Υδρογεωλογικές συνθήκες λεκάνης Θραψάνου Ν. Ηρακλείου Κρήτης. Ρύπανση και μόλυνση των υπόγειων νερών

Abstract

The subject of this treatise is the aquifer of the Hydrogeological basin of Thrapsano. Thrapsano is a plateau with average altitude +350m. It develops along the west and southwest foot of Lassithi Mountains (mountain chain of Dikti) and borders a plain land of nearly 52,000 stremmas (1 str = 1,000 m2); it is located about 35 km SE of the city of Heraklion. The basin of Thrapsano was created from the paleo-relief which was formed in the area during the great post-alpine orogenetic movements, especially in the period between the Lower Paleocene and the Lower Pleistocene. These movements created the big tectonic trench “Heraklion-Phaestos” at the greated area, part of which is the area of the survey. The paleo-relief in that area is due to the vertical movements (downward and upward) mainly of the big fault zones which cross the area of: a) Nipiditos-Kastelli-Hersonissos and b) the almost transversal fault zone to the precedent of Nipiditos-Xeniakos. The actions of these faults zones at the eastern-southeastern part of the area resulted in the creation of the massive of Lassithi Mountains (western and southwestern foot of Dikti mountain) whilst the west-southwest part buried and formed the bottom of sedimentary basin for the post-alpine formations. The bottom structure by alpine impermeability formations and the Neocene and Tertiary deposits conduced to the creation of the aquifer on very high altitudes (average altitude + 250m). In the area of the basin of Thrapsano torrents flow their surface water coming from the western and southwestern foot of Lassithi Mountain. The torrents are the following: a. Baboulanis b. Yerakini Lagada c. Panaghia d. Arkalochoriou. e. Vonis-Thrapsanou. Moreover, the surface water of the whole basin area has got three outlets, one on the southern part and two on the northern-western part. The preparation of this study includes the survey and the study of the area which is hydrologically determined and defined by the main watershed of the discharge basins which flow their surface water to the area of the basin regarding: - the geological and tectonic structure of the defined area, its transformed geo-relief and the hydrological and mainly hydrogeological conditions which prevail in the area of the basin, - the quantitative characteristics (piezometric surface, hydraulic parameters, movement of ground water) of the aquifer of the basin were also being investigated and studied; emphasis was given to the investigation of the qualitative characteristics of the ground water of the aquifer of the basin in order to ascertain the possibility of its exploitation and the degree of pollution and contamination. The quaternary deposits of the basin of Thrapsano, define its hydrogeological boundaries and form its main aquifer. But, Neogene water perforated deposits are being participating to the formation of the Integrated Aquifer of the basin. They are being lying under to a great part of the quaternary deposits. Also, a considerable part of the aquifer in southern and southeastern part of Thrapsano’s basin is being discharged from the spring of Migilissi. To the central NE and NW, the aquifer appears unmixed (only quaternary deposits are developing) whilst to the remaining greater part it appears mixed (neogene deposits participate to the ground aquifer, too). This hydrogeological differentiation of the aquifer has effect to the hydrochemistry of the restored and under restoration ground water; moreover the problem of the CaCO3 instability occurs to the mass of the mixed aquifer. - More than 25 years have gone since the exploitation of the aquifer of the basin of Thrapsano but no investigation has been done regarding its capacity, its qualitative characteristics or the mechanism of its diet and operation. There are forty-five (45) littered drillings under operation which irrigate 52,000 stremmas of agricultural land and, at the same time, they cover the water supply of three (3) Municipalities, twenty (20) Communities and twenty-four (24) settlements of the area i.e. 14,000 inhabitants approximately. Additionally, the Municipality of Heraklion supplements its needs in water from the aquifer of the basin; two (2) more Water Supply Associations (Episkopi and Lefkochori) including Communities and Settlements beyond the area of investigation and study are being served from the same aquifer. The surface of the Integrated Aquifer is almost horizontal to the northern and central part of the basin showing the very slow movement of the ground water. Besides, the calculated velocity of the ground water at the deposits of the important torrent of Baboulani (to the north) is hardly up to 0,7m per day. The aquifer water has been dated with the method of natural isotope of Tritio (T or 1H3) and it’s been confirmed the dynamic form of the piezometric surface of the aquifer. Regarding the age of the ground water of the basin, a great differentiation appears between the northern and the southern part of the aquifer. More specifically, it was defined that the age of the ground water to the northern and northeastern part comes up to 21.2 ±2.9 years; to the central part, the age remains almost the same with small odds coming up to 17.1±2.5 years, whilst to the southern part the age of the discharged water from the spring of Migilissi is smaller and comes up to 5.2±2.2 years. Important polluting activities occur to the area of the basin of Thrapsano. At first, there are the agricultural cultivations with the excessive use of highly toxic substances such as pesticides and fertilizers. The pollution through filtering or percolation is being transferred and ends up to the aquifer of the basin. Even residues from the olive factories, house and solid waste are being led to this aquifer. In other words, we come to the conclusion that the aquifer of the basin of Thrapsano is the receiver of organic pollutants in big quantities mainly. The total average annual polluting organic load going to the area of the basin is being estimated about: 800,000 tons. After the examination of the hydrogeological conditions we come to the conclusion that the mechanism of diet and operation of the basin of Thrapsano ensures the enrichment of the aquifer with the direct filterings of hydrometeors, mainly in form of rain falling onto the surface of the basin, or as water flowing to the torrents beds, especially to the most important of all, the torrent of Baboulanis. These quantities of water enrich every year the aquifer of the basin. The regulative balances had been estimated and found: 5.9 x 106 m3 of water. It was also examined exhaustively and with a thorough hydrochemical study, the important and acute problem of pollution and contamination of the surface and ground water of the basin of Thrapsano. The hydrochemical study of the aquifer showed that pollution and contamination exists at the basin of Thrapsano on a large scale; in parallel, the ground water has the problem of the chemical balance of the contained calcium carbonate. So, it arose that: I. the presence of organic substances to the aquifer (measured as oxidatibility), it has exceeded the upper acceptable accumulation as defined by the valid specification (80/778 Directive of the European Communities Council) for potable water for all state-members of the European Union. II. the problem of the calcium carbonate instability to the ground water was located to the area of the mixed part of the aquifer. However, the natural processings react with the phenomenon of self-purification against the human intervention to the hydrological cycle and the quantitative water downgrading. Thus, to the surface water, a part of the polluting organic load is being splitted by aerobic microorganisms into simple components consequently the initial organic load is being accordingly eliminated. The same phenomenon of self - purification was being observed at the area of the alluvial aquifer. The continuous production of CO2 to the ground water from organic substances causes the problem of the calcium carbonate hyper-saturation mainly to the area of the mixed aquifer. This surplus of CaCO3 in the ground water is being removed under proper conditions. Therefore an explanation could be given for the intensive problem of precipitation at the system of water transfer and distribution to the city of Heraklion which takes water from the drillings of the basin of Thrapsano. Finally, it is suggested to be taken measures of rational exploitation and upgrading of the water potential of the basin in order to be given an end to the uncontrolled -up today- excessive or marginal exploitation according to the sites of the aquifer of the basin as well as to its continuous qualitative downgrading. To deal with the enormous problem of the water resources pollution and contamination in the area, it is highly necessary to set up the management of the pollution and contamination with protective measures and modern methods of waste treatment (solid and liquid) which are being discharged to the area of the basin.

Στη παρούσα διατριβή οι εργασίες που έγιναν, αποσκοπούσαν στο προσδιορισμό των υδρογεωλογικών και υδροχημικών συνθηκών που επικρατούν στην έκταση της λεκάνης Θραψανού, παράλληλα με την εξέταση του σημαντικού προβλήματος της ρύπανσης και μόλυνσης του υδατικού δυναμικού της, κυρίως του υπόγειου. Από την αξιολόγηση των στοιχείων της στρωματογραφικής διάρθρωσης των υφισταμένων γεωτρήσεων στην έκταση της λεκάνης, σε συνδυασμό με τα αποτελέσματα της διεξαχθείσης έρευνας και μελέτης της περιοχής, μπορούν γενικά να εξαχθούν τα ακόλουθα συμπεράσματα: ■ η περιοχή αποτελεί λεκάνη τεταρτογενών αποθέσεων, η έκταση των οποίων καθορίζει και τα όριά της ■ στην υπόγεια υδροφορία της λεκάνης συμμετέχουν σε μεγάλο τμήμα της και νεογενείς σχηματισμοί, γεγονός που επιτρέπει τον διαχωρισμό του υδροφορέα της σε αμιγή και μεικτό ■ ο υδροφόρος ορίζοντας της λεκάνης σε όλη την έκτασή του είναι ελεύθερος και ενιαίος ■ το υδατοστεγανό υπόβαθρο στην έκταση του αμιγούς υδροφορέα στο ΒΑ/κό τμήμα αποτελεί το ημιμεταμορφωμένο σύστημα Σχιστολίθων-Φυλλιτών-Χαλαζιτών και στο ΒΔ/κό η διάπλαση του φλύσχη ■ Στο μεικτό υδροφορέα το υδατοσταγανό υπόβαθρο σε όλο το νότιο και νοτιοδυτικό τμήμα της λεκάνης εντοπίζεται επίσης στη διάπλαση του φλύσχη. Στο δυτικό τμήμα, όπως έδειξαν τα αποτελέσματα των γεωφυσικών διασκοπήσεων, η διάπλαση του φλύσχη βυθίζεται προς δυσμάς και στο ρόλο του υδατοστεγανού υποβάθρου συμμετέχει η νεογενής υδατοστεγανή αργιλική μάργα ■ η σχετικά μικρή ταχύτητα ροής του υπόγειου νερού στον αμιγή υδροφόρο συνεπάγεται τη μεγαλύτερη παραμονή του, που σταδιακά μειώνεται στην έκταση του μεικτού υδροφόρου μέχρι τη μικρότερη τιμή στην θέση της φυσικής εκφόρτωσης από τη πηγή Μιγκιλίσι, με βάση τα αποτελέσματα της χρονολόγησης του υπόγειου νερού με Τρίτιο ■ οι πολυάριθμες πηγές ρύπανσης και μόλυνσης εντός και εκτός της λεκάνης και κυρίως οι ανεξέλεγκτες από τις αγροτικές καλλιέργειες, έχουν ήδη υποβαθμίσει το υδατικό δυναμικό του υδροφόρου σε οριακό σημείο μη αναστροφής ■ οι συνεχείς ρυπάνσεις και μολύνσεις έχουν υπερκεράσει τον αξιόλογο βαθμό αυτοκαθαρισμού του υδροφόρου και έχουν ήδη επηρεάσει, εκτός των άλλων και τη χημική ισορροπία του συστήματος CaCO3-νερό στο υπόγειο νερό, ειδικά στην έκταση του μεικτού υδροφορέα στον οποίο η παρουσία του νεογενούς ασβεστολίθου είναι καθοριστική στη προσφορά CaCO3 από τη διαλυτοποίησή του. Η χρήση του υπόγειου νερού από το τμήμα αυτό του υδροφόρου, έχει δημιουργήσει το έντονο πλέον πρόβλημα των αποθέσεων CaCO3 κυκλώματα μεταφοράς και διανομής νερού ■ οι περισσότερες εκτελεσθείσες γεωτρήσεις και κυρίως οι λειτουργούσες, δεν εκμεταλλεύονται όλο το πάχος του υδροφόρου, ενώ δεν συνδέεται η παροχή εκμετάλλευσης με τις υδραυλικές παραμέτρους του υδροφόρου ■ υφίσταται καθεστώς ανορθόδοξης διαχείρισης του δυναμικού του υδροφόρου, που εστιάζεται κυρίως στο κατακερματισμό του σε διαφορετικούς φορείς αυτοδιοίκησης, αλλά και στη παράβλεψη των κατά θέσεις ιδιαιτεροτήτων του, όπως δείχνουν οι προσδιορισθείσες ανεπίτρεπτες πολύ μεγάλες τιμές απωλειών φορτίου, τόσο του υδροφόρου, όσο κυρίως των γεωτρήσεων άντλησης. - Τέλος από τα πιο πάνω αναφερόμενα κρίνεται επείγουσα και επιτακτική η ενιαία διαχείριση του υδροφόρου με επανακαθορισμό ενιαίας και συνολικής εκμετάλλευσής του, με βάση τη προτεραιότητα της προστασίας του και την αξιοποίηση των υδατικών πόρων και στην ευρύτερη περιοχή της όμορης πλούσιας και βαθειάς καρστικής υδροφορίας των πλακωδών ασβεστολίθων με έργα πλέον αλληλοσυμπληρούμενα.