The Science of the Total Environment 272 Ž2001.

217᎐230

Mapping the geogenic radon potential in Germany

J. Kemski a,U , A. Siehl b,1, R. Stegemannb, M. Valdivia-Manchego b

a
Kemski, Klingel & Veerhoff, ¨ on-Weichs-Str. 9a, D-53121 Bonn, Germany
b
Institute of Geology, Uni¨ ersity of Bonn, Nussallee 8, D-53115 Bonn, Germany

Abstract

Mapping the geogenic radon potential in Germany is a research project initiated by the German Federal Ministry
for the Environment, Conservation and Reactor Safety. The project was aimed to develop a standard methodology
for the estimation of a geogenic radon potential and to apply this method to map the region of Germany as an
overview for planning purposes. The regionalisation results from a distance-weighted interpolation of the site-specific
values of radon concentration in soil gas and in situ gas permeability of soils on a regular grid considering the
corresponding geological units. The map of Germany in a scale of 1:2 million is based on the radon concentration in
soil gas as an estimator of the geogenic radon potential assuming the ‘worst case’ of uniform highest permeability.
The distribution is subdivided into categories of low Ž- 10 kBqrm3 ., medium Ž10᎐100 kBqrm3 ., increased Ž100᎐500
kBqrm3 . and high Ž) 500 kBqrm3 . radon concentration. High values occur especially in regions with granites and
basement rocks of Paleozoic age, and are proven by measurements in 0.03% of the total area. Many of these regions
are also known for their enhanced indoor values. The class with increased values takes a portion of 7.86% and
likewise occurs mainly in regions with outcrops of folded and metamorphic basement, but also of some Meso- and
Cenozoic sediments with increased uranium contents andror higher emanation coefficients. For 67.3% of the
country, the radon concentration is classified as ‘medium’, and an assignment to specific geological units cannot be
made at the map scale considered. Low radon contents, where protective measures against radon are usually not
considered, are found in the geologically rather homogeneous part of northern Germany with unconsolidated
Cenozoic sediments, covering approximately 25% of the total country. It is of course not possible to predict the
indoor radon concentration of single houses from these maps, because construction type and structural fabric of
houses are essentially governing the extent to which subsoil radon potential affects the indoor concentration. Besides
this, in places with site-specific geochemical, structural and soil-physical properties, local radon anomalies may occur
which were not recorded in the course of the wide-meshed screening study. 䊚 2001 Published by Elsevier Science
B.V. All rights reserved.

Keywords: Radon mapping; Geology; Geogenic radon potential; Radon activity concentration in soil gas; Gas permeability

U
Corresponding author. Tel.: q49-228-6200910; fax: q49-228-6200913.
E-mail addresses: kemski@kkv-bonn.de ŽJ. Kemski., siehl@uni-bonn.de ŽA. Siehl., stegeman@geo.uni-bonn.de ŽR. Stegemann..
1
Tel.: q49-228-73-2399; fax: q49-228-73-9037.

0048-9697r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 8 - 9 6 9 7 Ž 0 1 . 0 0 6 9 6 - 9
218 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

1. Introduction of the residents. Maps of the geogenic radon

potential can be a predictive tool for the radon
concentration of houses to be built in develop-
The geogenic radon potential is the essential ment areas, but also to target the search for
parameter describing the subsurface as the main regions, where a higher percentage of existing
source for indoor radon concentrations indepen- houses with increased indoor radon values are to
dent on the construction features of buildings be expected. On the other hand, exclusion areas
ŽGunby et al., 1993; Kies et al., 1994; Kreienbrock can be defined, where no preventive measures
and Siehl, 1996.. Radon production and radon have to be considered.
supply are the essential criteria for evaluating the Using such maps of geogenic radon potential, it
geogenic radon potential. These parameters can has to be kept in mind that building-specific char-
be derived from the mapped distribution of geo- acteristics are always controlling radon entry from
logical rock units and their structural relation- ground into houses. Therefore, radon potential
ships. The investigation and evaluation of the maps cannot be used for the prediction of indoor
geogenic radon potential in Germany are based radon contents of individual houses. As far as
on field measurements of the radon concentra- accompanied indoor campaigns to soil gas mea-
tion in soil gas and the gas permeability of soils surements have been carried out, the correlation
under consideration of geological and soil physics between the geogenic prognosis and the actual
parameters ŽKemski et al., 1996a,b,c, 1998b.. Such measured indoor values was good ŽGundersen et
an approach is advisable, if ᎏ like in Germany al. 1988; Neznal et al., 1993a,b; Schumann 1993;
ᎏ exhaustive information about indoor radon Hulka et al., 1994; Kemski et al., 1998c; Lehmann
concentrations is not available and the geological et al. 1998..
conditions are rather inhomogeneous in large
parts of the country. In contrast to the regional
distribution of radon concentrations measured in
2. Measurement, classification and mapping
houses, radon concentrations in soil gas can be
regarded as a natural hazard parameter which is
independent of construction type, state and age A standardised in situ measuring procedure
of existing houses and the lifestyle characteristics using a single borehole probe for both radon and

Fig. 1. Measurement of radon activity concentration in soil gas and in situ gas permeability of soil.
 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 219

Fig. 2. Classification of geogenic radon potential by ranking of radon concentration in soil gas and gas permeability.

permeability measurements in 1 m depth was soil gas permeability, radon activity concentration
developed to ensure the regional representation in soil gas and radon potential class can be as-
and also the comparability of the field data col- signed. Grid-elements without measuring sites re-
lected ŽFig. 1.. At each measuring site, the maxi- ceive their values from the three nearest measur-
mum radon value and the arithmetic mean for ing sites lying in the same geological unit.
permeability from three boreholes are taken The spatial distribution of the main geological
ŽKemski et al., 1994, 1996b.. Because no exact units was taken as the basis for the selection of
functional relationship between these two representative measuring sites ŽFig. 4.. Taking the
parameters and the geogenic radon potential can test areas as training fields, a standard method-
be formulated, an empirical ranking classification ology for the estimation of the geogenic radon
is used ŽFig. 2.. Each of the two parameters is potential was developed. In areas with rather
subdivided into three classes, where the radon homogenous geological conditions, the measuring
concentration in soil gas is given greater weight point density is low Že.g. northern and north-east-
ŽKlingel and Siehl, 1993; Kemski et al., 1994.. By ern parts of Germany.. Measuring sites Ž508.
entry into the ranking matrix, a radon potential were used for this assessment. Areas with more
class between 1 and 6 is obtained. complicated hard rock geology and suspected
Generalisation of existing geological maps in higher geogenic radon potential were considered
various scales by combining geological units un- as test areas and mapped in detail. In these areas,
der lithostratigraphical and radon-relevant as- the density of the measurement grid was higher.
pects was a preliminary step before mapping the Measuring sites Ž627. were considered for the
radon potential ŽKemski et al., 1998a.. The gener- general radon potential map of Germany, using
alised geological map of Germany with the main only such sites whose values lie above the median
geological units is shown as an example ŽFig. 3.. of the geological subunits. In the geologically
Regionalisation of the geogenic radon potential more uniform areas, the average distance between
estimated from the measured values in the field the measuring points is approximately 25 km,
was realised by a distance-weighted interpolation while in the test areas it is approximately 5 km.
on a grid basis within geological units. Each grid Calculating the sum area of corresponding radii
element represents an area of 3 = 3 km, to which around every point, an entire coverage of Ger-
a series of attributes like geological properties, many of approximately 85% was reached.
220 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

Fig. 3. Generalised geological map of Germany as basis for the selection of representative test areas.
J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 221

Fig. 4. Location of measuring sites and test areas.

222 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

3. Radon potential map of Germany

Field measurements of the gas permeability of

soils on a nation-wide scale have revealed, that
the highest permeability of our classification is
the most frequent class. Therefore, only the radon
concentration in soil gas was taken as an estima-
tor of the geogenic radon potential of Germany,
assuming the worst case of highest soil permeabil-
ity all over the country. Besides this, we think that
field permeability measurements in radon map-
ping have only limited value, because the radon
flux from the geological environment into houses
is always governed by the coupling of the founda-
tion to the building ground, which has not much
to do with a permeability measured in an undis- Fig. 5. Log-normal distribution of radon concentration in soil
gas in Germany.
turbed soil in 1 m depth. The primary radon
supply from soil and bedrock is at best charac-
terised by the radon activity concentration in the the other hand helps to identify regions of exclu-
subsurface. sion.
In total, the distribution of the radon activity Areas with elevated radon concentrations in
concentration in soil gas is approximately log-nor- soil gas are of particular interest, when concern-
mal ŽFig. 5.. Grouping all measuring sites accord- ing future regulations for preventive measures in
ing to their geological age, the radon concentra- new buildings. Increased values of more than 100
tion in soil gas of sedimentary rocks turns out to kBqrm3 are common in soils above granites and
become generally higher with increasing age of silica-rich volcanic rocks, e.g. in the Bayerischer
the formations ŽFig. 6.. This is mainly caused by Wald, the Oberpfalz, the Fichtelgebirge, the
the stronger fracturing and mineralisation of the ¨
Erzgebirge, the Thuringer Wald and parts of the
older formations at their deeper erosion level. Schwarzwald. However, also some sedimentary
The oblique distributions of Quaternary and es- rocks are characterised by such high values: Black
pecially Weichselian sediments in northern Ger- shales of Palaeozoic age in the Vogtland and the
many with high 90% values are due to the con- northern part of the Rheinisches Schiefergebirge
tent of granitic material in the ice-drift sediments as well as sandstones and marls of Triassic age in
from Scandinavia. When compared in respect to ¨
the Thuringer Becken are well known examples.
the rock type, the silica-rich magmatic rocks Caused by the specific radionuclide contents of
Žgranites and rhyolites., as to be expected, show the glacier-transported granitic material, young
several times higher values than the unconsoli- glacial deposits in the northern part of Germany
dated sediments, while the consolidated sedi- have to be assigned to the same group.
ments are lying in between. The slightly elevated The spatial distribution of low radon activity
levels of gravels and silts can be explained as concentrations in soil gas is of great interest too.
above by their glacial inheritage. According to our experience, a significant limit
The present data basis allows regionalisation can be drawn with soil gas contents lower than 10
with different classifications. Fig. 7 shows a gen- kBqrm3. Almost in the entire northern part of
eral map with the classes ‘low’ Ž- 10 kBqrm3 ., Germany, radon concentrations do not exceed
‘medium’ Ž10᎐100 kBqrm3 ., ‘increased’ Ž100᎐500 this value. In these areas, building regulations for
kBqrm3 . and ‘high’ Ž) 500 kBqrm3 .. The map protective measures against radon in new
points to areas, where in unfavourable cases high dwellings have not to be considered.
indoor concentrations are likely to occur, and on In the remaining part of Germany, radon con-
 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 223

Fig. 6. Grouping of radon concentration in soil gas of sedimentary rocks according to geological age Žtop box. and rock type
Žbottom box..

centrations in soil gas between 10 and 100 For a statistically exhaustive prognosis in these
kBqrm3 are dominating. These are mainly areas regions, an essentially closer spaced measurement
with flat lying and faulted Mesozoic consolidated grid would be necessary. An example of a more
sediments, where further investigations of the ge- detailed classification and mapping of the local
ogenic radon potential should be carried out be- geogenic radon potential is discussed for one of
cause of the small-scaled variability of geological the regions with the highest radon signal in the
and soil-physical properties. soil, the Fichtelgebirge in Oberfranken.
Enhanced radon concentrations in soil gas
above 100 kBqrm3 appear particularly in Bayern,
¨
Sachsen, Sachsen-Anhalt and Thuringen, where 4. Case studies in the Fichtelgebirge
approximately between 12 and 47% of the respec- (Oberfranken)
tive area are concerned, for the whole of Ger-
many this portion amounts to approximately 8%. The Fichtelgebirge, situated in the north-east-
224 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

Fig. 7. General map of the radon concentration in soil gas in Germany.

 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 225

ern part of Bavaria near the border with the in soil gas and thus leading to classification in a
Czech Republic, is part of the Saxo-Thuringian lower radon potential class. Due to the high level
zone of the Central European Variscan Fold Belt. of gas permeability of the soil in all cases, the
The crystalline bedrock consists of Upper Paleo- classes of radon potential reflects this pattern.
zoic granites intruded into metamorphic Lower The regionalised radon concentration obtained by
Paleozoic rocks ŽFig. 8.. The geogenic radon po- the described raster-oriented interpolation
tential of the geological units was investigated at process based on the field measurements and
approximately 130 measuring sites by means of geological boundaries is shown in Fig. 9. The
the standardised procedure for soil gas and radon potential map ŽFig. 10. depicts the con-
permeability measurements mentioned above. tribution of permeability by the broadening of the
The highest radon concentrations in soil gas, at a areas covered by potential classes 4 and 5.
maximum value of up to 4 MBqrm3 , occur in the Radon potential maps are aimed to be used as
youngest granites and their accompanying quartz planning aids for administration purposes to esti-
veins. The Cambrian and Ordovician rocks are mate necessary further efforts and preventive
composed of alternating sequences of argilla- measures. Therefore, the information about the
ceous shale, quartzite and limestone rocks, radon potential has to be transferred to adminis-
showing significantly lower radon concentrations trative units. If these units are small and the

Fig. 8. Geological map of the Fichtelgebirge ŽOberfranken . with administrative boundaries and comparison of soil gas and indoor
measurements in municipalities.
226 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

Fig. 10. Geogenic radon potential classification in the Fichtelgebirge.

geological conditions are rather homogeneous, to their respective geogenic radon potential
the assignment should not be too difficult. But it classes. Hence, a more sophisticated approach is
is obvious from Fig. 8, that even within municipal- recommended to keep on the safe side, and at the
ity boundaries, quite heterogeneous geological same time to avoid unwarranted expenses.
situations with different radon potential can oc- In some of the municipalities of the Fichtelge-
cur. One approach would be the assignment of birge, long time measurements of radon concen-
the highest occurring geogenic radon potential trations in buildings were carried out in co-oper-
class to a municipality. In some cases, this could ation with the German Federal Office for Radia-
lead to being over precautious, because areas tion Protection ŽLehmann et al., in press.. Since
without development are to be ignored in this the measurement sites for radon concentrations
assessment, only existing or planned built-up ar- in soil gas did not correspond exactly with the
eas should be assigned to the respective mapped location of the measured houses, no direct corre-
classes ᎏ provided, the radon maps are detailed lation between the geological situation at the site
enough. In other cases, the highest geogenic radon of the building and the corresponding indoor
potential class mapped in the built-up area could radon concentrations could be calculated. In-
provisionally be assigned to the administrative stead, average values for municipalities were
unit, until a revision is made by targeted mea- compared. As to be expected, a positive correla-
surements comparable to the standardised proce- tion of the percentage of granite area with the
dure described above. In larger administrative median values of radon in soil gas in the munici-
districts with heterogeneous geological condi- palities was found, explaining also its correlation
tions, different built-up areas should be assigned with the median values measured in cellars ŽFig.
 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 227

Fig. 9. Radon concentration in soil gas in the Fichtelgebirge with measuring sites.

11.. A significant positive correlation exists Federal Republic of Germany is a topic of an

between radon concentrations in soil gas and in ongoing research project.
ground floor rooms. Based on the ground floor
ratio from the correlation graph, at soil gas con-
centrations of 500 kBqrm3 the indoor radon con-
5. Discussion
centration reaches the German ‘remediation
range’ of 1000 Bqrm3 ŽSSK, 1994.. With this
relation of 1:500, radon concentrations in soil gas An estimation of the variation of the radon
greater than 125 kBqrm3 may give rise to an concentration in the soil gas within groups of
exceeding of the upper limit of the ‘normal range’ rock-types is shown in Fig. 12. In higher hierar-
of radon concentration in dwellings in Germany chical groups with a greater number of measuring
of 250 Bqrm3. Such a validation of the geogenic sites, the standard deviation is approximately
prognosis by statistical evaluations of the correla- 7᎐12% of the median and points to a good repre-
tion between geologically soil gas radon measure- sentativity of the geology-confined sampling. The
ments near houses and of corresponding long-time detailed inspection, however, reveals that there
indoor measurements in a spot-to-spot assign- are some conspicuous regional differences
ment has also to take into account construction between similar rocks of same age. The terrestrial
features and the age of houses ŽLehmann et al., L ower Triassic Ž B untsandstein . in the
in press.. The elaboration of house-type specific Bitburg᎐Trier syncline received its sediment
empirical transfer functions to improve the esti- freight from the erosion of the already polycyclic
mation of radon contamination of houses in the reworked Paleozoic sediments of the Rheinisches
228 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230

Schiefergebirge and the Ardennen, almost barren

in radionuclides. On the contrary, sandstones of
¨
the same age and facies of the Thuringer Becken
contain a high portion of the debris of the crys-
talline Variscan basement in the south and the
Lower Permian ŽRotliegend., much richer in ura-
nium, which explains the three times higher me-
dian of the latter. But also granites show a great
variance. Consequently, it turns out that individ-
ual regions have their own ‘geogenic radon-sig-
nature’. Hence, a global extrapolation of the ge-
ogenic radon potential must always be safe-
guarded by comparative field measurements. The
position of the mapped borders between the
classes depends on the spatial distribution of the
measuring sites, so that with additional measuring
points at some positions a spatial shift of the
boundaries may occur in future. Especially for the
large areas with medium geogenic radon poten-
tial, more detailed field investigations are strongly
recommended. The well known fact should be
pointed out too, that the regional pattern of the
geogenic radon potential must not agree with the
regional distribution of long-time indoor mea-
surements. The latter depends on the foundation
depth, the insulation, the construction type, the
condition of the architectural fabric and finally on
the ventilation habits of the residents.
A comparison with radon potential maps of
neighbouring countries of the Federal Republic
of Germany shows a good agreement in the geo-
logically based prognosis. High indoor concentra-
tions above the Paleozoic rocks of the Ardennen
in the northern part of the Grand-Duchy of Lux-
embourg are well correlated with enhanced soil
gas contents above the Paleozoic rocks adjacent
¨
to the Prum᎐Bitburg᎐Trier area ŽKies et al.,
1994 . In contrast, low indoor values above the
.
Mesozoic rocks in the southern part of Luxem-
bourg correspond to rather low soil gas concen-
trations as in the German part of the
Bitburg᎐Trier basin.
Fig. 11. Statistics of municipality-related parameters: Median In the Czech Republic, a similar concept for
of radon concentration in soil gas as function of granite area mapping the geogenic radon potential like in
with regression line Žtop box.. Median of radon concentration Germany was established. A geology-based ap-
in cellars as function of granite area with regression line
Žmiddle box.. Correlation of maximum values of radon con-
proach with radon measurements in soil gas is
centration in ground floor rooms with soil gas and correlation used for the characterisation of distinct rock units.
line Žbottom box.. A comparison of the Czech and German results
 J. Kemski et al. r The Science of the Total En¨ ironment 272 (2001) 217᎐230 229

Fig. 12. Variation of radon concentration in soil gas above different geological units.

for adjoining areas show in general good agree- VII and the fourth International Workshop on the Geolog-
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