Transcript Document

Radon in groundwater
Analysis of causes and development of a
prediction methodology
Skeppström K.
PhD. Student
Dept. of Land and Water Resources Engineering, KTH
Layout of presentation
Radon (focus of Rn in groundwater)
Objective of project / Phases involved
Methodology
Results & Discussions
Radon
• Radioactive
• Colourless, odourless, noble gas
• Exists as 3 main isotopes:
•222Rn (uranium decay series, 238U),Half-life ( T1/2) = 3.8 days
•220Rn (Thorium decay series, 232Th), T1/2 = 56 seconds
•219Rn (Actinium decay series, 235U), T1/2 = 4 seconds
• Cancer risk
• 500 cases of lung cancer/year in Sweden; smokers have a
higher risk.
• Risk of developing of other cancers ?
Uranium decay series





238U
(parent)
226Ra

214Po
234Th
234Pa
234U
230Th
222Rn
218Po
214Pb
214Bi



+
+
210Pb

210Bi

210Po

206Pb
(stable)
Principal cause of radon problem
Geology
Genomsnittlig årlig stråldos i Sverige
Source: Statens Strålskyddsinstitut
Exposure routes
groundwater

Construction material
Soil gas / bedrock
(Granite)
Regulatory limits
(Sweden)
Radon in water
Radon in air
Radon > 1000 Bq/l
Radon: 400 Bq/m3
Gränsvärde för otjänligt
Riktvärde för radon i
befintliga bostäder
Radon > 100 Bq/l
Radon: 200 Bq/m3
Gränsvärde för tjänligt
med anmärkning
Gränsvärde för radon i
nya bostäder
Radon problems in water
Surface water
Groundwater
Dug wells
Drilled wells
(soil/sand aquifer)
(Hard rocks)
How radon in water is a
problem?
1000 Bq/l
in water
100 Bq/m3
in air
Dish washing
95 %
Shower
60 – 70 %
Bath
30 – 50 %
Washing machine
90 – 95 %
Tap water
10 – 45 %
WC
30 %
Radon in water
Prerequisites
Presence of parent
elements, 238U or
226Ra
Radon emanated
in mineral grain
escape in the
pore space
Recoil
Theory
Transport mechanisms
Pore space filled with
water- Radon dissolves
in the water
Water extracted from
drilled wells (fracture
water)
• Diffusion
• Convection
Dosimetry
How is it
a problem ?
• 1000 Bq/l is dangerous
Precipitation of 238U 234U, 230Th,
226Ra from water to surface of
fracture
Leaching of 238U and
234U
Emanation of 222Rn
Content of 238U in the rock:
10ppm
Concentration of 222Rn
in Bedrock: 0.33Bq/m3
rocks
222 Rn
Concentration of 222Rn in
groundwater: 5 milj Bq/m3
Radon Emanation
Radium atom
Radon atom

Mineral grain



Pore

Radon risk in Sweden
Groundwater radon risk map of Sweden
(after Åkerblom & Lindgren, 1997)
N
N
Radon content in
wells in the county
of Stockholm
20km
(White areas have
too few wells)
40km
0km
20km
40km
1000
Rn conc. (Bq/L)
0 to 100
100 to 500
500 to 1000
1000 to 64000
Rn (Bq/L)
0km
Radon risk areas
calculated using
kriging.
500
100
0
(Knutsson & Olofsson, 2002)
Any deduction?
Granite types of
rocks with high
uranium
concentration
High radon
concentration in
water
 not always the case
Hypothesis of project
The hypothesis stipulates that the occurrence of radon from
groundwater is governed by a number of well-defined factors
ranging from:
• Geological (bedrock, soil, tectonic structures, flow pattern and
surrounding environment)
• Chemical (oxidation reaction, other processes in water)
• Topographical (difference in elevation and slope that
determine flow pattern and renewal tendency and frequency)
• Technical (withdrawal system & frequency which determine
circulation as well as ventilation possibilities.
Purpose of research
Map processes and factors influencing
radon content in groundwater
Develop a prediction model, based on
statistics, that can be used to determine
areas at risk.
Study area
Phases of the project
Phase 1
Using GIS and multivariate analysis of
data to assess factors affecting radon
concentration – REGIONAL LEVEL
Phase 2
Phase 3
Detailed study at Ljusterö to determine
spatial & temporal variation of radon
concentrations due to a range of factors.
LOCAL SCALE
Development of risk prediction model
Phase 1
1. Data collection from:



Swedish National Land Survey (elevation and landuse
data)
Swedish Geological Survey, SGU (soil & bedrock
geology, fractures, radiometric)
Municipalities (data about wells and radon content)
2. Data transformation and extraction using
ArcGIS and its spatial analyst function
3. Statistical analyses including multivariate
analysis of data.
Factors considered
• Elevation
Variables
Derived factors
• Soil geology
•
Altitude difference
• Bedrock
•
Predominant soil,
bedrock, landuse
within a certain
vicinity e.g. 200 m
•
Slope of the terrain
• Fracture zone
• Landuse
• Uranium content
Geographical Information
System (GIS)
• GIS is a computer system for managing
spatial data.
• Purpose of GIS
•
•
•
•
•
•
Organisation
Visualisation
Spatial Query
Combination
Analysis
Prediction
Visualisations with GIS
Bedrocks
Soil
What is my objective?
For each well, relevant spatial patterns need to
be extracted from the factor maps
Data
obtained in
different
formats, e.g
ASCII,
point vector
GIS
Software: ArcMap
Spatial analyst function
Geostatistical software
To generate
continuous
surfaces with
a spatial
resolution of
50 m
+
Derive factors
Ultra edit
software
Methodology using GIS
Factors
Data preparation
Data extraction
Raster format
..
. ..
.
Topography
wells
Geology
Pixel size: 50m x 50m
Radiometric
Landuse
Continuous surface
Database
Wells
X
Y
Rn
Factor 1
Statistical methods
• Which method?
• Relate radon concentration with a large number of
variables
• Variables are both qualitative and quantitative in
nature
• Non-normal distribution of many variables
• Use of covariance and correlations ? Careful with
the interpretations
• Not much information about association between variables
• Non-linear associations can exist
• Very sensitive to ‘ wild observations- outliers ’
Statistical Analyses
Use of multivariate analysis of data
–
–
–
Each observational unit is characterised by several
variables.
It enables us to consider changes in several
properties simultaneously
Non normality of data (non parametrical tests)
Statistical Methods
1. Analysis of variance
2. Principal Component Analysis (PCA)
PCA method
• Eigenvectors of a variance-covariance
matrix
• Linear combinations of these variables
• Its general objectives:
• Data reduction (A small amount of k components
account for much of the variability of the data)
• Interpretation (may reveals relationships that were
not previously suspected)
Results of statistical analyses
Descriptive Statistics
Statistic parameters
Number of wells
Minimum radon concentration (Bq/l)
Maximum radon concentration (Bq/l)
Mean radon concentration (Bq/l)
Median value
Variance
Standard deviation
4439
4.0
63560
492
230
1505978
1227
Radon concentrations in Stockholm County
Boxplot
Median
25%-75%
Non-outlier range
ANOVA - Altitude
Anova - Relative altitude
ANOVA-Bedrock
ANOVA- Fracture
ANOVA - Soil
ANOVA- Landuse
ANOVA- Uranium
Summary of results
High radon concentration in drilled wells is related to:
– Low altitude
– Granite rocks
– Close distance to fracture
– When overlying geology is lera/silt
– Infrequent use of wells (summer houses)
– An overview of the terrain in the surrounding of the
wells (flat or hilly) is also of interest in connection to
groundwater flow tendencies and speed of flow.
Risk Variable Method
Preparation Phase
(Expert system)
Data collection
Statistical analyses
Expert assessment
Selection of
significant variables
Operational phase
(User Interface)
Define study area
Determination of
risk values
Determination of
uncertainty values
Suming up risk and
uncertainty values
Final Risk
Evaluation
Risk Variable Modelling (RVM)
V1 x R1 + V2 x R2 + V3 x R3 + ……….+ Vn x Rn = FRV
FRV = Final risk value
•
Where Vi= a risk value for a specific variable
(-2 to +2)
Ri = the rating of the variable (1 to 3)
Ratings after RVM
Altitude
2
Soil
2
Uranium
3
Landuse
2
Bedrock
3
Distance from
fracture
3
An example of a risk map
Field Studies
Field studies at Ljusterö
Why Ljusterö?
• Number of wells = 198
• 141 wells exceeding 500 Bq/l (71%)
• 96 wells exceeding 1000 Bq/l (48%)
• Radon concentration
• Mean = 1942 Bq/l
• Minimum = 50 Bq/l
• Maximum = 63560 Bq/l
Wells on ljusterö
predominant geology is gnejsgranitoid
What was done?
To choose 3-4 study areas on Ljusterö,
exhibiting drastic fluctuations in the radon
concentration and to perfom detailed study
at these locations
Detailed study
• Analysis of geology (bedrock type, fracture zones,
tectonic zones and fracture filling minerals, soil type and
soil depth)
• Altitude and other terrain considerations
• Analysis of technical factors (wells technical design,
hauling system, spatial temporal extraction patterns of
wells)
• Radiometric measurements of radiation (from soil
around wells as well as measurements of radiation in
wells and in tap water)
• Chemical analyses in water samples (U, Ra, Rn,
fluoride and other water components)