Ciampino carbonate high that acts as a reservoir for large quantities of gases (primarily composed of CO 2, H 2 S and radon) deriving from deep residual magmatic activity. These gases represent a high risk of indoor gas accumulation for the inhabitants of the area. Ra/ 232 Th Villa Doria Unit =.54 Rn kbq/m Rn/ 22 Rn activity ratio(vigna Fiorita) ADVECTIVE VF1 VF2 Ra/ 232 Th.54 Ra/232Th VF1 and VF2 permanent station significant differences in Rn and CO 2 concentrations and in the seasonal variability; VF1: mean CO vol.%; VF2: mean CO 2-7.1vol.%; 5 22 Rn kbq/m 3 DIFFUSIVE VF1: radon transport diffusive and advective; VF2 radon transport strictly advective.

9 SOIL GAS MEASUREMENTS ACROSS VIGNA FIORITA FAULT 1 2 (Giordano et al., 29) Ra/232Th Tavolato Unit = 1.4 PROFILE 1: 8 measures, 18 m length, 2 m equidistance PROFILE 2: 8 measures, 1 m length, 15 m equidistance 12th INTERNATIONAL WORKSHOP GARRM, September 16th 18th 214 Prague, Czech Republic

10 2 Ra/232Th Rn/22 Rn 1 1 2, 1,8 1,6 1,4 1,2 1,,8,6,4,2, Ra/232Th Tavolato Unit = 1.4 PROFILE 1: 8 measures, 18 m length, 2 m equidistance PROFILE 2: 8 measures, 1 m length, 15 m equidistance Ra/232Th 1.4 Rn/22 Rn (Giordano et al., 29) 2 2, 1,8 1,6 1,4 1,2 1,,8,6,4,2, Measures distance (m) 12th INTERNATIONAL WORKSHOP GARRM, September 16th 18th 214 Prague, Czech Republic CO2 (vol.%) Measures distance (m) CO2 (vol.%) SOIL GAS MEASUREMENTS ACROSS VIGNA FIORITA FAULT

11 SOIL GAS MEASUREMENTS PROFILES ACROSS VIGNA FIORITA FAULT Location of profiles Location of profiles Rn/ 22 Rn ratio map CO 2 concentration map AR1 Ra/232Th 1.4 AR3 AR5 AR2 Ra/232Th 1.4 AR4 AR6

12 RESULTS FROM FIUMICINO In the Fiumicino area in August 213, two boreholes at 35m depth, caused a gas blowout from a pressurized clayconfined gas pocket. Other past events of this type have also been reported in the area with gases mainly composed of CO 2 with traces of CH 4 and H 2 S. 2 1 (Ciotoli et al.,213) Soil gas measurements have been carried out on active pools in the roundabout (1) and on the adjacent area to the north (2, called circus land).

13 Rn/ 22 Rn MAP VS CO 2 CONCENTRATION IN THE ROUNDABOUT Leggend Rn/ 22 Rn Parameter Min Max Mean St. dev. Rn (Bq/m 3 ) Rn/ 22 Rn CO 2 (vol.%) Main blowout Secondary blowout Soil gas measure point with CO 2 value

14 Rn/ 22 Rn MAP VS CO 2 CONCENTRATION IN THE CIRCUS LAND Leggend Rn/ 22 Rn Main gas blowout Secondary gas blowout Soil gas measure point with CO 2 value Parameter Min Max Mean St. dev. Rn (Bq/m 3 ) Rn/ 22 Rn CO 2 (vol.%)

15 Rn (kbq/m 3 ) ADVECTIVE Rn/ 22 Rn vs. Ra/ 232 Th (roundabout) Ra/ 232 Th 1.7 DIFFUSIVE Rn (kbq/m 3 ) DIFFUSIVE Rn/22Rn Ra/232Th RADON TRANSPORT IN THIS AREA Ra/ 232 Th Backfill = 1.7 (roundabout) radon transport mainly diffusive; some measuring points have diffusive and advective mechanism; theese measuring points are located near gas blowout ADVECTIVE Rn (kbq/m 3 ) Ra/ 232 Th Rn (kbq/m 3 ) Rn/ 22 Rn vs. Ra/ 232 Th (circus land) DIFFUSIVE Rn/22Rn Ra/232Th Ra/ 232 Th Sand = 11.7 (circus land) radon transport strictly diffusive; large disequilibrium between Ra and 232 Th in the soil; in the three hotspots there is increase in the advective component where the highest soil CO 2 concentrations were recorded.

16 ASSESSMENT OF ENRICHMENT COEFFICIENT Generally Radon Emanation is the number of atoms of radon leaving the solid material divided by the amount generated from the sample. Where values higher than can be used to trace advective fluxes of deep gases (Schuman, 1993). In this study, we also obtained values greater than 1, which is actually an indication of Radon Enrichment. Enrichment Coefficient E.C. = C Rn C Ra ρ where: C Rn = soil radon activity concentration (Bq/m 3 ); C Ra = soil Ra content (Bq/kg); ρ = soil density (kg/m 3 ). ID measurements point soil Rn (Bq/m 3 ) Ra (Bq/kg) E.C. (ρ 12 kg/m 3 soil density) E.C. (ρ 14 kg/m 3 soil density) soil CO 2 (vol.%) Soil Radon transport VF1 Vigna fiorita* Diffusive-advective mixed VF2 Vigna fiorita* Strictly advective TFF1 T. Ficoncella* Diffusive-advective mixed TFF2 Ficoncella* Diffusive-advective mixed Roundabout MIN Fiu Diffusive Roundabout MAX Fiu Diffusive-advective mixed Circus (MIN) Fiu Diffusive Circus (MAX ) Fiu Diffusive *three years monitoring at permanent stations on a monthly basis

17 CONCLUDING REMARKS Faults and fractures are preferential pathways for strictly advective Rn uprise from deep sources. Advective movement is favoured by the presence of the carrier gas (CO 2 ), capable of carrying the radon from deeper to more superficial areas. For the recognition of deep sources the signal given by the 22 Rn is important, as the concentration tends to decrease significantly because of its low half-life. Rn/ 22 Rn ratio signal, together with the knowledge of the soil content of 232 Th and Ra of a given area is a stronger signal than the soil Rn concentration on its own. Evaluation of both the soil gases and its intrinsic permeability are vital for the investigation of unconformities and for risk assessment of indoor environments.

18 Thank you for your attention