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© Българско геологическо дружество и Българска академия на науките, 2021 https://bgd.bg/review-bgs
СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО,
год. 83, кн. 2, 2022, с. 11–16
REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY,
vol. 83, part 2, 2022, p. 11–16
https://doi.org/10.52215/rev.bgs.2022.83.2.11
Impact of the geological and hydrogeological settings on the radon potential
in Bulgaria
Dimitar Antonov1, Aglaida Toteva1, Sava Kolev1, Aleksey Benderev1, Mila Trayanova1,
Polina Andreeva1, Kremena Ivanova2, Simeon Valchev1
1 Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, “Akad. G. Bonchev” str., bl. 24;
E-mails: dimia@geology.bas.bg; aglaya.j@abv.bg; sava_kolev@geology.bas.bg; alekseybenderev@yahoo.com;
milatr@abv.bg; polina_a@geology.bas.bg; simeonwaltscheff@abv.bg
2 National Centre for Radiobiology and Radiation Protection, 1606 Sofia, Georgi Sofiiski str. 3; E-mail: k.ivanova@ncrrp.org
Влияние на регионалните геолого-хидрогеоложките условия върху
радоновия потенциал в България
Димитър Антонов1, Аглаида Тотева1, Сава Колев1, Алексей Бендерев1, Мила Траянова1,
Полина Андреева1, Кремена Иванова2, Симеон Вълчев1
1 Геологически институт, Българска академия на науките, 1113 София, ул. „Акад. Г. Бончев“, бл. 24
2 Национален център по радиобиология и радиационна защита, 1606 София, ул. „Св. Георги Софийски“ № 3
Резюме. Естественият радон (222Rn) е радиоактивен благороден газ, който се среща във всяка скала или почва поради съ-
държанието на радий (226Ra), част от семейството 238U, в литосферата. Един от показателите за оценка на влиянието на радона
върху определена площ/терен, включително и върху биотата, е т.нар. геогенен радонов потенциал. Различните видове скали и
почви имат различно съдържание на 226Ra, както и различна пропускливост. Радонът като газ има висока подвижност и се при-
движва от процесите на дифузия и конвекция с почвения газ през свързани и ненаситени с вода пори и/или пукнатини в скали и
почви. Следователно, геогенният радоновия потенциал на повърхността се влияе от геоложките особености (като съдържание
на уран в скалите), но и също така – от хидрогеоложките характеристики (като степен на водонасищане близо до повърхността).
Настоящото изследване разглежда основно различните по генезис плиткозалягащи подземни води в България въз основа на
публикувани източници от 1960 г. насам. След анализ на събраните данни са отделени няколко района, по отношение на лито-
ложките и геоморфоложки условия, и на база дълбочина 0–3 m на нивото на подземните води. В допълнение, е разработена ГИС-
базирана карта, свързваща скалите с очакван висок радонов потенциал и зоните с плитко залягащи подземни води.
Ключови думи: радон, регионална хидрогеология, плитко залягащи подземни води, видове скали.
Abstract. Natural radon (222Rn) is a radioactive noble gas that occurs in every rock or soil due to the content of radium (226Ra), part of
the 238U family, in the lithosphere. One of the indicators for evaluating the impact of radon on a certain area/terrain, including on the biota, is
the so-called geogenic radon potential. Different types of rocks and soils possess different 226Ra content and different permeability. Radon
gas has high mobility and is driven by diffusion and convection with the soil gas throughout connected and water-unsaturated pores and/or
cracks in permeable rocks and soils. Therefore the geogenic radon potential at the surface could be affected by the geological settings as a
content of uranium and also by the hydrogeological settings as a degree of saturation near the surface. The present study mainly deals with the
different by genesis shallow groundwater in Bulgaria based on the published since 1960’s sources. After analysis of the collected data several
distinct regions have been determined regarding lithological and geomorphological conditions, and the 0–3 m depth of the groundwater table.
In addition, a GIS-based map was elaborated relating the rocks with expected geogenic radon potential and shallow groundwater areas.
Keywords: radon, regional hydrogeology, shallow groundwater, rock types.
Introduction
Natural radon (222Rn) is a radioactive noble gas
that occurs in every rock or soil due to the content
of radium (226Ra), part of the 238U family, in the
geological environment (lithosphere). The gas is a
product of the decay of natural uranium, found to
varying degrees in a wide range of rocks and soils.
One of the indicators for evaluating the impact of
radon on a certain area/terrain, including on the
12
biota, is the so-called geogenic radon potential.
Different types of rocks and soils possess different
226Ra content thus the radon concentration varies
in distinct areas. Radon gas has high mobility and
is driven by diffusion and convection with the soil
gas throughout connected and water-unsaturated
pores and/or cracks in permeable rocks and soils.
Therefore the geogenic radon potential at the sur-
face could be affected by the geological settings as a
content of uranium, and also by the hydrogeological
settings as a degree of saturation near the surface.
There are established relationships, i.e. mutual
interactions in respect of the geogenic radon poten-
tial and moisture content of the medium, e.g. Pin-
ault and Bauron (1996), Hassan et al. (2011), etc.
In accordance with the large area relationships two
tendencies have been reported (Sakoda et al., 2011).
The first one represented results which concern the
increase of geogenic radon potential with increas-
ing of the moisture content of the media (Arvela et
al., 2016; Hellmuth et al., 2017). In this case, the
geological medium is represented by granites and
similar magmatic rocks. The second tendency is
in contradiction – the increase of the water content
leads to decrease of the geogenic radon potential or
so-called “screening effect” in the presence of sedi-
ment type of rocks, and soils (Jönsson, 2001; Sa-
koda, 2011).
In the recent years, in Bulgaria, there are several
survey studies at regional level relating the general
geology settings and the geogenic radon potential
(Ivanova et al., 2019; Antonov et al., 2020). In addi-
tion, a general review of the investigations, concern-
ing the hydrogeological settings on the territory of
Bulgaria with an emphasis on the shallow ground-
water spatial distribution, was made by Antonov et
al. (2021). This article is the next necessary step to
assess the regional hydrogeological conditions and
their probable effect over the geogenic radon poten-
tial at the surface. In addition, based on the analysis,
a GIS-based map was elaborated relating the rocks
with expected geogenic radon potential and shallow
groundwater.
Methodology of research
Although the area of Bulgaria is small, the assess-
ment of the regional hydrogeological conditions in
connection with an estimation of the geogenic ra-
don potential is a sophisticated task as the geologi-
cal settings are complex. Rocks of various ages, ori-
gin, mineral, and chemical composition are present.
Their special distribution and position are compli-
cated as a result of the complex tectonic structure of
the Balkan Peninsula (Dabovski et al., 2002). There-
fore the main issue is to distinguish zones which
differ one another in respect to the geogenic radon
potential increase or decrease due to the saturation
level of the host rocks and soils. For that reason, a
complex investigation was performed based on the
published monographs and hydrogeological reports
with subject hydrogeological conditions connected
with particular geological and geomorphological
settings of the Bulgarian territory. As the emphasis
of the study is shallow groundwater aquifers for the
first step of the methodology an appropriate geo-
morphological zonation has to be chosen. It should
be based “on territorial combination of forms of
the Earth’s surface with similar morphogenetic and
morphographic features, and supplies a synthetic
idea about the morphological evolution of the mod-
ern relief and the regularities in its territorial dif-
ferentiation.” (Yordanova, Donchev, 1997). Then,
as a second step, the data from the published maps,
monographs and hydrogeological reports were re-
lated to the geomorphological zones and finally a
potential regions or localities with shallow ground-
water aquifers were outlined.
Regional geological and geomorphological
settings
In order to evaluate the possibilities of water type ap-
pearance with the particular rock types on the whole
territory of Bulgaria condense but overall analysis
of the geomorphology, geological settings and tec-
tonic structures is performed. As a base, the com-
prehensive review made by Zagorchev (2009) was
used. As a first geomorphological zonation, which
is also one of the most popular, was considered that
one proposed by J. Galabov being published with
some evolved amendments in Kopralev (2002). In
it, four morphological regions are distinguished: “I.
Danubian hilly plain; II. Stara Planina zone with IIa.
Foothills of Stara Planina (Forebalkan, Prebalkan),
and IIb. Main Stara Planina chain; III. Transitional
strip (zone) with IIIa. Sredna Gora with the Cis-Bal-
kan basins (IIIb), the Kraishte (IIIc) and the Thra-
cian plain (IIId); IV. Rila-Rhodope massif with the
Rhodopes (IVa), Rila (IVb), Pirin (IVc), Osogovo-
13
Maleshevo Mountains (IVe) and the Sakar-Stran-
dzha Mountains (IVf)” (Kopralev, 2002).
Another suitable geomorphological zonation is
the one based fluvial and fluviolacustrine systems
that drained the Balkan area and produced at differ-
ent stages planation surfaces, river terraces, gorges
and other land forms (Zagorchev, 2009). In it, four
groups of fluvial systems are distinguished: “North-
ern Aegean (peri-Aegean) drainage basin: rivers
systems Aliakmonas (in Greece), Vardar/Axios (in
Macedonia and Greece), Struma/Strymon (in Bul-
garia and Greece), Mesta/Nestos (in Bulgaria and
Greece), Maritsa/Evros/Merich (in Bulgaria, Greece
and Turkey); Marmara Sea drainage basin; Danube-
Euxinian drainage basin that consists of Morava
river system (in Serbia and Bulgaria), western
Southdanubian river systems (between the Morava
and Iskar river system), Iskar river system, eastern
Southdanubian river systems; Euxinian (Black Sea)
drainage basin” (Zagorchev, 2009).
In addition to the above-mentioned review,
a GIS-based analysis on the spatial distribution
(as a percentage of absolute territory occupation)
of the rock types with expected geogenic radon
potential done by Antonov et al. (2020) was per-
formed (Fig. 1). That was made in order to relate
the future geogenic radon potential assessment
with the further analysis of the shallow ground-
water spatial distribution and occupation.
Hydrogeological localities
Based on both the geomorphological zonation and
archive book and monograph sources of hydrogeo-
logical characteristics of the shallow ground soils
and rocks in the territory of Bulgaria (Kamenov
et al., 1963; Angelova, 1994; Karastanev, Petrov,
1994), the following hydrogeological localities
were distinguished.
Western Thracian lowlands
The most extensive region in our country with shal-
low ground waters (0–4 m) is located in the South
Central Bulgaria. There, these waters occupy an
Fig. 1. A percentage distribution of the different rocks in Bulgaria with expected radon potential: 1, granites and granitoids;
2, gneiss and meta-granitoids; 3, shales; 4, sandstones; 5, carbonate sediments; 6, Cenozoic (Neogene and Quaternary) unconsoli-
dated and semi-consolidated siliciclastic rocks.
Note: * Rock types where high-level of geogenic radon potential is not being expected; ** Rock types with expected high-level of
geogenic radon potential.
Фиг. 1. Процентно разпространение на различните типове скали с очакван висок радонов потенциал: 1 – гранити и
гранитоиди; 2 – гнайси и мета гранитоиди; 3 – аргилити и алевролити; 4 – пясъчници; 5 – карбонатни скали; 6 – Неозойски
(Неоген и Кватернер) неконсолидирани и полуконсолидирани силицикластични скали.
Забележка: * Скали без или с много нисък геогенен радонов потенциал; ** Скали с очакван висок геогенен радонов по-
тенциал.
14
area of about 1,700 sq. kilometers from the vast ter-
race of the Maritsa River and its tributaries, and in
many places (Plovdiv, Pazardzhik and elsewhere)
cause swamping of large areas. They create serious
difficulties for the construction of deep construc-
tion foundations, as well as the sewerage network.
Significant areas with shallow water are also in the
Sofia field (about 400 sq. km – mainly in the ter-
races of the rivers Iskar and Lesnovska), in the Stara
Zagora field (about 420 sq. km – between the riv-
ers Sazliyka and Blatnitsa), and also in the Radomir
region, Kyustendil, Ihtiman and some other fields
in southwestern Bulgaria. In Northern Bulgaria, the
largest areas with shallow groundwater are those
in the Danube lowlands between Vidin and Silis-
tra, which cover a total of about 700 sq. kilometers.
The water level in them, especially near the river,
shows significant fluctuations – up to 3–4 meters
and more.
Depth of groundwater observed by
the irrigation of construction excavations
and foundations
The issue of irrigation of excavations and foun-
dations is especially relevant in the range of river
valleys, where the widest distribution of shallow
ground water. In the rock complexes of the pre-
Quaternary formations, which are rich in fissure
and karst waters, the water level is at a considerable
depth. Almost everywhere, they lie more than 10 m
below the surface and therefore the danger of hydra-
tion of the excavations is practically non-existent.
In the Pliocene basins, the groundwater level is at
Fig. 2. Location of the of the shallow groundwater in Bulgaria with respect to the rock types with expected radon potential:
1, granites and granitoids; 2, gneiss and meta-granitoids; 3, shales; 4, sandstones; 5, carbonate sediments; 6, Cenozoic (Neogene
and Quaternary) unconsolidated and semi-consolidated siliciclastic rocks; 7, shallow groundwater bodies; 8, in situ uranium ore
deposits; 9, surficial deposits
Фиг. 2. Местоположение на приповърхностните водоносни хоризонти с отчитане на скалите с очакван висок ра-
донов потенциал: 1 – гранити и гранитоиди; 2 – гнайси и метагранитоиди; 3 – аргилити и алевролити; 4 – пясъчници;
5 – карбонатни скали; 6 – Неозойски (Неоген и Кватернер) неконсолидирани и полуконсолидирани силицикластични ска-
ли; 7 – зони с плитко залягащи подземни води; 8 – in situ уранови находища; 9 – повърхностни находища
15
different depths and is related to both the relief and
the position of the sand lenses in the general com-
plex. For example, in the East Thracian Pliocene ba-
sin in some places, the water level is less than 2–3 m
from the surface, and elsewhere it exceeds 15–20 m
depth level of deep pressure horizons. In prolluvial
torrential cones, the groundwater level is most often
4 to 10 m below the surface, but somewhere it is
deeper (15–20 m). As a rule, to the periphery of the
cones, the waters become entangled and swamps
often occur, especially when the contoured alluvial
deposits have lower filtration properties.
Swamps
In Bulgaria they are observed mainly in the low-
lands and valleys. There are larger swampy areas
in the Danube lowlands, the Thracian plain, Sofia,
Burgas and part of the Trans-Balkan valleys and in
the terraces of some rivers, such as Rositsa and Yan-
tra, Kamchia, Struma. The reasons for the swamps
are most often the shallow groundwater level and
the support of the rivers or the scattered ground-
water outlet of the surface between the flooded and
non-flooded terrace. In the Burgas region and in the
valley of the Kamchia River the swamps are due to
surface waters.
Loess and loess-like sediments
They are usually attached to the lowest horizons
and do not pose a risk of flooding construction ex-
cavations. The depth of their level from the sur-
face is most often from 10 to 30 m. Only in the
southernmost parts of the loess formation, in the
range of loess and loess-like clays, groundwater
is sometimes attached to higher stratigraphic hori-
zons and can meet at a depth of up to 4–5 m from
the surface.
Based on the above data, the spatial distribution
of the shallow groundwater is shown as locations
over the territory of Bulgaria (Fig. 2) and as area of
occupation of the rock`s and soil`s territories with
expected geogenic radon potential (Table 1). The
latter represents the area of each rock and soil type
with expected geogenic radon potential in regards
to Fig. 1 analysis, and their area covered by the
shallow groundwater, respectively (the last col-
umn). Thus in the latter areas, the future investi-
gations of the geogenic radon potential could not
be performed properly due to the influence of the
pore water saturation. Especially, that is valid for
areas of alluvial deposits (Fig. 2) where about
18% of them are subject of eventual impact by
the shallow groundwater (Table 1). We are using
the term “eventual impact” due to the fluctuation
of the shallow groundwater caused mainly by the
weather`s seasonal characteristics.
Conclusion
The geogenic radon potential at a distinct area is
affected by the geological settings as a content of
uranium and also by the hydrogeological settings as
Table 1
Shallow groundwater area of occupation in respect of the rockʼs and soilʼs areas with expected radon potential
Таблица 1
Площ на зоните с плиткозалягащи подземни води, привързани към скалите с очакван висок радонов потенциал
Index Type Age Area, sq.km
Area of shallow
groundwater occupation,
sq.km
1 granites and granitoids Late Paleozoic (yPz2) 8159.744 5.708
2gneiss and
meta-granitoids Precambrian (yPe) 11652.745 11.458
3 shales lower–upper Permian
(2P1-2) 379.700 0.702
4 sandstones Ordovician 1114.809 N/A
5 carbonate sediments middle–upper Sarmatian
(kN1s) 10320.259 5.198
6 alluvial sediments Quaternary (aQ) 14561.284 2701.789
Note: indexes are according to Fig. 1ʼs legend
Забележка: индексите са съгласно легендата на фиг. 1.
16
a degree of saturation near the surface. In order to
assess the impact of the Bulgarian geological and
hydrogeological settings on the radon potential, re-
gional quantitative and qualitative geological and
hydrogeological analyses have been performed. The
most wide spread zone with rocks and soils with ex-
pected radon potential, namely an unconsolidated
and semi-consolidated siliciclastic rocks, covers
about 14 500 square kilometers of the territory of
Bulgaria. Itʼs also the most affected by the shallow
groundwater – about 18% of it is subject to satu-
ration. In contrast, for the case of the second most
wide spread zone (~11 652 square kilometers) – that
of gneiss and meta-gneiss, only 0.1% of its whole
territory is subject to eventual saturation caused by
the shallow groundwater. These studies will serve
in the further evaluation of the geogenic radon po-
tentialon the territory of Bulgaria.
Acknowledgements: This study is supported
by the National Science Fund of Bulgaria, in the
framework of Grant No КП-06-Н37/22/07.12.2019.
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Постъпила на 1.10.2022 г., приета за печат на 8.11.2022 г.
Отговорен редактор Йоцо Янев
