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© Българско геологическо дружество и Българска академия на науките, 2021 https://bgd.bg/review-bgs
Национална конференция с международно участие „ГЕОНАУКИ 2021“
National Conference with International Participation “GEOSCIENCES 2021”
Analysis of the hydrogeological conditions in Bulgaria in connection
with the radon potential
Анализ на хидрогеоложките условия в България във връзка
с изследване на радоновия потенциал
Dimitar Antonov1, Mila Trayanova1, Sava Kolev1, Aglaida Toteva1, Aleksey Benderev1,
Kremena Ivanova2, Simeon Valchev1
Димитър Антонов1, Мила Траянова1, Сава Колев1, Аглаида Тотева1, Алексей Бендерев1,
Кремена Иванова2, Симеон Вълчев1
1 Geological Institute, BAS, Acad. G. Bonchev str., bl. 24, Sofia; E-mails: dimia@geology.bas.bg; milatr@abv.bg;
sava_kolev@geology.bas.bg; aglaya.j@abv.bg; alekseybenderev@yahoo.com; simeonwaltscheff@abv.bg
2 National Centre for Radiobiology and Radiation Protection, Georgi Sofiiski str. 3, Sofia; E-mail: k.ivanova@ncrrp.org
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. Different types of rocks and soils possess different 226Ra content and
different permeability. Radon 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 radon potential of
the area could depend on hydrogeology and its particular settings. The study deals with the general characteristics of
the groundwater depths in Bulgaria based on the published since 1960’s sources. After analysis of the collected data
several distinct regions have been elaborated based on different depth of the groundwater table regarding lithological,
tectonic and geomorphological conditions. In addition, zones for screening assessment of groundwater table influence
of radon potential have been precised.
Keywords: radon, groundwater, water table, regional hydrogeology.
СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО,
год. 82, кн. 3, 2021, с. 201–203
REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY,
vol. 82, part 3, 2021, p. 201–203
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 geo-
logical 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
and in building materials. Different types of rocks
and soils possess different 226Ra content and differ-
ent permeability. Radon 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. There-
fore, the radon potential of the area could depend on
not only geology but and from hydrogeology and
other factors.
In respect of the radon potential and moisture
content of the medium, there are established rela-
tionships (e.g. Pinault, Baubron, 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 re-
sults, which concern the increase of radon poten-
tial with increasing 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 an opposite one – the increase of the wa-
ter content leads to decrease of the radon potential
or so-called “screening effect” (Jönsson, 2001; Sa-
koda et al., 2011). The latter case concerns sediment
type of rocks and soils. In Bulgaria, there is national
survey study representing the general regional geol-
ogy settings and the radon potential (Ivanova et al.,
2019) but there is absence of detailed investigation
of the hydrogeology and the radon potential tenden-
cies. This study is the first attempt to represent a
https://doi.org/10.52215/rev.bgs.2021.82.3.201
202
methodology and to assess the regional hydrogeo-
logical conditions with an emphasis on the shallow
groundwater in connection with radon potential fate
and evaluation.
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 radon potential
is a sophisticated task as the geological settings are
complex. Rocks of various ages, origin, mineral,
and chemical composition are present. Their special
distribution and position are complicated as a result
of the complex tectonic structure of the Balkan Pen-
insula (Dabovski et al., 2002). Therefore the main
issue is to distinguish zones which differ one an-
other in respect to the 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 hydrogeo-
logical conditions connected with particular geolog-
ical and geomorphological settings of the Bulgarian
territory. As the emphasis of the study is shallow
groundwater aquifers for the first step of the meth-
odology an appropriate geomorphological 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 morphologi-
cal evolution of the modern relief and the regulari-
ties in its territorial differentiation.” (Yordanova,
Donchev, 1997). Then, as a second step, the data
from the published monographs and hydrogeologi-
cal reports were related to the geomorphological
zones and finally a potential regions or localities
with shallow groundwater 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, a 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-Balkan basins,
IIIb, the Kraishte (IIIc) and the Thracian plain (IIId);
IV, Rila-Rhodope massif with the Rhodopes (IVa),
Rila (IVb), Pirin (IVc), Osogovo-Maleshevo Moun-
tains (IVe) and the Sakar-Strandzha Mountains (IVf)
(Kop ralev, 2002).
Another suitable geomorphological zonation is
the one based on fluvial and fluviolacustrine sys-
tems that drained the Balkan area and produced at
different stages planation surfaces, river terraces,
gorges and other land forms (Zagorchev, 2009). In
it, four groups of fluvial systems are distinguished:
Northern Aegean (peri-Aegean) drainage basin: riv-
ers systems Aliakmonas (in Greece), Vardar/Axios
(in Macedonia and Greece), Struma/Strymon (in
Bulgaria and Greece), Mesta/Nestos (in Bulgaria
and Greece), Maritsa/Evros/Merich (in Bulgaria,
Greece and Turkey); Marmara Sea drainage ba-
sin; 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).
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), the following hydrogeological localities
were distinguished.
Western Thracian lowlands. The most extensive
region in our country with shallow ground waters
(0–4 m) is located in the South Central Bulgaria.
There, these waters occupy an area of about 1700 km2
from the vast terrace 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 construction foundations, as well as the sewer-
age network. Significant areas with shallow water
are also in the Sofia field (about 400 km2 – mainly
in the terraces of the rivers Iskar and Lesnovska),
in the Stara Zagora field (about 420 km2 – between
the rivers 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 groundwa-
ter are those in the Danube lowlands between Vidin
and Silistra, which cover a total of about 700 km2.
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 flooding of excavations and foundations
203
is especially relevant in the range of river valleys,
where the widespread spatial distribution of shallow
ground water exists. 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
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 proluvial
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 lowlands and valleys. There are larger swampy
areas in the Danube lowlands, 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 does not pose a
risk of flooding construction excavations. The depth
of their level from the surface 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 horizons and can meet at a depth of up
to 4–5 m from the surface.
Conclusion
Perspective regional and local hydrogeological
studies have been started to characterize groundwa-
ter and its shallowness as a factor for the level of ra-
don potential, including further elaboration of a map
of the shallow ground water localities (0 to 3 m).
These studies will serve to establish a connection
between the radon potential and the geogenic fea-
tures on 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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