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 ?

Uranium decay series 238U (parent) 222Rn 206Pb (stable) 226Ra 218Po 214Bi   238U (parent) 214Po   234Th  210Pb   234Pa  210Bi    234U 210Po  +  206Pb (stable)  230Th  + 

Principal cause of radon problem Geology

Genomsnittlig årlig stråldos i Sverige Source: Statens Strålskyddsinstitut

 Exposure routes groundwater 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 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 - Radon emanated in mineral grain escape in the pore space Recoil Theory Prerequisites Presence of parent elements, 238U or 226Ra Transport mechanisms Diffusion Convection Pore space filled with water- Radon dissolves in the water Dosimetry 1000 Bq/l is dangerous Water extracted from drilled wells (fracture water) How is it a problem ?

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 222 Rn Concentration of 222Rn in Bedrock: 0.33Bq/m3 rocks 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)

(Knutsson & Olofsson, 2002)

Any deduction?  not always the case 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 Phase 2 Phase 3 Using GIS and multivariate analysis of data to assess factors affecting radon concentration – REGIONAL LEVEL Phase 1 Detailed study at Ljusterö to determine spatial & temporal variation of radon concentrations due to a range of factors. LOCAL SCALE Phase 2 Development of risk prediction model Phase 3

Phase 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) Data transformation and extraction using ArcGIS and its spatial analyst function Statistical analyses including multivariate analysis of data.

Factors considered Elevation Derived factors 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 To generate continuous surfaces with a spatial resolution of 50 m + Derive factors Data obtained in different formats, e.g ASCII, point vector GIS Software: ArcMap Spatial analyst function Geostatistical software Ultra edit software

Methodology using GIS

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 Analysis of variance 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 4439 Minimum radon concentration (Bq/l) 4.0 Maximum radon concentration (Bq/l) 63560 Mean radon concentration (Bq/l) 492 Median 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 Method Preparation Phase (Expert system) Data collection Statistical analyses Expert assessment Selection of significant variables Determination of risk values Determination of uncertainty values Suming up risk and uncertainty values Final Risk Evaluation Preparation Phase (Expert system) Operational phase (User Interface) Define 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)

Distance from fracture Ratings after RVM Altitude 2 Soil Uranium 3 Landuse Bedrock Distance from fracture

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)