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Dissolved Oxygen decrease near the bottom of the Inner Saronikos Gulf affected
by the Athens Sewage Outfall
Alexandra Pavlidou, Ioannis Hatzianestis and Roza Psyllidou-Giouranovits
Hellenic Center for Marine Research, Institute of Oceanography, Anavyssos, Greece ([email protected], +302291076347)
ABSTRACT
In this work, the depletion of dissolved oxygen near the bottom of the Inner Saronikos Gulf caused by the
sewage discharges from the Psittalia Sewage Treatment Plant of Athens is studied. Evidence of the sewage
plume diffusion is given by examining the distribution of the concentrations of coprostanol, a common fecal
sterol, in the surface sediments of the area. A deterioration of the quality of the Inner Saronikos Gulf has been
confirmed.
INTRODUCTION
The environmental quality of Saronikos Gulf has been studied since 1987 within the framework of monitoring
programs of Hellenic Center for Marine Research, providing important evidence of environmental change,
especially after the operation of the Sewage Treatment Plant on the Psittalia Island. Since 1994, the sewage
generated by the city of Athens (population approx. five millions) has been primarily treated in Psittalia Treatment
Plant, diverting the effluent from the untreated shoreline discharged to sea-surface, to primarily treated
deepwater by using multi-port diffusers at the depth of 63 m. Since the end of 2004, the sewage of Athens city
has been secondary treated. Approximately, more than 800.000 m3 d-1 of treated waste is discharged into the
inner Saronikos Gulf, carrying ~100 x 106gC d-1. This area is practically flat with a mean depth of ~90 m, and a
volume of ~14 km3. Apart from the treated sewage, no other potential sources of anthropogenic inputs exist in
the area of the Inner Gulf.
Fig 2: : Vertical distribution of Ammonium concentrations (μM)
souwest of Psittalia (S7) in September 2003.
Fig. 3: Vertical distribution of Ammonium concentrations (μM) southeast of
Psittalia (S7) in September 2003.
Fig. 1: Bathymetric map of Saronikos Guf and sampling
sites.
METHODOLOGY
Fig. 4: Horizontal distribution of DO concentrations (mL/L) near
the bottom of the Inner Saronikos gulf in January 2003.
S11
S13
6.50
6.00
6.00
5.50
5.50
5.00
5.00
4.50
4.50
RESULTS AND DISCUSSION
A strong pycnocline appears in the Inner Saronikos Gulf during the warm period (May-November) with surfaceto-bottom temperature change of ~27 – 15 ºC [7]. During the stratification period the sewage plume discharging
from Psittalia Sewage Plant is trapped within the seasonal pycnocline whereas, during the mixing period
(December-April) it reaches the sea-surface. Throughout the year, the spreading of the plume is governed by
advection caused by the prevailing circulation pattern at the particular layer where the plume floats. The
circulation in the Inner Saronikos is mainly wind-driven and predominantly cyclonic that is induced by northerly
winds. As a result the sewage plume tends to spread mostly to the south-west of Psittalia outfall (station S7). The
south-westward spread has been supported by the observed nutrient distributions (Figs. 2,3).
Low Dissolved Oxygen (DO) values (< 3.00 mL/L) were detected near the bottom of the Inner Saronikos Gulf,
mainly at the stations located southwest of Psittalia, as the biochemical result of the oxidation of the organic
matter which is carried by the wastewater effluents into the Inner Saronikos Gulf (Figs 4-6).
Temporal variability of DO for the period 1992-2010 showed decreasing trend of the DO concentrations near the
bottom of the stations located southeast of Psittalia Sewage outfall (Fig. 7). Additionally, it seems that there is a
systematic variation pattern of the DO values throughout a year, with a significant increase during February –
March, due to the homogenization of the water column and the oxygenation of the deep layers (Fig. 8).
The study of fecal sterols in the sediments (coprostanol values, coprostanol/cholesterol and
coprostanol/coprostanol+cholestanol rations) confirms the sewage dispersion pathways (Fig. 9). According to
these results, although the whole area in a distance ~14 km from the outlet is contaminated by human wastes,
the sediments in a direction southwest of Psittalia were more seriously affected than in the southeast direction.
Additionally, sediment data before the operation of Psittalia Treatment Plant (1993), after the operation of the
Primary Treatment Plant (2004) and after the operation of the secondary phase of the Treatment (2007), showed
deterioration of the quality of the Inner Saronikos Gulf (Figs 10-11).
4.00
4.00
3.50
3.50
3.00
3.00
2.50
2.00
Fig. 6: Vertical distribution of DO concentrations (mL/L)
southwest of Psittalia (S7) in October 2003.
Fig. 5: Vertical distribution of DO concentrations (mL/L)
southeast of Psittalia (S7) in October 2003.
2.50
2.00
1.0
Coprostanol/Cholestanol+Coprostanol
DO and nutrient data used in the present study were obtained from four to twelve cruises per year for the period
1992 to 2010, in Saronikos Gulf, using mainly the research vessels «Aegaeo» and «Filia» of the Hellenic Center
for Marine Research (HCMR). Five selected stations in the inner Saronikos Gulf were used (Fig. 1). Seawater
samples were collected from standard depths (surface, 10m, 20m, 50m, 75m and near bottom) with Niskin
bottles, either mounted on a rosette or individually on a hydro wire. Nutrient determination was performed with a
BRAN+LUEBBE II autoanalyzer according to standard methods [1,2]. The determination of ammonium and
phosphate was performed using Perkin Elmer 20 Lambda and 25 Lambda spectrophotometer according to
standard methods [3,4]. DO was measured on board with the Winkler method [5,6].
Sediment samples in Saronikos gulf were taken in 1993, 1999 and 2007. For sterol analysis, freeze - dried
sediment samples were Soxhlet extracted for 24 hours with a mixture of methanol-dichloromethane 2:1, and the
extracts were cleaned up by saponification and fractionated by column chromatography on activated silica gel.
The fraction containing the sterols was derivatized (silylation) with a mixture BSTFA-TMCS 4:1 and the sterol
determination was performed by gas chromatography – mass spectrometry. The following sterols were
quantified: coprostanol, epicoprostanol, cholesterol and cholestanol.
S7
0.9
S8
Highly Polluted
S13
0.8
0.7
S11
0.6
S16
0.5
0.4
0.3
0.2
Unpolluted
0.1
0.0
0
Fig. 7: Temporal variation of DO (mL/L) near the bottom of
station S11, southeast of Psittalia during 1992-2010.
Fig. 8: Temporal variation of DO (mL/L) near the bottom of
station S13, southwest of Psittalia during 1992-2010.
5
10
15
20
Distance from the sewage outlet (Km)
Fig. 9: Coprostanol/Cholestanol+Coprostanol ratio at the stations of
the Inner Saronikos gulf in surface sediments in 2007.
References
Fig. 10: Coprostanol/Cholesterol in surface sediments of
Inner Saronikos gulf in 1993, 1999 and 2007.
Fig. 11: Coprostanol/Cholestanol+Coprostanol in surface
sediments of Inner Saronikos gulf in 1993, 1999 and 2007.
1. Mullin, J. B., Riley, J. P., 1955. The colorimetric determination of silicate with special
reference to sea and natural waters. Analytica Chimica Acta, 12, 162-176.
2. Strickland, J. D. H., Parsons, T. R.,1977. A practical handbook of sea water analysis.
Fisheries Research Board of Canada, 167, 310p.
3. Koroleff F., 1970. Revised version of “direct determination of ammonia in natural
waters as indophenol blue” Int Con Explor Sea C M 1969/ C:9 ICES information on
techniques and methods for sea water analysis. Interlab Rep 3:19–22
4. Murphy, J., Riley, J. P., 1962. A modified solution method for determination of
phosphate in natural waters. Analytica Chimica Acta, 27, 31-36.
5. Carpenter, J. H., 1965(a). The accuracy of the Winkler method for the dissolved
oxygen analysis. Limnology and Oceanography, 10, 135-140.
6. Carpenter, J. H., 1965(b). The Chesapeake Bay Institute technique for dissolved
oxygen method. Limnology and Oceanography, 10, 141-143.
7. Kontoyiannis, H., 2010. Observations on the circulation of Saronikos Gulf: a
Mediterranean embayment sea-border of Athens/Greece. Journal of Geophysical
Research, Vol. 115, C06029, 23 pp.

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