LAND - SEA INTERACTION. SOUTHERN DOBROGEA COASTAL POTABLE WATER SOURCES AND UPPER QUATERNARY BLACK SEA LEVEL CHANGES

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Section Geology
https://doi.org/10.5593/sgem2021/1.1/s01.010
LAND – SEA INTERACTION. SOUTHERN DOBROGEA COASTAL
POTABLE WATER SOURCES AND UPPER QUATERNARY BLACK SEA
LEVEL CHANGES
Assist. Prof. Dr. Glicherie Caraivan
Dr. Dan-Lucian Vasiliu
Geogr. Jenica Bujini
Dr. Bogdan-Adrian Ispas
National Research-Development Institute for Marine Geology and Geoecology – GeoEcoMar,
Romania
ABSTRACT
The coastal area is the field of continuous interaction of marine hydrodynamic factors
with those that act on the land. Since the prehistory, human communities have settled in
this area, having access to both marine and land resources, drinking water being the
most important.
The Black Sea level is the drainage base level to which the surface waters of its
hydrographic basin flow, but it also influences the groundwater dynamics.
Southern Dobrogea is a typical geologic platform unit, divided in several tectonic
blocks by a regional WNW – ESE and NNE – SSW fault system.
Four drinking water sources have been identified: surface water, phreatic water,
medium depth Sarmatian aquifer, and deep Upper Jurassic – Lower Cretaceous aquifer.
Surface water sources are represented by several springs emerged from the base of the
loess cliff, and a few small rivers, barred by coastal beaches.
The cyclic Upper Quaternary climate changes induced drastic remodeling of the Black
Sea level and the corresponding shorelines.
During the Last Glacial Maximum (MIS 2), the shoreline retreats eastwards, reaching
the 100-120 m isobaths. In these conditions, the surface drainage base level was very
low.
The Holocene Transgression (MIS 1) determined a sea level rise up to the modern one.
During the Greek colonization, the rising sea level caused the salinization of the
previous drinking water phreatic sources. In these conditions, in the Roman Age, a new
hydraulic infrastructure had to be developed, using aqueducts for available inland water
delivery.
Keywords: land-sea interaction, aquifer, potable water sources, climate and Black Sea
level changes
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INTRODUCTION
The coastal area is the field of continuous confrontation and interaction of marine
hydrodynamic factors with those that act on the land. Since the prehistory, human
communities have settled in this area, having access to both marine and land resources,
drinking water being the most important.
The Black Sea level is the drainage base level to which the surface waters of its
hydrographic basin flow, but it also influences the groundwater dynamics.
South Dobrogea is located in the southeastern extremity of Romania, on the western
coast of the Black Sea (Fig. 1).
Fig. 1. Location of the studied area (from Google Earth)
MATERIALS AND METHODS
The textural, mineralogical and faunal features of the samples taken from drillings and
Black Sea surface sediments were analysed in the GeoEcoMar laboratories. Depth and
salinity were estimated based on the sedimentary and faunal criteria, thus allowing the
determination of several transgressive and regressive sequences.
In 2010, in the framework of the Programme of Romanian coast rehabilitation managed
by the Dobrogea Littoral Water Basin Administration, a new drilling well, 50 m in
depth, was made in the northern part of the Mamaia barrier beach. Undisturbed sample
materials were collected, and significant mollusc shells were selected for 14C absolute
age determinations. The radiocarbon dating process was performed at DAT (Accelerator
Department Tandem) from IFIN-HH (HERAS Project), based on an AMS (Accelerator
Mass Spectrometry) technique.
RESULTS AND DISSCUSION
1. Regional geological framework.
South Dobrogea geological unit is bounded to the north by Capidava – Ovidiu fault.
South Dobrogea has a specific platform features, with a Pre-Cambrian crystalline
basement and a sedimentary cover (Palaeozoic – Quaternary deposits).
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Regional WNW – ESE and NNE – SSW fault systems divide the South Dobrogea
structure in several tectonic blocks with uneven thickness and differing positions of the
stratigraphic limits (Fig. 2).
Several geological formations have been identified in outcrops and boreholes.
The basement, including Archaic gneisses (Ovidiu Series), Lower Proterozoic high-
grade metamorphic rocks (Palazu Series) and Upper Proterozoic (Vendean) volcano-
sedimentary rocks (Cocoșu Formation), is dipping southwards and westwards.
The sedimentary cover started with thick Mesozoic deposits, followed by thin
Sarmatian – Quaternary deposits.
Mesozoic is represented by locally developed Triassic deposits, consisting of reddish to
yellowish or dark sandstones, argillaceous shales, limestones, oolites, breccias and
conglomerates [1].
The Upper Jurassic – Lower Cretaceous carbonate rocks crop out along the Capidava –
Ovidiu Fault (Fig. 2).
Fig. 2. Simplified geological-structural map, showing the main tectonic units of
Dobrogea area
Upper Cretaceous (Cenomanian – Senonian) is represented by conglomerates,
sandstones, silty marls (Cenomanian + Turonian) and chalk with concretionary cherts
and sandstones and conglomerates in the basal part (Senonian).
The Sarmatian deposits are well represented, being more or less like a continuous layer
that covers the eastern part of South Dobrogea. It consists mainly of limestones with
some detritic levels.
The Quaternary succession covers most of the South Dobrogea surface. These deposits
start with a reddish argillaceous level (Lower Pleistocene), covered by up to 40 m of
loess deposits (Middle – Upper Pleistocene). Along the main streams, the recent alluvial
sediments are present [2].
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2. Hydrogeological considerations
In the study area, four water sources have been identified: surface water, phreatic water,
medium depth groundwater (the Sarmatian aquifer), deep groundwater (the Upper
Jurassic – Lower Cretaceous aquifer).
Surface water is represented by several springs, occurring at the base of the cliffs, and
coastal lakes. Chemical analyses of samples from these sources show slight influences
from the sewerage system. Therefore, they couldn’t be used nowadays as drinking water
source.
The phreatic aquifer, developed at the base of the loessoid deposits, on the impervious
red clay overlapping the Sarmatian limestones, was investigated by many wells (Fig. 3).
Groundwater is mainly potable but the pumping rates at the installation are too low,
around 1 l/s. Thus, from the quantitative point of view, this aquifer is not appropriate as
drinking water source.
The Sarmatian aquifer
In the area of Constanţa city, the medium depth aquifer is located in the altered and
karstified Sarmatian limestones, locally covered by bentonitic clay. The thickness of the
Sarmatian deposits varies between 2 and 68 m, in the zone of Constanţa city, increasing
southward.
Groundwater flow in the Sarmatian limestones is mixed. The aquifer is locally
unconfined, in the zones where it is covered by by loess deposits, or locally confined, in
zones where it is covered by clayey loess deposits [6].
The aquifer is supplied from the Bulgarian territory, where the whole structure is at
higher elevations, and by recharge from precipitation. Before the 1990s, an irrigation
system was also in function. Starting from 1990, the irrigations were gradually reduced
and nowadays they are not used anymore.
The aquifer discharges to the east (Fig. 4), to the Black Sea, and to the Danube – Black
Sea canal, which intercepts the Sarmatian deposits on the last 5 - 6 km, before reaching
the Black Sea.
Fig. 3. Piezometric map of loess aquifer [6,
modified]
Fig. 4. Piezometric map of Sarmatian
aquifer [6, modified]
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The aquifer is intercepted up to 60 – 65 m deep. It is confined when covered by
bentonitic clay and unconfined when the bentonitic clay is missing.
The water table in the abstraction wells from the Sarmatian aquifer is, usually
ascending, with stabilized depths between 34.5 and 36 m. The pumping rates at the
installation are between 3.33 l/s and 6.25 l/s and the optimal exploitable rates are
between 3.64 l/s and 7.84 l/s for a well, the estimated average hydraulic conductivities
are between 3.48 m/day and 6.90 m/day, corresponding to 16 to 30 m thick total
screened intervals.
From the qualitative point of view, groundwater from the Sarmatian aquifer, from 100
to 155 m deep abstraction wells, exceeds the standard for nitrate content (80 mg/l) and
for filtrate residual (over 2000 mg/l). The bacteriological analyses show that the
abstraction wells in the industrial zone exceed considerably the standard for total
coliforms and faecal coliforms.
The Upper Jurassic – Lower Cretaceous aquifer complex
The Upper Jurassic – Lower Cretaceous aquifer complex, located in the limestone and
dolomite deposits, is generally confined and it is affected by the regional WNW – ESE
and NNE – SSW fault systems mentioned above (Fig. 5). The aquifer can become
locally unconfined in the western and northern parts of South Dobrogea, where the
Lower Cretaceous deposits crop out [5, 6]. In the southern and eastern parts of South
Dobrogea, the deep aquifer complex is separated from the Sarmatian aquifer by the
Senonian aquitard consisting mainly of chalk and marl.
The natural boundary of the Upper Jurassic – Lower Cretaceous aquifer is the Capidava
– Ovidiu Fault. In the northern compartment of the fault, the Upper Jurassic have low
thickness and are overlapped on green shales with high elevations and low permeability,
that make a barrier for the groundwater flow, thus deviating it towards east [10].
The piezometric heads (Fig. 5) show that the Upper Jurassic – Lower Cretaceous aquifer
is supplied from the Bulgarian territory, where the Upper Jurassic deposits crop out [9].
The aquifer discharges to the east, to the Black Sea, and also to northeast, in the Lake
Siutghiol. The aquifer is also supplied by vertical percolation from the Sarmatian
aquifer or, in the western part of South Dobrogea, from effective infiltration. Along the
coast, the piezometric heads of the Upper Jurassic – Lower Cretaceous aquifer are
Fig. 5. Piezometric map of Upper Jurassic – Lower Cretaceous aquifer [6, modified]
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higher than the ones of the Sarmatian aquifer, so there is an upward vertical flow.
This aquifer has been intercepted by means of several wells in the study area, at depths
between 50 and 300 m. The exploitable potential of this karstified aquifer complex is
expressed by the differing exploitable rates in the wells mentioned above, reflecting the
anisotropic character of the fissuring porosity.
Thus, the abstraction rates at the installation of the wells are around 6.6 l/s and 22 l/s in
different tectonic blocks. the tectonic block 5, around 7.5 l/s in the tectonic block 10 and
around in the tectonic block 13. The optimum abstraction rates vary between 5.5 and
267.3 l/s, while the average hydraulic conductivities vary between 1.84 and 332.73
m/day, corresponding to the screened intervals with total thicknesses between 109 and
154 m.
The Upper Jurassic – Lower Cretaceous aquifer has drinking water (G. Caraivan, 2006).
The groundwater quality and the abstraction rates indicate that this aquifer should be
used for the drinking water supply in the Constanţa County.
3. Climate and sea level changes.
The cyclic Upper Quaternary climate changes induced drastic remodeling of the Black
Sea level and the corresponding shorelines [3, 4] (Fig. 6).
Fig. 6. Black Sea level changes in the last 50000 y. BP [4]
Neolithic settlements from Dobrogea, such as Hamangia (5200 - 4600/4500 y. BP) and
Gumelnița (4600/4500 - 4000 y. BP) were formed near the main watercourses,
especially at the mouths of the paleovaleys into the Black Sea, transformed then into
lakes and marine estuaries: Mangalia, Tătlăgeac. The Hamangia settlements were
grouped around the main freshwater sources: the Casimcea, Carasu, Nuntași, Babadag,
or springs from Istria, Fântânele, Corbu de Jos, Canara – Ovidiu, Cișmele, Limanu etc.
[7].
During the Greek colonization, the rising sea level caused the salinization of the
previous drinking water phreatic sources (Fig. 7). In these conditions, during the Roman
Age, a new hydraulic infrastructure had to be developed, using aqueducts for available
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inland water delivery (Fig. 8).
Fig. 7 Greek pit (Histria fortress, north of
Constanța)
Fig. 8 Roman aqueduct, 4th century AD,
still active today (Corbu village, north of
Constanța)
CONCLUSION
Marine stratigraphic and sedimentological studies, as well as groundwater monitoring
data have highlighted the land-sea interaction between sedimentation processes and
groundwater dynamics with climate change.
The Black Sea level variations during the Late Quaternary are also reflected in the
change of the surface water drainage basic level, but also of the underground waters.
Consequently, during the last lowered level of the Black Sea (LGM / MIS 2) the flood
waters (Danube in this case) intensified their erosional action, both on land and on the
emerged continental shelf, transporting huge amounts of sediments, which they
deposited at the top of the continental slope. Piezometric levels were very low in both
phreatic water and Deepwater aquifers, as well.
With the onset of the Holocene (MIS 1) the sea level begins to rise, decreasing the
transport capacity of running water. The shoreline migrates to land by clogging the
mouths of rivers at the entrance to the sea. New sedimentary bodies are formed (Danube
delta, coastal barriers, clogging of former bays).
Consequently, Neolithic human settlements migrated to land following the trend of the
sea. The groundwaters, especially the phreatic, more accessible in coastal areas were
affected by their salinization. This explains the decline of ancient fortresses, e.g. Histria,
Tomis, Mangalia. The inhabitants were forced to develop new hydrotechnical water
supply systems, transporting water through aqueducts from greater distances (up 25km).
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ACKNOWLEDGEMENTS
The present study was supported by the project “Cross border Maritime Spatial
Planning for Black Sea – Bulgaria and Romania – MARSPLAN-BS II”, Grant
Agreement: EASME/EMFF/2018/1.2.1.5/01/S12.806725 – MARSPLAN-BS II.
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