Simultaneous monitoring of soil radon and moisture at different depths in a Fukushima forest site, Japan

Transcription

1 The 22 nd International Symposium on Environmental Biogeochemistry Simultaneous monitoring of soil radon and moisture at different depths in a Fukushima forest site, Japan Misato Ohno*, Ryoko Fujiyoshi, Kazumasa Okamoto, and Kikuo Umegaki, Graduate School of Engineering, Hokkaido University, Sapporo, JAPAN *Pacman@eng.hokudai.ac.jp

2 1. Introduction: Radon in soil Exhalation Diffusion& Advection WHO handbook on indoor radon In Geophysical exploration, Soil & atmospheric sciences Radon Radioactive Water soluble Chemically inert Radon Isotopes 222 Rn 219 Rn 22 Rn 222 Rn Series 23 U 232 Th 238 U Emanation Half-life (T 1/2 ) 3.9 s s 3.82 d 238 U 226 Ra α-decay energy (MeV)

3 2. Purposes I. to elucidate possible relationships of soil radon and moisture with several parameters including soil properties and meteorological ones, II. to evaluate dynamic properties of soil radon and also moisture quantitatively, III.to apply radon as a tracer for soil gas and water transportation. And further, to evaluate the fate of fallout nuclides in Fukushima forest site in our country by using soil radon as a tracer of moving soil gas and water in the future. 2

4 Temperature ( ) Precipitation, Snow (mm h -1 ) 3. Location Fukushima No.1 NPP Location: Fukushima University, Japan. (37.68N, 14.4E) Stand: Temperate deciduous forest (Oaks, Pine, Chesnut etc.) Mean Annual Temperature: 13 Precipitation: 12 mm Snow fall: 19 cm (Japan Meteorological Agency) Date Time series plots of Temperature, Precipitation and Snowfall in Fukushima. (May 214 Jun 21) 1 1 3

5 Soil Depth (m) 4. Monitoring Procedures Since August 213~ Radon Activity concentration Temperature Barometric pressure (VDG, Algade, France) Since May 214~ Soil moisture (Soil moisture sensor, North One Co. Ltd., Japan) Φ 9 mm Schematic diagram of setting probes (Left: Rn, Right: Moisture) 4

6 . Outline of the previous studies: Soil Properties Depth profile of soil properties ( 226 Ra content, ph, Water content, and porosity: Aug. 21, 213 Homogeneous soil Depth (m) ²²⁶Ra (Bq kg ¹), ph, Water Content, Porosity (%) 2 4 ²²⁶Ra(Bqkg ¹) ph Water Content (%) Porosity (%) 222 Rn (kbq m ³) 222 Rn (kbq m ³) dc dz = 1 (kbq m 4 ) Depth (m) dc dz = 4 (kbq m 4 ) Depth (m) Depth distribution profiles of 222 Rn in soil in 213; Upper (Jun.-Nov.), Lower (Dec.-Feb.)

7 Atmospheric Pressure (hpa) 222 Rn (kbq m -3 ).Outline of the previous studies: Transport of 222 Rn in soil The effect of Typhoon on 222 Rn 97 Sep. 1 Sep. 1 Sep. 2 Sep. 2 Date (213) Effective Diffusion Coefficient of Rn (D e ) D e = 1 J dc dz [m2 s 1 ] cm 6 cm 3 cm (hpa) Typhoon No.18 Sep : 96 (hpa), 22 (mm h -1 ) Rn flux (J) J = 2~ [Bq m 2 s 1 ] D e = 4. 7~ [m 2 s 1 ] C: Rn Activity concentration (Bq m -3 ), z: Depth of the soil (m) 6

8 Moisture (%) Moisture (%) Moisture (%) 6. Results: Time series plots of Rn and moisture (May 28 ~ Nov. 1 in 214) 222 Rn (kbq m -3 ) 222 Rn (kbq m -3 ) 222 Rn (kbq m -3 ) 3 Depth.3 m 2 1 May. 28 Jun. 17 Jul. 7 Jul. 27 Aug. 16 Sep. Sep. 2 Oct. 1 3 Depth.6 m 2 1 May. 28 Jun. 17 Jul. 7 Jul. 27 Aug. 16 Sep. Sep. 2 Oct. 1 3 Depth 1 m 2 1 May. 28 Jun. 17 Jul. 7 Jul. 27 Aug. 16 Sep. Sep. 2 Oct. 1 Summer 1, 2, 3 Fall 1, Rn & water dynamics events in soil Rainfall intensity.9 ( >mm h -1 ) Summer 1 (Jun 28 ~ Jul 3) 2 (Jul 9 ~ 14) 3 (Jul 17 ~ 24) Fall 1 (Oct ~ 13) 2 (Oct 13 ~ 21) 7

9 Precipitation (mm h -1 ) Precipitation (mm h -1 ) Moisture (%) 6. Results: Water Infiltration on soil radon Rainfall.3 q z Elapsed time (h) Elapsed time (h) Rn (kbq m -3 ) Water flow (q z ) q z = (m s 1 ) from Gravitational drainage Depth ( ) Changing moisture (Upper) and 222 Rn (Lower) in a rainfall event 8

10 Precipitaiton (mm h ¹) 6. Results: Dissolution rate of 222 Rn in water Saturated Decreasing Rn level after saturation (Summer 1) Rn (kbq m -3 ) Elapsed time (h) Precipitation, Depth 1. m Depth (.6 m.3 m ) A t = A exp( λ + S t) Rn A(t): Rn Concentration (Bq m -3 ) λ: Decay constant (s -1 ) S: Dissolution rate (s -1 ) 9

11 Precipitaiton (mm h ¹) Rn-222 Bq/m 3 ) Rate of increase (Bq m -3 h ¹) Dissolution rate (h ¹) 6. Results: Temporally rise and Dissolution rate of Soil 222 Rn Rate of increase in soil radon Dissolution rate of radon in water m.6 m.3 m Summer Event Summer Fall Summer Details of precipitation in each events Total Precipitation(mm) Summer Fall Duration (h) Intensity (mean, mm h -1 ) Event Fall

12 6. Results: Effect of decreasing moisture on soil radon after rain Soil moisture (%) Rn (kbq m ³) Precipitation (mm h ¹) Precipitation (mm h ¹) Exfiltration q z Elapsed time (h) Water flow q z : Diffusion - controlled (Evaporation & Capillary rise) Depth ( ) Changing moisture (Upper) and 222 Rn (Lower) after a rainfall event 11

13 Dh (θ) (m² s ¹) 6. Results: Comparing moisture diffusivity with soil Rn transport in soil 1.E- 1.E-6 1.E-7 Moisture diffusivity Soil Moisture θ (%) Summer 1 Summer 2 Summer 3 Fall 1 Fall 2 q z = K h θ D h θ dθ dz q z : Flow rate K h : Hydraulic conductivity D h : Moisture (Hydraulic) Diffusivity dθ/dz: Moisture gradient D h θ =. 1 7 ~ [m 2 s 1 ] Effective Diffusion Coefficient of Rn D e = 4. 7~ [m 2 s 1 ] Dynamics of Rn and Moisture in soil Diffusion-controlled Time lag between Rn and Moisture 12

14 7. Conclusion I. Soil radon and moisture in soil show an opposite correlation at precipitation II. There is some time lag between soil 222 Rn and moisture at the beginning and at the end of precipitation. III.Same order of magnitude was obtained for the diffusivity of both radon and water in soil. 13

15 Acknowledgements: We could appreciate Prof Akira Watanabe (Fukushima University Japan) for helping field works in our Fukushima forest site, and also Dr. Claude Bertrand (Algade, France) for giving valuable advices for Rn measurements. 14