Natural radiation
in the geological anomaly of
Poços de Caldas Plateau,
Minas Gerais, Brazil
P. Bossew (1), N.C. Da Silva (2), R.J. Oliveira (2,3)
(1) German Federal Office for Radiation Protection, Berlin, Germany
(2) Laboratório de Poços de Caldas (LAPOC), Brazilian National Commission
for Nuclear Energy (CNEN), Poços de Caldas, MG, Brasil;
(3) Universidade Federal de Alfenas (UNIFAL), campus Poços de Caldas
Rationale & background
• Poços de Caldas Plateau – a geological
anomaly
• high natural radiation known for long
• Scattered indoor Rn data since ca. 20
years
• 2004-2013 large project:
“Projecto Planalto Poços de Caldas”:
assessment of ambient dose rate,
indoor Rn, cancer incidence.
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Content
•
•
•
•
Location
Geology
Ambient dose rate
Uranium mine and Morro
de Ferro
• Indoor radon
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Location
Brazil
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Morphology
Google Earth
Lakes
Urban area
forest
field
Zaine 2011, p.56
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View of the city
Poços de
Caldas City,
ca. 150,000
inhabitants
view from the
caldera rim,
North of the
city
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Geology, 1
most important:
QAPF diagram
for plutonite
Tinguaite, Foaite:
nepheline syenite,
Tinguaite = volcanic
Foiaite = plutonic
Further rocks: lavas (fonolite),
pyroclastic rock (tuff), sediments,
arenite (erosion of more ancient
rock); Lujaurite = type of foiaite,
mineral enriched (RE, Th, U, etc)
Origin: Cretaceous subvolcanic intrusion into the
Proterozoic crystalline basement. Collaps  caldera;
inside: several volcanic structures. NRE-2014
Prague2014-pb140906
Hirosaki 22-25.9.2014
Very specific for the region:
Hydrothermal alteration, forming
the so-called potassic rocks,
partly highly mineral enriched.
Origin of the particular radiation
levels.
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Geology, 2
Hydrothermal alteration
Leads to the “potassic rocks”
• Iso-volumetric substitution of minerals by the action of mineral-enriched
thermal waters, connected to the volcanic activity
• Texture of the original rock preserved
• Mainly affected: Tinguaite, Phonolite; somewhat less: Foiaite
• Irregularly distributed over the entire planalto (= the region encircled by
the caldera);
But also continuous occurrences, in particular along the circular structures
(ancient volcanoes?) inside the caldera
• Radioactive anomalies mostly connected to hydrothermal alteration
- 2 additional processes: fracturation, weathering
- Weathering: thermal stress, chemical weathering (oxidation, dissolution,
hydrolysis, hydration)
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Geology, 3
Tinguaite
from the
uranium mine
Reduced side:
U content
Redox front
Fonolite
so-called
potassic
rocks,
formed by
hydrothermal
alteration of
foiaite and
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tinguaite
Oxidized
side:
low U
Lujaurite
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Ambient dose rate: regional pattern
Complexo
Varginha (proterozoic
basement)
carborne survey, 5L plastic scintillometer,
1 second measurement, connected to
GPS, automatically recorded.
ca. 530,000 values
syenite bodies
Grupo Pinhal
(protezoic basement)
Planalto de Poços de Caldas
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Ambient dose rate: Poços de Caldas Plateau
map of radioactive anomalies
morro de Ferro
mina Osamu Utsumi
dose rate map generated by ordinary kriging using
geology as external deterministic predictor
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Garda 1990, p.81
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Extreme values
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Uranium mine, 1982-1995,
18
now under remediation 12 of 27
U mine: Mineralization
U mine:
effects of
hydrothermal
alteration
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U mine: potential environmental problems
seepage of
acid drainage
water from
waste rock
piles
possible
resuspension of
dry material
possible spill over from
retention basins into two
local rivers, used for
drinking water
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Morro de Ferro
Th-Rare Earth deposit on the slope of a hill;
only several 10 m thick.
100000
gamma spectrum
232Th ff. ~20 Bq/g
226Ra ff. ~0.7 Bq/g
10000
counts
1000
100
10
1
0
200
400
600
800
1000
1200
1400
1600
1800
2000
energy (keV)
origin: hydrothermal
alteration +
weathering
Studied as natural
analogue to nuclear
waste deposit, for
investigating
transport and
migration processes
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Indoor radon
30000
20000
25000
Poços de Caldas
20000
10000
Poços de Caldas
15000
Bq/m³
340
10000
320
Caldas
0
300
Aguas
da Prata
5000
Bq/m³ indoor
annual mean
ground floor
-10000
13 to 50
50 to 100
100 to 200
200 to 500
500 to 685
-20000
Andradas
-30000
280
260
0
240
220
-5000
Caldas
180
-10000
-15000
Aguas da
Prata
160
140
120
-20000
Andradas
-10000
0
10000
20000
30000
40000
200
-25000
-30000
statistics:
-10000
0
10000
20000
30000
40000
577 values annual mean, 464 in ground floor
AM±SD (all) = 104 ± 81 Bq/m³
interpolated map of standardized indoor
GM */ GSD (all) = 81 Bq/m³ */ 2.0
concentration, living and sleeping rooms in
approximately log-normal
ground floor, in contact with ground,
empir. prob(C>100 Bq/m³) = 40%
inhabitants who sleep with closed windows.
empir. prob(C>300 Bq/m³) = 3%
Ordinary block kriging, 1 km  1 km grid
seasonal maxima > 1000 Bq/m³ found
demographically adjusted:
AM=101, GM=90 Bq/m³.
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100
80
60
Indoor radon - Brazil
Natal, <40
Malanca et al. 1997
Lages Pintada:
up to several 1000
Campos et al. 2011
up to >100
Malanca & Gaidolfi 1997
Belo Horizonte:
up to 2600
low
moderate
high
very high
Santos et al. 2008, 2009a,b
• So far no national Rn survey of Brazil
• In regional studies regionally very
high values found
• Poços de Caldas study:
- important because it is radon prone
area;
- can serve as pilot study for larger
surveys.
Poços de Caldas:
up to >1000
Veiga et al. 2003
Rio de Janeiro:
up to 200
Magalhaes et al. 2003
Campinas:
up to >250
Canoba et al. 2002
Country
São Vicente / Santos:
up to 600
Austria
Czech Republic
Curitiba:
Denmark
mostly <100
Corrêa 2011
Finland
Lithuania
Macedonia
Netherlands
Norway
Slovenia
Approximate population adjusted mean
radon concentrations in ground floor rooms Spain
in different European countries compared to Switzerland
U.K.
the Poços de Caldas project.
Poços de Caldas Project
Lima Marques et al. 2006
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estimated mean in
ground floor, Bq/m³
133
258
96
260
59
120
23
135
166
118
200
89
108
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Conclusions
• Poços de Caldas is a region with elevated natural radiation
• However, the levels of ambient dose rate and indoor Rn are
not exceptionally high in residential areas
• There are a few anomalies and radiation hot spots, but not inhabited
• So far no elevated cancer incidence could be found,
compared to other Brazilian regions
• Planned: further investigations, e.g. soil Rn
• The region is scientifically interesting:
- very particular geology
- possibility to study radioecological processes
• Challenges:
- mapping is not easy because of high spatial variability
- correlation radiation ~ geology is party weak
- correlation dose rate ~ indoor Rn also weak
Space for further research!
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