Περιβαλλοντική γεωχημική μελέτη του Σαρωνικού κόλπου

Abstract

The environmental study of the Saronikos gulf was based on the investigation of the organic and inorganic geochemistry of the gulf sediments.Surface sediments were collected from 15 sampling stations during 5 oceanographic cruises. Analyses of sediments included determinations of granulometry, organic carbon, calcium carbonate, hydrocarbon and trace element concentrations. The hydrocarbons analysed were aliphatic compounds (their sum, the unresolved complex mixture - UCM, the n-alkanes with 14-25 carbon atoms, the “isoprenoids” alkanes and some hopanes) and 24 polycyclic aromatic hydrocarbons (PAH). Hydrocarbon analysis performed by gas chromatography-mass spectrometry (GC-MS). Twenty trace elements were determined in sediments, namely Co, Cr, As, Fe, Sb, Ba, U, Sc, Th, Rb, Cs, Ta, Hf, La, Sm, Tb, Yb, Ce, Eu and Lu. Metal determinations were carried out by neutron activation analysis.The field and laboratory study revealed the following:The Elefsis bay is highly influenced by organic pollution, with hydrocarbon concentration levels comparable to those from other industrialized and urbanized coastal areas. The area near to the sewage outfall suffers from moderate organic pollution, while in the rest of Saronikos gulf the hydrocarbon levels are low.The aliphatic hydrocarbons represent the major part of the total hydrocarbons in sediments and their origin is mainly petrogenic. This conclusion is supported by the results of UCM, the ratio of UCM to resolved compound, the ratio pristane to phytane, the distribution of n-alkanes and the identification of specific hopanes having petroleum origin. Terrestial inputs of hydrocarbons are recognizable in the composition of n-alkanes’ group. Hydrocarbon inputs from plankton have not been identified.The study of individual PAH concentrations in combination with the application of several diagnostic criteria (such as the distribution of compounds according to their molecular weight, the sum of pyrolytic compounds, the ratio methylphenanthrenes to phenanthrene, fluoranthene to pyrene) revealed that PAH mixture has mainly pyrolytic origin in sediments. Petrogenic inputs of PAH compounds attributed mainly to naphthalene, phenanthrene and dibenzothiophene series. The apparent predominance of pyrolytic PAH over petrogenic PAH can be explained both in terms of high contributions of related sources, and better preservation of pyrolytic compounds. Biogenic inputs in the PAH group are not discernable in sediments.With the exception of the Elefsis bay, the concentrations of trace elements in the Saronikos gulf sediments are generally within natural levels. In the case of sampling of June 1994, the sediments of station 3 in the Elefsis bay presented high concentrations of As, Sb, Fe, Cr, Ba and U, which could probably, be attributed to a certain pollution incident. It is noteworthy that, in the same station the PAH concentrations were also very high. Moreover, the periodical anoxic / suboxic conditions (during the summer) in the Elefsis bay seem to favour the formation of sulfides.The application of several normalization procedures to the trace element data was useful to spot disturbances in trace element concentrations and facilitated the identification of their anthropogenic or geological origin. In addition, the distribution of trace elements in three geochemical fractions of sediments was investigated, using two extraction methods, one with 50% HCl and a second with 0.05N EDTA. The trace metals in each geochemical fraction had different degrees of release from the solid phase. Five groups of elements were defined, having similar distribution patterns in the three geochemical fractions (namely, group 1: Hf, group 2: Ta, Sc, Th, Rb, Cs, Yb, Tb, La and Sm, group 3: Ce, Cr, U, As, Ba, Lu, group 4: Eu and Sb , group 5: Fe and Co). The first two groups consist of trace elements that are usually incorporated in the crystal lattices. Groups 3 to 5 consist of trace elements, which have a significant proportion (different in each group), associated with the authigenic phases in the sediments. The latter groups are more significant, from an environmental point of view, than the former.The application of factor analysis revealed four main geochemical factors, which control the distribution of trace elements in the sediments of Saronikos gulf. These factors are the lithogenic factor, the organic factor, the clay factor and the factor of Fe oxides / hydroxides. The distribution of hydrocarbons in the sediments is controlled by the organic factor. Factors of natural origin (typical in non-disturbed sediments) are the lithogenic factor, the clay factor and the factor of Fe oxides / hydroxides. In contrast, the organic factor is anthropogenic and is significant in the Elefsis bay and in the area near the sewage outfall (for the hydrocarbons). The carbonate factor is, in all cases, in negative relationship with the other geochemical factors. The grain size of the sediments influences the distribution of the trace elements. The fine material (silt and clay) is in positive relationship with the natural geochemical factors.An estimation of the degree of toxicity of the Saronikos gulf sediments was performed. Several “sediment quality criteria” from the literature were used for this purpose. It appears that, the Elefsis bay has the greatest probability of observing adverse biological effects in the organisms. The toxicity in the Elefsis bay sediments seems to decrease from the western to the eastern part. The area near the sewage outfall is also of potential concern with regards to the sediment toxicity, but to a lesser degree than the Elefsis bay. Finally, the rest of the Saronikos gulf has a rather low probability for toxicity.

H περιβαλλοντική και γεωχημική μελέτη των ιζημάτων του Σαρωνικού κόλπου είχε ως σκοπό τον καθορισμό της προέλευσης των ιχνοστοιχείων και των υδρογονανθράκων καθώς και του τρόπου ενσωμάτωσής τους στα ιζήματα. Επίσης, μελετήθηκαν οι γεωχημικές διεργασίες που πραγματοποιούνται στο συγκεκριμένο περιβάλλον, καταγράφηκαν τα επίπεδα ρύπανσης των ιζημάτων του κόλπου και εκτιμήθηκε ο βαθμός τοξικότητάς τους. Για την επίτευξη των παραπάνω στόχων συλλέχθηκαν επιφανειακά ιζήματα από ένα δίκτυο συνολικά 15 σταθμών στον Σαρωνικό κόλπο, κατά τη διάρκεια 5 ωκεανογραφικών πλόων. Στα δείγματα των ιζημάτων προσδιορίστηκαν: η κοκκομετρία τους, η περιεκτικότητά τους σε οργανικό άνθρακα και ανθρακικό ασβέστιο, καθώς και οι συγκεντρώσεις ορισμένων υδρογονανθράκων και ιχνοστοιχείων. Οι υδρογονάνθρακες που προσδιορίστηκαν ήταν οι αλειφατικοί υδρογονάνθρακες (το σύνολό τους, καθώς και το «μη διαχωρισμένο σύνθετο μίγμα», τα κανονικά αλκάνια με 14 έως 35 άτομα άνθρακα, τα «ισοπρενοειδή» αλκάνια και ορισμένα οπάνια) και 24 πολυκυκλικοί αρωματικοί υδρογονάνθρακες (ΠΑΥ), με 2 έως 6 αρωματικούς δακτυλίους. Ο ποσοτικός προσδιορισμός των υδρογονανθράκων στα ιζήματα έγινε με αέρια χρωματογραφία / φασματομετρία μάζας.