influence of volcanic subsoil on indoor radon levels in ... · garrotxa. radiation measurements 44,...
TRANSCRIPT
V. Moreno1, J. Bach2, C. Baixeras1, Ll. Font1
1Unitat de Física de les Radiacions, Departament de Física. 2Unitat de Geodinàmica Externa i d’Hidrogeologia, Departament de Geologia.
Universitat Autònoma de Barcelona
Influence of volcanic subsoil on
indoor radon levels in Olot (NE Spain)
Prague, Sept 18th-20th, 2012
Contents:
2. Methodology and Instrumentation 2. Methodology and Instrumentation
4. Conclusions and Perspectives 4. Conclusions and Perspectives
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives
3. Experimental Results 3. Experimental Results
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 1
The Volcanic Region of La Garrotxa:
– Quaternary volcanic materials
over a Tertiary substratum.
– System of faults (Amer fault with
evidences of recent activity).
1: Quaternary sediments,
2: Neogene and Quaternary volcanic rocks,
3: Neogene sedimentary rocks,
4: Palaeogene sedimentary rocks,
5: Mesozoic sedimentary rocks,
6: Palaeozoic granitoids,
7: Palaeozoic metasedimentary rocks,
8: Faults,
9: Thrusts.
Simplified geological map of the north-eastern Catalonia
1 2 3 4 5 6 7 8 9
0 10 km
Olot
Vall d'enBas
St. FeliuPallarols
LesPlanes Amer
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 2
Blowholes in the Volcanic Region of la Garrotxa:
– Definition: Natural holes in the volcanic soil that blow cold air in warm days.
– Traditionally, very appreciated by inhabitants.
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 3
Previous surveys in the Volcanic Region of la Garrotxa:
Type of survey
2002
2003
2004
2005
2006
2007
Measurement
points
Type of
measurement CRn (Bq·m-3)
Indoor radon: Preliminary survey 27 Integrated 14 – 717
Indoor radon: Survey 125 Integrated 15 – 1465
Radon and Thoron: Blowholes 26 Punctual 7 – 9460
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Apr - Jul Jul - Nov Nov - Feb/Mar Feb/Mar - May/Jun
CR
n s
ea
so
n / C
Rn w
inte
r
Measurement period
Volcanic subsoil: 42 sites
Non volcanic subsoil: 11 sites
0
500
1000
1500
Apr - Jul Jul - Nov Nov - Feb/Mar Feb/Mar - May/Jun
CR
n (
Bq
m-3
)
Measurement period
Blowhole 1
Blowhole 2
Baixeras, C., Bach, J., Amgarou, K., Moreno, V., Font, Ll., 2005. Radon levels in the volcanic region of La Garrotxa. Radiation Measurements 40,
509-512.
Moreno, V., Baixeras, C., Font, Ll., Bach, J., 2008. Indoor radon levels and their dynamics in relation with the geological characteristics of La
Garrotxa. Radiation Measurements 43, 1532-1540.
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 4
Blowholes of the Volcanic Region of la Garrotxa (2007):
Geological
unit vair (m·s-1)
CRn-222
(Bq·m-3)
CRn-220
(Bq·m-3)
1 0.01 – 0.83 7 – 9460 39 – 2540
2 0.01 – 0.36 5 – 1100 8 – 1850
3 0.01 – 0.08 7 – 93 112 – 1020
Moreno, V., Bach, J., Baixeras, C., Font, Ll., 2009. Characterization of
blowholes as radon and thoron sources in the volcanic region of La
Garrotxa. Radiation Measurements 44, 929-933.
3
2
1 Located in different geological units:
1. Batet and Olot plain lava flows
2. Garrinada and Montsacopa volcanoes
3. Bosc de Tosca lava flows
26 blowholes identified:
19 outdoor
7 indoor
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 5
Blowholes in Batet and Olot
plains lava flows (Unit 1):
Natural holes in a volcanic scoria levels under a
slim and massive lava flow stratum.
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 6
Blowholes in Garrinada and
Montsacopa volcanoes (Unit 2):
Natural holes among the pyroclastic materials
that form the volcanic cones.
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 7
Blowholes in Bosc de Tosca
lava flows (Unit 3):
Artificial holes or cavities in the field walls build
over the broken and permeable surface of the
lava flow.
1. Introduction, Motivation and Objectives 1. Introduction, Motivation and Objectives 8
Objectives:
The main objective for the present work is to study in more detail radon dynamics in
this volcanic region.
– Confirm the seasonal variation obtained with punctual and integrated
measurements.
– Measure continuously radon levels in dwellings with higher values.
2. Methodology and Instrumentation 2. Methodology and Instrumentation 9
Selected dwellings:
– Located over volcanic materials (unit 1) and with high
annual averaged CRn:
• A house having a former indoor well (covered):
CRn= (1254 ± 74) Bq·m-3
• A house having a blowhole:
CRn= (429 ± 34) Bq·m-3
Measurements::
– Integrated measurements (1 – 4 months)
– Continuous measurements:
• Initially: 1 week in both dwellings
• Later (more detailed study of the blowhole): some months
Indoor
well
2. Methodology and Instrumentation 2. Methodology and Instrumentation
Passive detector:
– FzK dosimeter based on Makrofol track-etch
detector (polycarbonate).
Active detectors:
– PRASSI from Silena (Lucas Cell)
– RAD7 from Durridge Co (Semiconductor)
Electrochemically etched (KOH 6N (1:1) ethanol).
Conditions:
T = 40 ºC
Frequency: 3kHz
Electric field strength : 31 kV·cm-1
Duration: CE (4 h), ECE (1.5 h)
Semi-automatic track counting system: CCD + ImageJ
10
Calibrations:
– Active detectors (manufacturers and INTE).
– Passive detectors (HPA): Makrofol
Curve and calibration factor ( < 600 cm-2):
Makrofol = (0.66 ± 0.05) cm-2/kBq·m-3·h
Intercomparisons:
2. Methodology and Instrumentation 2. Methodology and Instrumentation
0 100 200 300 400 500 600 700 800 9000
100
200
300
400
500
600
n (
tr·c
m-2)
Rn
(kBq·m-3
·h)
Makrofol
Center
2004
2005
2006
2007
2008
2009
2010
2011
2012
Type of detector
HPA (United Kingdom) Makrofol
INTE (Spain) Passive and active
Saelices en Chico (Spain) Passive and active
BfS (Germany) Passive
11
Dwelling having an indoor well:
– Integrated radon measurements
(Makrofol detectors):
• Higher radon levels in summer.
• Same behaviour of houses with
blowholes.
3. Experimental Results 3. Experimental Results
0
200
400
600
800
1000
C R
n (
Bq·m
-3)
Time
0
500
1000
1500
2000
Apr - Jul Jul - Nov Nov - Feb/Mar Feb/Mar - May/Jun
CR
n (
Bq
m-3
)
Measurement period
12
– Continuous radon
measurements in
winter
(PRASSI monitor):
• Daily variations
Dwelling having a blowhole:
– Integrated radon measurements
(Makrofol detectors) at:
• 60 cm above the blowhole
outlet.
• Different distances to the
blowhole outlet across the
room.
3. Experimental Results 3. Experimental Results
0
200
400
600
800
1000
J F M A M J J A S O N D
CR
n (
Bq
·m-3
)
Time
2002
2004
2005
2006
2007
2008
2009
0
100
200
300
400
500
600
700
0 1 2 3 4 5 6 7 8 9
C R
n (
Bq
·m-3
)
Distance to the blowhole (m)
Summer
Winter
Mean room value
13
Dwelling having a blowhole:
– Continuous measurement of radon and thoron at the outlet of the blowhole (PRASSI
and RAD7 monitors simultaneously):
– Comparison with punctual measurements (RAD7):
3. Experimental Results 3. Experimental Results
0
2000
4000
6000
8000
10000
CR
n (
Bq·m
-3)
Time
PRASSI
RAD 7 (Rn-222)
Rn-220 73.1
CCC
7RAD222Rn
PRASSiRn
220Rn
Date T (ºC)
(±0.01)
RH (%)
(±0.01)
vair (m·s-1)
(±0.01)
222Rn (Bq·m-3) 220Rn (Bq·m-3) CRn – 220 /
01 Jun. 14.0 81.4 0.06 (71 ± 18)·10 (40 ± 32)·10 0.56
03 Aug. 13.7 95.4 0.72 2682 ± 50 (168 ± 14)·10 0.63
24 Nov. 8.9 77.3 - 7 ± 7 39 ± 22 5.57
7RAD222RnC
Airflow (CRn-222 > 200 Bq·m-3):
CRn-220 / CRn-222 = 0.6
No airflow (CRn-222 < 200 Bq·m-3):
CRn-220 / CRn-222 = 3.5
14
Dwelling having a blowhole:
– Continuous measurements of radon at the outlet of the blowhole (PRASSI monitor):
• Important daily and seasonal variations.
• What can explain this behaviour?
3. Experimental Results 3. Experimental Results
0
2000
4000
6000
8000
10000
12000
CR
n (
Bq·m
-3)
Time
2008
2009
No d
ata
No d
ata
No
da
ta
15
Dwelling having a blowhole:
– Relation with atmospheric air temperature (T out).
• Threshold temperature = T soil = 12 ºC
• If T out > T thres at least for 3 h: rapid increase of radon levels.
• Soil-outdoor temperature difference is the most influent parameter for indoor
radon levels in dwellings with blowholes.
3. Experimental Results 3. Experimental Results
-10
0
10
20
30
40
0
2000
4000
6000
8000
10000
12000
T (º
C)
CR
n (B
q·m
-3)
Time
C Rn
T out
T thres
Winter Spring Summer Autumn
16
4. Conclusions and Perspectives 4. Conclusions and Perspectives 17
Conclusions:
– Continuous measurements carried out confirm previous seasonal results and show
important daily variations.
– The behaviour of radon levels in a house having an indoor well and another having a
blowhole is opposite to that observed in houses in general and similar to underground
places (caves, tunnels, mines,..)
– The main radon entry mechanism in the studied houses is a convective flow due to the
soil-outdoor temperature differences, which drive the seasonal variations.
– The results of this volcanic region could be observed in other volcanic regions with
similar characteristics.
4. Conclusions and Perspectives 4. Conclusions and Perspectives 18
Perspectives:
– Continue exploring the origin of the radon in this region (volcanic rocks, lower
substrates with high radium content (granites), mantle degassing (and transport
through fractures in crust, as the Amer fault).
– Measure other gases that can have deep (CO2, H2S, …) or shallow origins (He, …) at
the identified blowholes.
– Measure soil radon and thoron concentrations and radon and CO2 fluxes at different
volcanic materials of the region.
– Model radon concentration dynamics in the studied indoor blowhole.
Thank you