Radon in groundwater Analysis of causes and development of a prediction methodology

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 ?

214Po

210Bi

210Pb

206Pb (stable)

238U (parent)

234Th

234Pa

234U

230Th

226Ra

222Rn

218Po

214Pb

214Bi

Uranium decay series

210Po

+

+

Principal cause of radon problem

Geology

Genomsnittlig årlig stråldos i Sverige

Source: Statens Strålskyddsinstitut

Exposure routes

Construction material

Soil gas / bedrock (Granite)

groundwater

Regulatory limits(Sweden)

Radon in water Radon in air

Radon > 1000 Bq/l

Gränsvärde för otjänligt

Radon: 400 Bq/m3

Riktvärde för radon i befintliga bostäder

Radon > 100 Bq/l

Gränsvärde för tjänligt med anmärkning

Radon: 200 Bq/m3

Gränsvärde för radon i nya bostäder

Radon problems in water

Surface water Groundwater

Dug wells

(soil/sand aquifer)

Drilled wells

(Hard rocks)

How radon in water is a problem?

1000 Bq/l

in water100 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-

Water extracted from drilled wells (fracture

water)

Radon emanated in mineral grain

escape in the pore space

Pore space filled with water- Radon dissolves in the water

Transport mechanisms

• Diffusion

• Convection

Prerequisites

Presence of parent elements, 238U or 226Ra

Recoil Theory

How is it a problem ?

Dosimetry

• 1000 Bq/l is dangerous

Precipitation of 238U 234U, 230Th, 226Ra from water to surface of

fractureLeaching of 238U and 234U

Emanation of 222RnContent of 238U in the rock:

10ppm

Concentration of 222Rn in groundwater: 5 milj Bq/m3

Concentration of 222Rn in Bedrock: 0.33Bq/m3

rocks

222 Rn

Radon Emanation

Mineral grain

Pore

Radon atomRadium atom

Radon risk in Sweden

Groundwater radon risk map of Sweden(after Åkerblom & Lindgren, 1997)

0 k m 2 0 k m 4 0 k m

Radon content in wells in the county of Stockholm

N

Rn conc. (Bq/L)

0 to 100 100 to 500 500 to 1000 1000 to 64000

0

100

500

1000

Rn

(B

q/L

)

0 k m 2 0 k m 4 0 k m

Radon risk areascalculated usingkriging.

N

(W hite areas havetoo few wells)

(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 1Using GIS and multivariate analysis of

data to assess factors affecting radon

concentration – REGIONAL LEVEL

Phase 2Detailed study at Ljusterö to determinespatial & temporal variation of radon concentrations due to a range of factors.LOCAL SCALE

Phase 3 Development of risk prediction model

Phase 11. 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

• Soil geology

• Bedrock

• Fracture zone

• Landuse

• Uranium content

Variables Derived factors

• Altitude difference

• Predominant soil, bedrock, landuse within a certain vicinity e.g. 200 m

• Slope of the terrain

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

GISSoftware: ArcMap

Spatial analyst functionGeostatistical software

To generate continuous

surfaces with a spatial

resolution of 50 m

+

Derive factors

Data obtained in

different formats, e.g

ASCII, point vector

Ultra editsoftware

Methodology using GIS

Topography

Geology

Radiom etric

Landuse

R a ster fo rm a t

P ix e l s ize : 5 0 m x 5 0 m

C o n tin u o u s su rface

Factors

. ....

.. .

.... wells Wells X Y Rn Factor 1

Data preparation Data extraction Database

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 Methods1. Analysis of variance2. 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 4439Minimum radon concentration (Bq/l) 4.0 Maximum radon concentration (Bq/l) 63560Mean radon concentration (Bq/l) 492Median value 230 Variance 1505978 Standard deviation 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 MethodData collectionData collection

Statistical analysesStatistical analyses Expert assessmentExpert assessment

Selection of significant variables

Selection of significant variables

Determination of risk values

Determination of risk values

Determination of uncertainty valuesDetermination of uncertainty values

Suming up risk and uncertainty

values

Suming up risk and uncertainty

values

Final Risk EvaluationFinal Risk Evaluation

Preparation Phase(Expert system)

Operational phase(User Interface)

Define study areaDefine study area

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 ljusteröpredominant 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)