geological structure and seabed morphology of the stoupa submarine groundwater discharge system,...

ORIGINAL ARTICLE

Geological structure and seabed morphology of the Stoupasubmarine groundwater discharge system, Messinia, Greece

G. Rousakis • A. P. Karageorgis • P. Georgiou

Received: 14 September 2012 / Accepted: 31 October 2013 / Published online: 17 November 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Detailed marine geological–geophysical survey

of the submarine groundwater discharge (SGD) system at

Stoupa, Messinia (Greece) was conducted as part of an

offshore study aiming at the evaluation of the discharge

rate, the quality of the water and the investigation of

potential ways for exploitation. Systematic mapping of the

seafloor included swath bathymetry, seismic profiling and

side-scan sonar imaging in order to reveal the precise

morphology of the submarine discharge site, to better

understand the structure of the SGD system and the nature

and thickness of the sedimentary cover, and finally to

provide the necessary data for a potential exploitation

design. The SGD system is located in an E–W trending

ellipsoidal depression characterized by two depth maxima

at 27 and 29 m. This depression has been developed on the

hanging wall of a N–S trending fault, whilst the ground-

water discharges occur at the base of a 10-m-high steep and

faulted rocky slope developed on conglomerates or lime-

stone formations, also occurring in the coast. Recent sand

deposits cover the seabed around the depression. The

complex morphology of the discharge site, the steep slopes,

and the rapid changes (due to erosion with subsequent

slope collapse) during enhanced water flow periods, do not

favor submarine constructions for the exploitation of the

SGD system.

Keywords Stoupa � Greece � Submarine

groundwater discharge � Karstic aquifer �Water demand

Introduction

Submarine groundwater discharges (SGDs) are known since

the time of the Greek geographer Strabo (who lived from 64

BC to 21 AD), when he mentioned a submarine spring (fresh

groundwater) 4 km offshore Latakia, Syria. The spring was

accessible by boats and spring water was being collected

(utilizing a lead funnel and leather tube) and transported to

the city, constituting a source of fresh water for the locals

(Taniguchi et al. 2002; UNESCO 2004).

The lack of drinking water worldwide has directed the

attention of scientists to submarine spring systems, through

which in many cases potable water of good quality flows

into the sea (Burnett et al. 2003; Fleury et al. 2007).

According to recent calculations, large quantities of

freshwater (*240 km3 year-1) flow into the sea through

the SGD systems and correspond to approximately 6 % of

the total annual river discharge (UNESCO 2004). How-

ever, the exploitation of this fresh water is technically

difficult because of its contact with sea water.

In the Mediterranean region many SGDs have been

traced in Spain, France, Italy, Slovenia, Croatia, and

Greece, in areas generally dominated by karstified lime-

stones of various ages. The exploitation, management and

protection of karst aquifers is quite difficult because of the

extreme variability of their hydraulic properties, which are

almost impossible to be determined at a local scale (Bak-

alowicz 2011).

In the Mediterranean region, one of the few well-studied

SGDs lies in the Mar Piccolo (Gulf of Taranto, Southern

Italy) (Calvino and Stefanon 1969; Stefanon 1973). In this

area, there are about 30 submarine springs with one of

them discharging at a rate of *0.8 m3 s-1; these springs

are fed by groundwaters derived from the Cretaceous

limestones and calcarenites.

G. Rousakis (&) � A. P. Karageorgis � P. Georgiou

Hellenic Centre for Marine Research, Institute of Oceanography,

46.7 km Athens-Sounio Avenue, 19013 Anavyssos, Greece

e-mail: [email protected]

123

Environ Earth Sci (2014) 71:5059–5069

DOI 10.1007/s12665-013-2910-1

In Greece, the Anavalos submarine spring system in the

southeastern coast of Peloponnese (at Anavalos-Kiveri),

which discharges with a mean rate of *8 m3 s-1, is con-

sidered as a typical example of SGD (Tiniakos et al. 2005

and references therein; Breznik and Steinman 2011). So

far, this is the only SGD in Greece continuously exploited

for irrigation purposes since the 1970s.

Due to the extensive presence of carbonate forma-

tions in Greece, SGDs occurrence is common throughout

the country. However, they are poorly studied, whilst

continuous monitoring of the quantity and quality of

emanating waters was, until recently, completely lacking.

Recently, monitoring of radon progenies in SGDs was

carried out at four sites in the northern Mediterranean, i.e.,

Cabbe (Monaco), Chalkida, Korfos and Stoupa-Kalogria

(Greece) (Tsabaris et al. 2010; Karageorgis et al. 2011).

The Stoupa-Kalogria site was studied in more detail for

in situ radionuclide characterization using the underwater

gamma-ray spectrometer KATERINA (Tsabaris et al.

2008, 2012). The authors conclude that the use of radon

isotopes as tracers of SGD, as well as indicators of mixing

processes between groundwater and seawater, is advanta-

geous for the continuous in situ monitoring of SGDs in

coastal areas.

The SGD in Stoupa-Kalogria is an impressive single-

point source of fresh/brackish water and is the subject of

the present study. The main goal of the current study is to

present in detail the bathymetry, the morphological fea-

tures of sea-bottom surface and the composition of sub-

bottom strata in the SGD system and its surrounding area.

Further, the future potential exploitation of the Stoupa-

Kalogria SGD system is assessed.

Regional setting

Submarine groundwater discharge of karstic aquifers is the

result of karstification below the present sea level during

periods of low sea level in the Pleistocene. The karst sys-

tem in the area of SW Taygetos (SW Peloponnese, Greece)

has been developed in the Cretaceous, Triassic and Jurassic

limestone formations, while the underground waters flow

out at coastal springs such as those located in the Karda-

mili, Stoupa and St. Nikolaos settlements. Pleistocene

marine formations consisting of marls, marly limestones

and conglomerates as well as of Quaternary deposits pre-

vail along the coastal region of the eastern Messinian Gulf

(Stamatis et al. 2011).

Many coastal and submarine groundwater discharges are

found in Stoupa-Messinia with the majority of them loca-

ted SW of the Kalogria Bay (Fig. 1a). The submarine

groundwater discharge in the aforementioned area was

extensively investigated by the Hellenic Centre for Marine

Research (HCMR) during the time interval 2009–2010.

Continuous monitoring of flow velocity, temperature and

salinity throughout the year revealed a maximum discharge

rate of 1.25 m3 s-1 and minimum salinity values of

1–2 psu during the rainy periods, whereas during the dry

periods the discharge rate decreased to 0.2 m3 s-1 and

salinity increased to [12 psu (Karageorgis et al. 2011).

Methods and equipment

The investigation of seabed bathymetry and morphology

and sub-bottom profiling were undertaken simultaneously

Fig. 1 a Google map of the study area with the submarine groundwater discharge location; and b map of swath bathymetry and geophysical

measurement tracklines

5060 Environ Earth Sci (2014) 71:5059–5069

123

(in November of 2009), using a private small vessel

(named ‘ORION’) cruising at speeds of 2–2.5 knots. For

the complete coverage of the study area, the survey was

conducted along 20 tracklines running perpendicular to the

coast (in an E–W direction) and along two tracklines run-

ning parallel to the shore (in a N–S direction), with the

overall length of the performed survey being approxi-

mately 12.5 km (Fig. 1b). The main tracklines were par-

allel to each other and *50 m apart, whereas closer to the

SGD the trackline spacing was reduced to *20 m in order

to enhance data reliability. Navigation and positioning

during the field operations were accomplished with dif-

ferential GPS, using a Trimble R8 L1/L2 receiver, with

continuous corrections via land stations utilizing the ‘Real

Time Kinematic’ technique in order to obtain position

accuracy in the range of a few centimeters.

The bathymetric data were collected using an L3 Nautic

SB1185 multibeam system. The 180 kHz SB1185 system

has been designed for operation at water depths down to

600 m and transmits 126 beams arrayed over an arc of

153�. The spacing between soundings (beam footprint) is a

function of received beam width, water depth and beam

incidence angle, resulting in beam dimensions of

1.5� 9 1.5�. The swath of sea floor insonified along each

trackline was approximately eight times the water depth

(depths\100 m). A Coda Octopus motion sensor was used

to compensate for the vessel’s motion (i.e., roll, pitch and

heave) during transmission and reception cycles with an

accuracy of *0.05� for the roll and pitch and *5 cm for

the heave. A conductivity, temperature and depth instru-

ment (CTD) was used several times during each survey day

to determine the sound velocity within the water column,

so that each acoustic path could be ray-traced to the sea-

floor, and thus to correct for refraction in the water column.

A Geoacoustics Boomer profiler and a Geoacoustics

side-scan sonar (100/500 kHz) were used for the sub-bot-

tom profiling and surficial mapping, respectively. The

Boomer profiler operated at a frequency of 0.7–2.0 kHz

and at a shooting rate of 125 ms and pulse duration of

2 ms, achieving an across-track resolution of about

0.5–1.2 m. The swath width of the side-scan sonar was

about 200 m (100 m per channel) at a shooting rate of

200 ms (5 pings s-1). The dual frequency side-scan sonar

tow-fish was kept 5–20 m above the seabed succeeding an

across-track resolution of about 50 cm.

Results

Bathymetry

The seabed morphology is characterized by smooth slopes

in the NW part of the study area, at depths of 20–40 m. In

the central part of the study area, a rocky ridge of greatly

variable height with steep slopes occurs (SW–NE). At

depths between 35 and 45 m, this ridge rises up to 20 m

above the seafloor. The rocky ridge extends 500 m to the

NE and, then, smoothes out at a depth of 11.5 m

(remaining beneath the seabed) where only the appearance

of low height scattered rocky outcrops is noticed (Fig. 2a)

The exact location and detailed bathymetry of the sea-

floor depression in the SGD site are illustrated in Fig. 2b.

The shape of the depression looks like an ellipsoid with its

main axis in an E–W direction and its secondary one in a

N–S direction, whilst two depth maxima at 29 and 27 m

are identified. The main spring discharges fresh/brackish

water and is located in the eastern edge of the depression,

where a maximum depth of 29 m is observed at the base of

an almost vertical faulted slope (Fig. 2b). A second spring

is located in the western part of the depression and is

characterized by a maximum depth of 27 m. The maximum

depth of the depression, considering as reference level the

surrounding 19-m deep seabed, is 9.5 m and is observed at

the eastern edge of the main spring. The depth decreases to

5 m in the south side of the depression, where two smaller

springs are found. The depression walls become less steep

to the west and south. Finally, the surrounding sea-bottom

is smooth and almost flat but towards the west and beyond

the depth of 21 m it becomes sharper.

To the west of the Kalogria Beach and at depths lower

than 13–14 m (east of the rocky ridge), the bottom

becomes gradually smoother towards the shallow water and

only in the shallowest part some outcrops of low height

appear. In addition, the seafloor of the coastal area east of

the SGD is covered by rocks and patches of Posidonia (P.

oceanica) meadows in its shallower section.

Acoustic sub-bottom profiling

The processing and interpretation of sub-bottom profiles

from the discharge site and its surrounding area revealed a

hard bedrock (acoustic basement for the Boomer profiler)

overlain by recent sediments or outcropping as rocky for-

mations. Particularly, in the discharge site, the bedrock

outcrops in the area of the main SGD where a fault occurs

displaying a morphological offset of about 10 m. This fault

forms the eastern boundary of the depression (Fig. 3).

The sub-bottom profile 111 (Fig. 3) shows that sediment

thickness west of the SGD reaches up to 11 m but close to

the spring it decreases to about 5 m. At the western end of

the profile, at 50 m depth, sediment cover diminishes and

the bedrock emerges on the seafloor.

The hard bedrock has been detected in all the W–E

profiles located north and south of the main depression and

is covered by 3- to 4-m-thick sandy deposits. The discov-

ered fault can be also traced to the north of the depression,

Environ Earth Sci (2014) 71:5059–5069 5061

123

forming a smoother morphological discontinuity which is

draped by sediments.

The processing of seismic profiling made in the wider

area north of the discharge site revealed as a major struc-

ture a SW–NE trending bedrock ridge (Figs. 4, 5). More

specifically, this bedrock ridge demonstrates a 30-m-high

morphological displacement along its western flank (see

profile 7 in Fig. 4) and continues for several meters to the

east. Then, it breaks off by a smaller ridge (the morpho-

logical offset is about 10 m) where the bedrock is covered

by sandy sediments. The small depression formed in the

bedrock surface east of the ridge has been filled with sandy

sediments with a maximum thickness of 12 m. According

to profile 6 (Fig. 5), towards the north, the bedrock is

exposed on the seafloor, with a larger horizontal develop-

ment to the W–E direction, which reaches a length of

250 m. On the top of the ridge, we observe a small

depression (graben) with steep walls formed by small faults

trending north-south. To the east, the hard bedrock plunges

more mildly under the seafloor and forms a basin which has

been filled with sandy sediments of a maximum thickness

of 11 m at some positions in the center of the basin. In

addition, profile 6 (Fig. 5) shows that near the north end of

the Kalogria Beach the bedrock approaches the sea-bottom

surface. This coastal section is covered with conglomerate

formations and, possibly, these formations correspond to

the acoustic bedrock illustrated in profile 6. Finally, the

bedrock ridge continues NE for about 200 m, north of

profile 6, and smoothes out forming a rocky seabed (see

profile 2 in Fig. 6). In a short distance from the shoreline,

the bedrock dips seawards beneath a 5- to 14-m-thick pile

of sandy sediments (Fig. 6).

Fig. 2 a Detailed bathymetric map of the study area; and b 3-D view bathymetry of the discharge area WSW of the Kalogria Beach


123

Mapping of morphological features

Side-scan sonar profiles were processed and merged to

create the mosaic presented in Fig. 7. The most striking

feature is the SW–NE trending bedrock ridge represented

by the strongly reflecting (dark) section in the mosaic

(Fig. 7). This ridge forms a physical barrier that stops the

sediment transport seawards. Sandy material is being

trapped in the Kalogria Bay and deposited in the exterior

area of the depression.

The narrowest sector of the depression in the SGD site is

marked by an intense reflection in the acoustic mosaic

because of its steep walls and the hardness of its rocky

bottom. Around the depression, in the area of the main

discharge (mainly to the west), the seafloor is covered by

sandy sediments (see the light gray reflection in Fig. 7)

since all erosion products resulting from the strong flow of

the discharging water are transferred there.

In addition, sand covers the sea-bottom surface to the

east of the high elevation occurring in the area west of the

Kalogria Beach. However, some small rock formations

appear near the beach. Also, the seabed seems to be sandy

in its deeper parts to the west, beyond the rocky ridge.

The seafloor to the north of the Kalogria Beach appears

to be rocky with patches of Posidonia meadows at shallow

depths, but most of the NW part of the wider area is cov-

ered with sand up to higher water depths.

Finally, in the south side of the area surrounding the

SGD system a continuous development of rock formations

with a general direction ENE–WSW is visible. This area

Fig. 3 Boomer sub-bottom profile 111 through the center of the discharge, running E–W. The hard bedrock (Boomer acoustic basement) is

shown under thick sandy sediments as well as the fault east of the discharge site


123

forms the north side of Stoupa’s shoal, which continues to

the Stoupa Beach and to higher water depths as well, WSW

of the study area.

Discussion and conclusions

Geomorphology of the SGD

The marine geological–geophysical survey of the near-

shore area of Stoupa (WSW of the Kalogria Beach)

revealed a remarkable depression in the seafloor, which

hosts a fresh/brackish submarine spring system. The most

remarkable feature identified is an impressive single-point

SGD located at a depth of 29 m. The depression is char-

acterized by a rocky bottom and steep (almost vertical)

walls of up to *10 m height.

The bedrock in the study area is largely covered with recent

sand deposits and crops out on the seafloor forming ridges

parallel to faults. According to the geomorphological con-

figuration of the bedrock and the geological observations in

the broader coastal area, we suggest that it consists of Pleis-

tocene conglomerates which outcrop in the shoreline next to

the Kalogria Beach. It is also possible that the acoustic base-

ment (bedrock) recorded in the sub-bottom profiles may rep-

resent the underlying Mesozoic limestone formations, which

occur at the coast and further inshore and are covered

unconformably by conglomerates of post-alpidic age. There-

fore, we may infer that the Stoupa-Kalogria submarine dis-

charge is derived from the karstic aquifer that has been

developed in limestone formations or conglomerates.

In the discharge area, it is apparent that the depression’s

bottom is composed of hard bedrock from which water

emanates. The underwater springs are single-point sources of

Fig. 4 Sub-bottom profile 7, north of the discharge site showing the outcrop of the bedrock and the faults that border it to the west


123

fresh/brackish water with variable discharge rates, scattered

along the coast at depths generally less than 30 m. The

consideration of our data together with observations from

divers shows that all SGDs are associated with rocky out-

crops on the seabed. Hence, it may be deduced that SGDs in

the study area are formed in places where the bedrock is

brittle and fragmented by faults, allowing groundwater

intrusion through cracks and cavities.

Erosional features and consequences

Water discharging at high velocities ([1 m s-1; Kara-

georgis et al. 2011) causes intense erosion of limestone/

conglomerate formations, which are materials susceptible

to mechanical and chemical weathering and erosion.

Intense erosion is probably responsible for the rapid change

of the depression’s morphology considering the successive

observations during the time interval 2006–2009. A col-

lapse of an underwater cave ceiling and the subsequent

rock and debris transport to the depression’s bottom was

evidenced from divers’ observations and relevant photo-

graphs. It is speculated that the recorded changes of the

underwater relief were triggered by the heavy and contin-

uous rainfalls in February 2009, which intensified the

SGD’s activity (Karageorgis et al. 2010).

Types of exploitation structures and examples

Structures for the exploitation of coastal karstic aquifers

have been summarized by Biondic et al. (2005) and include

Fig. 5 In sub-bottom profile 6, the appearance of the hard bedrock on the bottom is more extensive and smoother than in profile 7. A depression

bounded by small faults on top of the bedrock ridge is clearly identified


123

well networks, galleries, dams, pits, and trenches. According

to the aforementioned study, submarine safe exploitation of

the submarine springs may be accomplished by the utiliza-

tion of ‘siphon-like’ or ‘bell-shaped’ capping structures,

which accurately regulate the volume of the pumped water.

In the Anavalos submarine spring system (which dis-

charges at a mean rate of *8 m3 s-1), located near the

southeastern coast of Peloponnese (Greece), the construc-

tion of dams in the sea has prevented seawater intrusion

into the aquifer, thus isolating freshwater (Tiniakos et al.

2005; Breznik and Steinman 2011). The resulting ‘fence’

has allowed the formation of a freshwater lake in the sea,

which has been used for the irrigation of the adjacent

coastal lands (Zektser 1996; Burnett et al. 2003; Fleury

et al. 2007 and references therein). Tiniakos et al. (2005)

argue that the major problem associated with the long-term

exploitation of the Anavalos spring system is the salini-

zation of the aquifer due to: (1) earthquakes; (2) over-

pumping from boreholes; and (3) disturbance of the water

balance after the implementation of additional irrigation

projects in the catchment area.

Is the exploitation of Stoupa submarine spring feasible?

The economically advantageous exploitation of SGDs is a

multi-faceted issue that requires careful study before any

exploitation attempts are applied. Biondic et al. (2005) list

the following factors in priority order: water demand, total

discharge volume and rate, depth at the spring mouth,

distance of the spring from the coast, local meteorological

Fig. 6 The hard bedrock in sub-bottom profile 2 is exposed on the seafloor at shallow depths and is partly covered by 5- to 14-m-thick sandy

sediments


123

conditions, morphology of the spring and number of spring

mouths, geomorphology of the seabed, and coastline

geomorphology.

In addition, a major factor is the water quality, which in

many cases, including the Stoupa SGD, is variable throughout

the year, with freshwater discharging during winter and

brackish water during the rest of the year. If the salt content of

the spring water makes it unsuitable for drinking purposes and

irrigation, it could be used in combination with desalination

processes. Recently, Karagiannis and Soldatos (2008), in a

Fig. 7 Side-scan sonar mosaic of the seafloor of the survey area with the main geomorphological features


123

critical review of desalination methods and the associated

costs, conclude that the desalination cost of brackish water is

50 % less than is required for seawater desalination. Mem-

brane methods, mainly reverse osmosis systems, are the

optimal choice for the desalination of brackish water, due to

lower energy consumption and recent technological advances.

In that perspective, the Stoupa SGD might be of some eco-

nomical importance.

The water demand in Stoupa town and the neighboring

settlements is very high and becomes more severe during

the summer period, when tourism activities almost double

the population to *6,000 inhabitants. However, local

geomorphological conditions, at and around the SGD,

make submarine engineering works rather expensive and

possibly risky: (1) the bottom relief at the SGD is char-

acterized by steep/vertical slopes and the presence of active

faults; (2) the main SGD is covered by a solid limestone

rock with dimensions of 5 9 4 9 1 m. (observations and

measurements from divers; Karageorgis et al. 2011); (3)

scattered rocks, debris and loose sediments cover the sea-

bed; and (4) the underwater morphology changes due to the

extremely high water flow velocities during raining peri-

ods, particularly after heavy rainfall.

An effort was made to evaluate the cost/benefit of a

potential installation that could capture the Stoupa SGD.

However, a thorough literature review revealed a tremen-

dous lack of data regarding the cost of such an installation

with its maintenance needs. Even at the well-known SGDs

at Anavalos-Kiveri, which are still under exploitation,

economic figures are not available. Therefore, the final

benefit from the exploitation of the Stoupa SGD cannot be

effectively determined.

In conclusion, prior to any construction, bottom con-

solidation works would be necessary to remove rocks,

debris and any other obstacles at and around the main

submarine spring. However, due to the difficulties descri-

bed above, the construction of any structure (either pilot or

permanent) on the seafloor, aiming to capture the fresh/

brackish water, would be seriously hampered. Finally,

present knowledge suggests that the exploitation of the

Stoupa SGD is not feasible.

Acknowledgments We are grateful to Ch. Liapakis and V. Balis for

their assistance during field work. The assistance of I. Panagiotopo-

ulos in the revision process is acknowledged. We wish to thank four

anonymous reviewers for valuable comments and suggestions, which

improved the manuscript.

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