Environmental Conditions in a Polluted Lagoon.
Implications for Decontamination Planning and Management
SOTERIOS P. VARNAVAS
Department of Geology
University of Patras
Patras 26 500
GREECE
Abstract: In situ measurements and laboratory work showed that in Aetoliko, Greece lagoon a
major source of pollutants is the freshwater discharging in the lagoon by the pumping system.
It is suggested that precipitation of pollutants like P, Cd, Pb, Mn, Fe, Cr, etc. by construction
of an artificial pond will greatly improve the quality of waters in the lagoon.
The lagoon water layers were identified. Large quantities of pollutants are float on the top of
an intermediate cold layers, in the form of particulate matter. Below 9m down the lagoon
seafloor anoxic conditions occur. Near the lagoon floor pH is lower compared with the waters
of the upper layers. This work stresses the necessity for actions which must be taken for the
lagoon decontamination.
Key-Words: - lagoons, decontamination, methods, toxic gases, toxic, metals, particulate,
matter, anoxic,environments.
1. Introduction
The Aetoliko lagoon on the west Greek
Coasts is an important marine environment [1]. Domestic sewage from the
town of Aetolikon and from a number of
adjacent villages are the main sources of
pollutants in the lagoon. Another major
source of pollutants is the existing
pumping system on the west coast of the
lagoon bringing freshwaters in the lagoon.
These are surface waters which are
collected in a pool prior to their discharge
in the lagoon. Other sources of pollutants
are related to human activities such as
olive oil mills, the effluents of which are
discharged in the lagoon through streams
[2,3,4]. In this lagoon occasionally
significant quantities of toxic gases are
released from the seafloor, which escaping
through the water column cause fish death
and put in danger the local people.
The aim of this work is to assess the
environmental conditions occurring in the
lagoon, determine the sources of major
pollutants, define their behavior in the
lagoon and suggest appropriate methods of
decontamination.
2. Results and Discussion
2.1 Freshwater - seawater interaction
processes
The freshwaters discharged in the lagoon
through the pumping system are loaded
with toxic elements related to chemicals
used in the adjacent cultivated soils.
Therefore, the study of the influence of
these waters on the lagoon is of great
importance. In order to achieve this, the
following methodology was used: a) in
situ measurements were carried out for pH
conductivity, temperature, b) water
sampling was carried out. Both,
measurements and sampling were carried
out at increasing distance from the site of
discharge and at different seasons, c) The
quantity of suspended solids in the waters
at different sampling sites was measured.It
was isolated and analyzed for a number of
elements such as Cd, Pb, Cn, Cr, Zn, Mn,
Al, Si, Fe and Ca[5,6].
An investigation of the fresh water seawater interaction processes showed that
at the transition from the freshwater to the
seawater with slight increase of the
salinity there is a sudden increase in the
phosphorus value. This phenomenon was
observed in December, January and May.
Under the same conditions there is a
tendency for metals to increase in the
particulate matter. This is a result of in
crease of the degree of transfer of the ions
from the dissolved to the solid form. The
degree of incorporation of the metals
studied in the solid form decreases in the
following order: Mn>Zn>Cu>Cd>Fe.
These are useful observations which can
be used in planning the decontamination
of the lagoon.
Figure 1 shows the vertical variability of
temperature at the deeper zone of the
lagoon on 19.3.2005. It is seen that
temperature decreases from the seasurface
down to 8 m depth, the rate of decrease in
creasing at 5m. At 8m temperature reaches
its lowest value, remaining constant down
to 14m. At 15m there is a sudden increase
in the temperature. Below this depth
temperature tends to increase, with a
change in rate at 19m.
2.2.2
Conductivity
The conductivity profile shows a distinct
peak at 8m, (Figure 2.).Below this depth,
changes in the rate of coductivity were
observed at 14m and 19m
Station 4 19/3/05
Station 4 19/3/05
Conductivity ( mS/cm )
23 24 25 26 27 28 29 30 31 32 33 34
23,524,525,526,527,528,529,530,531,532,533,534,535
Depth ( m )
2.2. Water masses.
In order to define the existing water
masses in the lagoon temperature and
conductivity measurements at increasing
water depth were carried out with an
AANDERAA RCM9 Recording Current
Meter.
Therefore, it is feasible with this method
to locate thin water layers.
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Temperature (o C )
9
10
11
12
13
14
15
16
17
18
0
2
Fig. 2: Vertical variability of conductivity
(19/3/05)
4
6
21/6/1999
8
Depth ( m )
10
12
Conductivity ( mS/cm )
14
15 17 19 21 23 25 27 29 31 33 35 37
16
18
22
24
26
28
Fig.1: Vertical variability of temperature
(19/3/05)
2.2.1 Temperature
depth ( m )
20
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Fig. 3: Vertical variability of conductivity
(21/6/99)
Station E9
D. O. (mg/l)
0
1
2
3
4
0
2
4
Depth (m)
On the basis of the temperature and
conductivity vertical variations the
following water masses were defined: i) a
surface layer (0-4m) ii) 6-6m iii) 6-8m iv)
8-14m v) 14-19m vi) 19m - seafloor.
It is interesting to note that the sharp
conductivity peak at 8m depth coincides
with the ceiling of the cold layer (8-14m),
peaks of suspended particulate matter,
particulate Fe, Cr, Zn concentrations. It is
therefore implied that large quantities of
contaminants are float in form of a thin
layer above the cold layer, floating in the
lagoon. It is considered that this
knowledge is of importance in decision
making plans for decontamination and
management of the lagoon. Similar peaks
in particulate Fe, Cr, Zn were located at
4m depth, which coincide with a change in
the rate of temperature decrease, and in the
seafloor waters with the highest
conductivity value.
6
8
10
12
14
16
Fig. 5: Vertical variability of dissolved
oxygen (21/6/99)
2.2.4 pH
Significant decrease in pH values were
observed in the waters near the seafloor
compared with the upper part of the water
column.
2.2.3 Dissolve doxygen (D.O.)
Dissolved oxygen measurements were
carried out in the hot period (June) in the
deeper zone of the lagoon which showed
that D.O. decreases from the seasurface
down to 8m depth (ceiling of the cold
layer). Just below this depth (8.5m) D.O.
is below 1 mg/l, while below 9m down to
the seafloor D.O. is zero.
21/6/99
pH
Depth ( m )
7
Station E5
D.O. (mg/l)
0
1
2
3
4
5
7,5
8
8,5
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
0
1
2
Depth (m)
3
Fig. 6: Vertical variability of pH (21/6/99)
4
5
6
7
8
9
Fig. 4: Vertical variability of dissolved
oxygen (21/6/99)
3
Metal concentration variations
in the water column
Particulate matter analyzed from
different water levels in the lagoon
showed the following:
i) the particulate matter shows increase in
the bottom waters, near the surface and
in the intermediate water masses.
ii) Particulate Fe, Cr, Zn, show
significant peaks at 4m, 9m, depth
and near the seafloor.
iii) Pb, Mn show similar vertical
variability with the above group of
metals, except that the 9m peak is
very weak.
iv) Cr shows a tendency to decrease with
depth down to 20 m depth with very
slight increase at 9m depth. It exhibits
a very pronounced peak near the
bottom.
4 Conclusions
In situ measurements and laboratory
work allowed the determination of the
existing environmental conditions in the
Aetoliko lagoon, which are of
importance in planning its decontamination and management.
A major source of contaminants is the
fresh water discharged in the lagoon by
the pumping system. The study of the
behavior of these contaminants, mainly
phosphorous and toxic metals (i.e Pb,
Cd, Cr et.) suggests that removal of these
contaminants will greatly improve the
quality of the freshwater and in turn of
the lagoon. The removal of the
contaminants can be achieved by
planning and construction of an artificial
pond for this purpose.
It has been revealed that at intermediate
water depths there is a cold layer at the
top of which pollutants (i.e Cd, Pb, Cr,
Mn etc) float in the form of particulate
matter. Similar layer rich in pollutants
also exists in shallow layers and near the
seafloor. These are characterized by
sharp peaks in conductivity.
The deep waters, where release of toxic
gases takes place, are characterized by
low pH relative to the waters of the upper
part of the column. Further geochemical
study is needed prior to take any action
for improving the quality of the seafloor
environment[7,8].
Below 9m depth dissolved oxygen
reaches zero value down the seafloor
(32m). It is therefore revealed that a large
part of the water masses in the lagoon are
under anoxic conditions.
It should be stressed that actions must
be taken for decontamination of the
lagoon for the following reasons:
i) The lagoon is a food source
environment;
fishing
of
large
quantities of fish throughout the year
takes place.
ii) Although it is not known whether
toxic metals get into the food chain,
their presence in large amounts may
affect directly or indirectly the human
health.
iii) The periodic discharge of toxic gases
from the lagoon floor, exept for other
negative impact may cause under
certain
environmental
and
meteorological
conditions human
poisoning.
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