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 . 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. 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