The mixing zone developed at freshwater-seawater interface is one of the most important features in complex coastal hydrogeologic systems, which controls subsurface flow and reactive transport dynamics. Freshwater-seawater mixing-zone development is influenced by many physical and chemical processes, such as characteristics of geologic formation, hydrodynamic fluctuations of groundwater and seawater levels, fluid-rock interactions, and others. Wide mixing zones have been found in many coastal aquifers all over the world. However, the mechanisms responsible for wide mixing zones are not well understood. In this thesis, two hypotheses were proposed to explain wide mixing zones in coastal aquifers: (1) kinetic mass transfer coupled with transient conditions, which create the movement of the mixing zone, may widen mixing zones; and (2) aquifer stratification may widen the mixing zone. The hypotheses were tested by both multiscale numerical simulations and laboratory experiments. Numerical simulations were based on a variable-density groundwater model by varying mass transfer parameters, including immobile porosity, mobile porosity, and mass transfer coefficient, and the hydraulic conductivity contrast between aquifer layers. Laboratory experiments were conducted in a quasi-two-dimensional tank, where real beach sands were installed and foodstuff dyes were used to visualize the development of freshwater-seawater mixing zone. Major conclusions included (1) the mixing zone can be significantly widened when the mass transfer timescale and the period of transient boundary is comparable due to the nonequilibrium mass transfer effects; and (2) a thick mixing zone occurs in low-permeability layer when it overlays upon a fast flow layer. These results not only improve the understanding of the dynamics of mixing-zone development and its associated geochemical processes in coastal aquifers, but also identify hydrogeologic conditions for the model of sharp-interface approximation to be valid. In addition to better understanding the mechanisms and dynamics of mixing zone, this thesis also investigates cost-effective management of coastal groundwater resources. To protect and conserve limited water recourses in coastal regions, interest in aquifer storage and recovery (ASR) has been growing in recent years. ASR is a promising strategy for water resources management and has been widely used in many contaminated and saline aquifers. However, its performance may be significantly constrained by mass transfer effects due to the mobilization of solutes initially residing in immobile domains. Better understanding of kinetic mass transfer effects on ASR is needed in order to aid the decision-making process. A numerical model is developed to simulate ASR performance by combining the convergent and divergent dispersion models with a first-order mass transfer model. By analyzing the concentration history at the pumping well, we obtain simple and effective relationships for investigating ASR efficiency under various mass transfer parameters, including capacity ratio and mass transfer timescales, and operational parameters. Based on such relationships, one can conveniently determine whether a site with mass transfer limitations is appropriate or not for ASR and how many ASR cycles are required for achieving a positive recovery efficiency (RE).

Philosophiae Doctor - PhD / The increasing population in search for better social and economic development in coastal areas puts groundwater resources under pressure because of the high fresh water demand for domestic and agriculture use. Seawater intrusion is widely recognised as major concern in coastal aquifers across the globe and is influenced by multiple factors, being climate variation which is projected to adjust recharge of groundwater because of decreased precipitation patterns and to increase sea level variations and over-abstraction due to high freshwater demand as a result of increased population and agricultural practices, thereby inducing salinization in groundwater. The coastal aquifer in Heuningnes Catchment is not exempted from these issues and salinization is a major concern in the area affecting groundwater quality. In Heuningnes Catchment and South Africa in general there is limited knowledge on the application of integrated approach for assessing groundwater quality especially salinization mechanism in coastal aquifers. The main goal of this research is to test and demonstrate the viability of using joint interpretation approach of geophysics, geochemical and geological information to investigate groundwater quality in coastal aquifers thus improving on the understanding of using such approach. This work offers the initial thorough assessment of groundwater quality and understanding of the salinity sources and hydro-geochemical processes governing the chemical composition of groundwater in the region. Thus provide advice on the fitness of this water for consumption and irrigation purposes. Thirty-two groundwater samples were collected and analysed for (Mg2+), (Ca2+), (Na+), (K+), (Cl-), (SO42), (HCO3-), (pH, TDS and EC). To estimate fitness of groundwater quality for consumption purposes WHO (2011) and SANS241 guideline were used and for irrigation utilization the water quality indices (EC), (Na+%), (SAR), (RSC), (KR) and (MR) were used. Statistics approaches were employed to ascertain the primary geochemical processes governing the chemical composition of groundwater in the research region. Lastly, the spatial distribution maps were created by means of ArcGIS. Electrical resistivity method was used to map the extent of saline distribution within the subsurface. The findings of this study revealed that groundwater in the region is alkaline type and TDS, EC, Na+, Cl- exceeded WHO and SANS241 guidelines for consumption water. The geophysical investigation revealed that the sandy clay/clayey sand, fine sand and fractured sandstone units make up the coastal aquifer systems within the area. Further, revealed that these aquifers were saturated with fresh, saline or brackish water depending on the subsurface layer. The presence of saline and brackish was confirmed by the chemical results which indicated a Na+-Cl- type as a dominant water type. Also classifying groundwater based on EC and TDS supports these findings. The foremost hydro-geochemical processes that controls the salinity and quality of groundwater in the study region as indicated by Gibbs plot are water-rock interaction followed by evaporation process. Furthermore, analysis of hydrochemical data also proposes that weathering of silicate minerals, ion-exchange and dissolution of carbonate minerals amended ion concentration in groundwater thus influencing salinisation in certain parts of the study region. Also assessment of ionic-ratios displays influences of marine sprays as well as seawater, on the chemical structure of groundwater within the Heuningnes Catchment aquifer. Valuation of groundwater quality and its fitness for ingestion and irrigation purposes, the results indicated that shallow groundwater in the area is not suitable for any use; however, groundwater from deeper boreholes was found fresh and appropriate for irrigation and household purposes. Findings of this study indicated that salinity is the major groundwater quality issue for this area and that monitoring of groundwater quality in Heuningnes Catchment is limited. The absence of consistent monitoring program on groundwater quality makes it difficult to ascertain long term trends on groundwater quality parameters. Therefore, this study emphasizes the need for regular groundwater quality monitoring to assess the trends of these parameters in order to make an informed decision as to what can be done for mitigation purposes.

<p>Fresh groundwater quality and availability in coastal areas is affected by seawater intrusion into coastal aquifers, and coastal water quality and ecosystem status may be significantly affected by groundwater pollutants that are transported into coastal waters by submarine groundwater dis-charge (SGD). This thesis uses an overall regional perspective for investigating: i) seawater intru-sion and its possible control in sustainable coastal groundwater management; ii) SGD and its relevant quantification as one interacting part among the diverse main regional pathways of freshwater and tracer/pollutant inputs from land to sea; and iii) the integrated system functioning of both i) and ii) as main components of the same coastal groundwater system.</p><p>Results show that intensive pumping rates may be maintained for a long time before major re-gional seawater intrusion problems are recognized by too high salinities in pumped groundwater. After such late recognition, pumping wells are no longer useful and a common strategy of mov-ing groundwater pumping further upstream from the coast only increases the extent of the salt-water intrusion zone into the aquifer. An alternative strategy may be to control seawater intrusion through artificial groundwater recharge, for instance by sufficiently treated wastewater, which may considerably reduce long-term trends of salinity increase in pumped groundwater, even for small artificial recharge rates compared to pumping rates. In general, account for natural spatial-temporal variability and randomness may be essential for relevant prediction of groundwater dynamics for management purposes. Spatial and temporal randomness effects, however, may not be additive, but rather largely overlapping, with either spatial or temporal randomness being the dominating part that must be accounted for in predictive groundwater dynamics calculations. Aquifer depth is identified as an important control parameter in this context, yielding much greater temporal randomness effects in shallow than in deep aquifers.</p><p>Combined simulation results suggest a simple, approximately linear regional relationship between total SGD and its hydrologically determined freshwater component. Tidal oscillation may signifi-cantly affect such linear dependence of steady-state SGD, but primarily for low SGD conditions. High SGD appears to depend mainly on a dominant freshwater component, which effectively counteracts density-driven flow of seawater into the aquifer and thus decreases also effects of sea-level oscillation on the seawater component of total SGD. Comparative analysis between different SGD estimation methods in different reported high-SGD regions of the world indicates possible anomalously large regional SGD estimation from tracer concentrations in coastal waters, by confusing different main pathways of groundwater flow and pollutant inputs to the sea.</p>

Made available in DSpace on 2016-08-19T17:47:12Z (GMT). No. of bitstreams: 1 LUIS ALFREDO LOPES SOARES.pdf: 9086971 bytes, checksum: 8228f0b447540ace59bc3dd69d26d6f0 (MD5) Previous issue date: 2004-04-26 / The salinization vulnerability of groundwater in the Itaquí-Bacanga area, caused by the super exploitation in the coastal aquifers usually creates the phenomenon of seawater intrusion, justified by the increasing of the freshwater/ seawater interface, which can bring a scenario of general contamination. Instead of using traditional methods for contamination evaluation, which are not applicable in this case, different methods must be applied using much more complex knowledges about hydrogeology and hydrochemistry that demand more tools for collecting hydrogeochemistry parameters. In the northwest coast of the island of São Luís, Itaquí-Bacanga area, some zones in the aquifer system are already seriously damaged by reasons explained above, causing deactivation of some wells through salinization. From the analysis of lithologic profiles, physicochemical parameters and integrating geotopographics and chemical datas, a vulnerability map was built, also considering historical features. / A vulnerabilidade à salinização das águas subterrâneas na área Itaquí Bacanga, consequência da super explotação de aquíferos costeiros provoca, em geral, o fenômeno de intrusão salina, decorrente do avanço da interface água doce/água salgada, podendo ocorrer uma situação de contaminação generalizada. O uso de metodologias tradicionais para avaliação de contaminação não se aplica a esta situação. Em consequência, utiliza-se metodologia distinta e bem mais complexa, envolvendo conhecimentos de hidrogeologia e hidroquímica, que necessitam de numerosas ferramentas para a obtenção de parâmetros hidrogeoquímicos. Na costa noroeste da ilha de São Luís, na área Itaquí Bacanga, o sistema aquífero encontra-se, em alguns setores, gravemente afetado por este fenômeno, já tendo ocorrido desativação de poços salinizados. A partir da análise de perfis litológicos, análises físico-químicas e de seções integrando dados geotopográficos e químicos, foi gerado um mapa de vulnerabilidade que considerou também situações históricas.

Fresh groundwater quality and availability in coastal areas is affected by seawater intrusion into coastal aquifers, and coastal water quality and ecosystem status may be significantly affected by groundwater pollutants that are transported into coastal waters by submarine groundwater dis-charge (SGD). This thesis uses an overall regional perspective for investigating: i) seawater intru-sion and its possible control in sustainable coastal groundwater management; ii) SGD and its relevant quantification as one interacting part among the diverse main regional pathways of freshwater and tracer/pollutant inputs from land to sea; and iii) the integrated system functioning of both i) and ii) as main components of the same coastal groundwater system. Results show that intensive pumping rates may be maintained for a long time before major re-gional seawater intrusion problems are recognized by too high salinities in pumped groundwater. After such late recognition, pumping wells are no longer useful and a common strategy of mov-ing groundwater pumping further upstream from the coast only increases the extent of the salt-water intrusion zone into the aquifer. An alternative strategy may be to control seawater intrusion through artificial groundwater recharge, for instance by sufficiently treated wastewater, which may considerably reduce long-term trends of salinity increase in pumped groundwater, even for small artificial recharge rates compared to pumping rates. In general, account for natural spatial-temporal variability and randomness may be essential for relevant prediction of groundwater dynamics for management purposes. Spatial and temporal randomness effects, however, may not be additive, but rather largely overlapping, with either spatial or temporal randomness being the dominating part that must be accounted for in predictive groundwater dynamics calculations. Aquifer depth is identified as an important control parameter in this context, yielding much greater temporal randomness effects in shallow than in deep aquifers. Combined simulation results suggest a simple, approximately linear regional relationship between total SGD and its hydrologically determined freshwater component. Tidal oscillation may signifi-cantly affect such linear dependence of steady-state SGD, but primarily for low SGD conditions. High SGD appears to depend mainly on a dominant freshwater component, which effectively counteracts density-driven flow of seawater into the aquifer and thus decreases also effects of sea-level oscillation on the seawater component of total SGD. Comparative analysis between different SGD estimation methods in different reported high-SGD regions of the world indicates possible anomalously large regional SGD estimation from tracer concentrations in coastal waters, by confusing different main pathways of groundwater flow and pollutant inputs to the sea. / QC 20101012

Η λεκάνη του ποταμού Γλαύκου εκτείνεται επί των δυτικών κλιτύων του Παναχαϊκού, στο ΒΔ/κό τμήμα του νομού Αχαΐας, NA/κά της πόλης των Πατρών καταλαμβάνοντας συνολική έκταση 101,67 Km2 και αποτελεί την κύρια πηγή κάλυψης των υδατικών αναγκών της. Χαρακτηρίζεται από μέτρια ανάπτυξη υδρογραφικού δικτύου δενδριτικού τύπου αποστράγγισης και κατά συνέπεια υψηλή διαπερατότητα των πετρωμάτων και σημαντική κατείσδυση, σε βάρος της επιφανειακής απορροής. Οι σπουδαιότεροι από τους χείμαρρους που διαρρέουν την περιοχή έρευνας είναι ο Ξηροπόταμος, ο Φίλιουρας, η Σούτα, του Βερβενίου και το Ελεκιστριάνικο, οι οποίοι συμβάλουν εντός του Γλαύκου. Το αλπικά πτυχωμένο υπόβαθρο της λεκάνης περιλαμβάνει μια ακολουθία προορογενετικών ιζημάτων (πελαγικοί ασβεστόλιθοι, ραδιολαρίτες, κ.α.) της ζώνης Πίνδου, και μια συνορογενετική κλαστική ακολουθία (φλύσχης), που αποτελείται από εναλλαγές ιλυολίθων, ψαμμιτών και σπανιότερα κροκαλοπαγών. Τα στρώματα της ζώνης Πίνδου αναδύθηκαν με την τελική φάση των πτυχώσεων στο Κάτω Ολιγόκαινο, όπου έλαβε χώρα η επώθηση της Πίνδου, υπό μορφή καλύμματος, πάνω στη ζώνη Γαβρόβου – Τριπόλεως. Τα μέτωπα των επωθήσεων μεταξύ των λεπιών και οι άξονες των πτυχώσεων έχουν διεύθυνση ΒΔ – ΝΑ με κατεύθυνση της κίνησης από τα ανατολικά προς τα δυτικά και άμεση σχέση με την πλειονότητα των καρστικών πηγών. Στην περιοχή του Πατραϊκού κόλπου επικρατεί ένα ρομβοειδές σύστημα ρηγμάτωσης με μεγάλης κλίσης και ΑΒΑ/κής και ΔΒΔ/κής διεύθυνσης κανονικά ρήγματα που δημιουργεί ένα μωσαϊκό κατατεμαχισμένων τεκτονικών μπλοκ και μια γενική ανύψωση της περιοχής που συνεχίζεται μέχρι σήμερα. Τη διάνοιξη των κοιλάδων της ευρύτερης περιοχής έρευνας, η οποία συχνά συνοδεύεται από κατολισθήσεις, την καθορίζουν 4 δέσμες νεοτεκτονικών ρηγμάτων, ΒΔ/κής, ΒΑ/κής, ΔΒΔ/κής, και ΒΒΑ/κής διεύθυνσης. Το Πλειοτεταρτογενές επικάλυμμα που εμφανίζεται στην παράκτια και λοφώδη περιοχή αποτελείται από δύο λιθοστρωματογραφικές ενότητες. Η κατώτερη συνίσταται από αργίλους και άμμους, που αποτέθηκαν σε ένα λιμναίο έως υφάλμυρο περιβάλλον ιζηματογένεσης κατά τη διάρκεια του Πλειόκαινου έως το Κατώτερο Πλειστόκαινο και η ανώτερη από Καλάβριας ηλικίας δελταϊκά και χερσαία κροκαλοπαγή. Γενικά οι προσχώσεις χαρακτηρίζονται από λιθολογική ανομοιομορφία τόσο κατά την κατακόρυφη όσο και κατά την οριζόντια διεύθυνση και το πάχος τους παρουσιάζει βαθμιαία αύξηση από τα βορειότερα και νοτιότερα όρια προς το κεντρικό τμήμα της λεκάνης, όπου και υπερβαίνει τα 200 μ. Το ετήσιο ύψος βροχόπτωσης (1071 mm) στην περιοχή έρευνας είναι αρκετά υψηλό για τα δεδομένα της χώρας μας και κυμαίνεται από 640 – 900 mm στις πεδινές περιοχές, ενώ στις ορεινές περιοχές φθάνει τα 1560 mm. Τα τελευταία χρόνια παρουσιάζεται μια εποχιακή μετακίνηση των βροχοπτώσεων από τον χειμώνα προς την άνοιξη και το φθινόπωρο, η οποία συνεπάγεται απώλεια του διαθέσιμου νερού της ενεργής κατείσδυσης, λόγω αυξημένης εξατμισοδιαπνοής. Από υδρολογικής άποψης η λεκάνη του Γλαύκου δέχεται ετησίως κατά μέσο όρο 108,89 106 m3 νερού από βροχόπτωση, εκ’ των οποίων ποσοστό 48,9 % επανέρχεται στην ατμόσφαιρα με τις διαδικασίες της εξάτμισης και διαπνοής, 28,6 % απορρέει επιφανειακά και 22,5 % κατεισδύει, κυρίως σε σημαντικούς από υδρογεωλογική άποψη σχηματισμούς (ασβεστόλιθους και τεταρτογενείς αποθέσεις). Χαρακτηριστικό υδρογεωλογικό γνώρισμα της περιοχής έρευνας αποτελούν οι ημιαυτόνομες υδρογεωλογικά καρστικές ενότητες, που αναπτύσσονται λόγω της λεπιοειδούς και ρηξιγενούς τεκτονικής στην περιοχή και διαφοροποιούν την κίνηση του υπόγειου νερού. Σχετικά με τις προσχωματικές αποθέσεις του Τεταρτογενούς ο ελεύθερος υδροφόρος ορίζοντας της ανώτερης ζώνης κάμπτεται προοδευτικά και μεταπίπτει σε επάλληλους υπό πίεση υδροφόρους ορίζοντες στην παράκτια ζώνη. Στις αρχές της δεκαετίας του ’90 στην παράκτια περιοχή, λόγω της υπεράντλησης, διαπιστώθηκαν αρνητικές τιμές της πιεζομετρικής επιφάνειας σε απόσταση έως 3 Κm από την ακτή, με συνέπεια την αλμύρινση του προσχωματικού υδροφόρου από τη διείσδυση της θάλασσας. Η μείωση των αντλήσεων στα μέσα της δεκαετίας του ’90 οδήγησαν στην άνοδο των πιεζομετρικών στάθμεων και στην επανεμφάνιση φαινομένων αρτεσιανισμού των γεωτρήσεων της παράκτιας ζώνης. Από την επεξεργασία των αντλητικών δεδομένων διαπιστώθηκε η εμφάνιση υψηλών τιμών του συντελεστή της υδραυλικής αγωγιμότητας τόσο στους προσχωματικούς υδροφόρους της παράκτιας ζώνης, όσο και στους καρστικούς σχηματισμούς. Μικρότερες ταχύτητες κίνησης του υπόγειου νερού εμφανίζουν οι υδροφόροι των αποθέσεων του Νεογενούς, αλλά και των προσχώσεων της νότιας όχθης του ποταμού Γλαύκου σε σχέση με αυτούς της βόρειας όχθης, λόγω της επικράτησης των λεπτομερών έναντι των αδρομερών υλικών. Ο σημαντικότερος παράγοντας τροφοδοσίας του προσχωσιγενή υδροφόρου της λεκάνης του Γλαύκου είναι η πλευρική τροφοδοσία κατά μήκος της κοίτης του ποταμού και εν’ συνεχεία αυτός της πλευρικής τροφοδοσίας από τους ασβεστόλιθους, ενώ λιγότερο σημαντικοί εμφανίζονται οι παράγοντες του απευθείας κατεισδύοντος νερού από τις βροχοπτώσεις και από το περίσσευμα του προς αρδευτική χρήση νερού. Με βάση το ισοζύγιο εισροών – εκροών των προσχωματικών υδροφόρων της λεκάνης του Γλαύκου, για τα υδρολογικά έτη 1999 – 2002 προέκυψε μέσο ετήσιο πλεόνασμα 6,5*106 m3 νερού. Γενικά, αν εξαιρεθεί η παράκτια ζώνη της λεκάνης του Γλαύκου, η ποιότητα του νερού των προσχωσιγενών υδροφόρων, καθώς και των νεογενών αποθέσεων, αλλά και των καρστικών σχηματισμών της λεκάνης, κρίνεται καλή. Στην πλειονότητά τους τα υπόγεια νερά, όπως και τα επιφανειακά και τα πηγαία, εμφάνιζαν τον τύπο Ca-HCO3, που περιλαμβάνει γενικά νερά με καλή τροφοδοσία και ανανέωση κυρίως κατά μήκος των αξόνων αποστράγγισης. Η χωροχρονική μετακίνηση της ζώνης υφαλμύρινσης στην παράκτια ζώνη, την περίοδο 1999 – 2002, διαπιστώθηκε εκτός από την αυξημένη κατανομή διάφορων ιόντων και με την χρησιμοποίηση των συντελεστών S.A.R., Revelle και Schoeller, αλλά και των λόγων rBr-/rCl- και rBr-/rI-. Στη διάρκεια της περιόδου 1999 – 2002, προέκυψε μια αισθητή μείωση της ρύπανσης από τη διείσδυση θαλασσινού νερού στην παράκτια περιοχή. Σημαντικό ρόλο στην επιτάχυνση του ρυθμού της αποκατάστασης των παράκτιων υδροφόρων της λεκάνης του Γλαύκου διαδραμάτισε η πληθώρα γεωτρήσεων αυτόματης ροής στην παράκτια ζώνη, οι οποίες παροχετεύουν περί τα 3,5*106 m3 ρυπασμένου νερού, σε ετήσια βάση, προς τη θάλασσα. Η θέση και η χωροχρονική μετακίνηση της ζώνης υφαλμύρινσης στην παράκτια ζώνη διαπιστώθηκε επίσης από γεωηλεκτρικές έρευνες που πραγματοποιήθηκαν με τις διατάξεις Schlumberger, Wenner – Schlumberger και Pole – Pole. Από τη γεωηλεκτρική έρευνα διαπιστώθηκε ότι η διείσδυση του υφάλμυρου νερού σπανίως εμφανίζεται με τη μορφή μετώπου, αλλά ακολουθεί επιλεκτικές οδούς μέσω των αδρομερέστερων σχηματισμών, ενώ οι λεπτομερείς αργιλικοί σχηματισμοί παίζουν τον ρόλο υδραυλικού φραγμού. Τέλος, με τη μέθοδο της σεισμικής ανάκλασης προσδιορίστηκαν η γεωμετρία και το βάθος συνάντησης του υποβάθρου στην παράκτια ζώνη. Επισημαίνεται η ανάγκη τεχνητού εμπλουτισμού του προσχωσιγενή υδροφόρου ορίζοντα με τις χειμερινές απορροές του Γλαύκου για την ταχύτερη αποκατάσταση της ποιότητας του νερού και την αύξηση του υδατικού δυναμικού της λεκάνης. Εφαρμόστηκε η μέθοδος DRASTIC για την εκτίμηση της τρωτότητας των υπόγειων νερών απέναντι σε εξωτερική ρύπανση, όπου με βάση τις τιμές του δείκτη DRASTIC κατασκευάστηκε χάρτης τρωτότητας της περιοχής. Περιοχές υψηλής τρωτότητας εντοπίζονται στο παράκτιο τμήμα του υδροφορέα, ενώ αντίθετα περιοχές μέσης τρωτότητας στο Α/κό τμήμα του υδροφορέα. Τέλος, κατασκευάστηκε μαθηματικό ομοίωμα των υδροφορέων της προσχωματικής λεκάνης του Γλαύκου με την εφαρμογή του κώδικα MODFLOW παρέχοντας έτσι ένα όσο το δυνατόν πιο αξιόπιστο πρότυπο διαχείρισης του υπόγειου υδατικού δυναμικού της περιοχή έρευνας. / The Glafkos’ river basin is extended on the west side of the Panaxaikon mountain in the northwest area of the prefecture of Achaia, southeast of the city of Patras, occupying a total extent of 101,67 Km2 and constituting the main source of Patras’ water needs. It is characterized by the mediocre growth of the hydrographical network (a dendrite type of draining) and consequently the high permeability of rocks and the important infiltration at the expense of the runoff. The most important torrents that flow through the region of research are Xiropotamos, Filiouras, Souta, Verveniou and Elekistrianiko, which contribute inside the Glafkos River. The Alpine folded basin’s basement includes a sequence of pre-orogenetic sediments (marine limestone, radiolarites, etc) of the Pindos zone, and a clastic orogenetic sequence (flysch), which is constituted by alternations of silt rocks, sandstones and rarely conglomerates. The layers of the Pindos zone emerged at the final phase of the folding tectonics in the Lower Oligocene, where the Pindos’ thrust occurred in the form of a nappe, above the Gavrovo – Tripolitsa zone. The foreheads of the thrusts between the slices as well as the axes of folds have a NW – SE direction, with the direction of movement from east to west, and have a direct relation to the majority of the karstic springs. In the region of Patraikos Gulf, what prevails is a rhomboid faulting system with high slope and ENE, WNW directed normal faults that create a mosaic of fragmentized tectonic blocks and a general rising of region that continues until today. The opening up of the valleys in the wider region of research, which is often accompanied by landslides, is determined from 4 bunches of neotectonic faults directed NW, NE, WNW and NNE. The Plio-quaternary cover that appears in the coastal and hilly region is constituted by two stratigraphical units. The lower one consists of clays and sands, which were deposited in a lacustrine to brackish environment of sedimentation during the Pliocene to the Lower Pleistocene and the upper one consists of Calabria dating from the deltaic and land conglomerates. Generally the alluvium deposits are characterized by lithological heterogeneity both at the vertical and the horizontal direction and their thickness presents a gradual increase from the northerner and southerner limits to the central department of the basin, which exceeds 200 m. The annual height of rainfall (1071 mms) in the region of research is quite high for our country’s data and fluctuates from 640 to 900 mms in flat regions, whereas in the mountainous regions it reaches 1560 mms. During the last years, what takes place is a seasonal displacement of rainfall from winter to spring and autumn, which leads to the loss of the available water of the active infiltration because of increased evaporotranspiration. From a hydrologic aspect, the Glafkos basin receives annually, on average, 108,89 106 m3 of the water from rainfall, from which 48,9 % comes back to atmosphere with the processes of evaporation and transpiration, 28,6 % rises on the surface and 22,5 % infiltrates, mainly in important, from a hydrogeological aspect, formations (limestones and quaternary depositions). The semi-independent hydrogeologically karstic units, which are developed because of the laminated and faulty tectonics in the region and differentiate the movement of groundwater, constitute a characteristic hydrogeological feature of the region of research. In relation to the alluvium deposits, the unconfined aquifer of the superior area is progressively bent and falls into successive under pressure (confined) aquifers in the coastal area. At the beginning of the ‘90s in the coastal region, because of the overpumping, negative values of the piezometric surface in a distance up to 3 Km from the coastline were realised, having as a consequence the salinity of coastal alluvium aquifer from the seawater intrusion. The reduction of pumping in the middle of the ‘90s led to the rise of piezometric levels and to the reappearance of the phenomena of artesian wells in the coastal area. Due to the elaboration of the pumping data, what was realised is the appearance of high values of the hydraulic conductivity’s factor in the alluvium aquifers of the coastal zone as well as in the karstic aquifers. The aquifers of the Neogene formations as well as the alluvium aquifers of the southern bank of the river Glafkos present a smaller speed of movement of groundwater as compared to those of the northern bank, because of the predominance of the fine-grained clastics. The most important factor of the recharge of the Glafkos’ basin alluvium aquifer is basically the costal recharge from the river bank and then follows the inductive recharge from the limestones, whereas the direct infiltrated water from the rainfall and from the surplus of the irrigation water seem to be the least important factors. Based on the balance of the inflows – outflows of the Glafkos’ basin alluvium aquifer, an average annual surplus of 6,5*106 m3 of water has resulted, during the hydrologic years 1999 –2002. Generally, the quality of water in the alluvium aquifers as well as in the aquifers of Neogene formations, but also that in the karstic aquifers is considered to be good, if the coastal area of the Glafkos’ basin is excluded. Most of the groundwater, as well as the surface and spring water, presented the geochemical type Ca-HCO3, which generally includes water of good recharge and renewal, mainly along the drainage axes. The time-space movement of the brackish zone in the coastal area, in the period 1999 –2002, was realised not only by the increased distribution of various ions but also by the utilisation of factors S. A. R., Revelle and Schoeller, as well as the ratios rBr-/rCl- and rBr-/rI-. During the period 1999 – 2002, a perceptible reduction of the pollution from the saline intrusion in the coastal region took place. The abundance of wells of automatic flow into coastal area, which drain 3,5*106 m3 of brackish water, annually, to the sea played an important role in the acceleration of the restoration of the coastal basin aquifers of the river Glafkos. The position and the time-space movement of the brackish zone in the coastal area was realised also by geoelectrical researches that took place with the dispositions Schlumberger, Wenner – Schlumberger and Pole – Pole. Geoelectrical research revealed that seawater intrusion rarely appears in the form of a forehead, but follows selective paths via the coarse-grained clastics, while the fine-grained clastic formations play the role of a hydraulic barrier. Finally, the geometry and the depth of the basement in the coastal area were determined with the application of the seismic reflection method. The need of artificial enrichment of the alluvium aquifer from the wintry flows of the Glafkos River is pointed out, for a more rapid restoration of the water quality and the increase of the Glafkos’ basin water potential. The DRASTIC method was applied for the estimation of the vulnerability of the groundwater towards exterior pollution, where based on the DRASTIC indicator’s values, a map of the vulnerability of the region of research was constructed. Areas of high vulnerability are located in the coastal part of the aquifer, while areas of medium vulnerability are located in the eastern part of the basin. Finally, a mathematic model of the Glafkos’ basin alluvium aquifers was constructed with the application of code MODFLOW, thus providing an as much as possible reliable model of the management of the Glafkos’ basin groundwater potential.

Seawater-fresh groundwater natural equilibrium conditions in the aquifers of the Sel&ccedil / uk sub-basin may be disturbed by the discharge occurred during the past 30 years in order to supply water for irrigation and domestic purpose usage. Two dimensional density dependent cross sectional saturated flow and solute transport simulations were carried out to determine whether seawater intrusion has occurred in the Sel&ccedil / uk sub-basin due to the imposed discharge or the determined salt-water in the western section of the aquifers represents natural interface equilibrium conditions. The numerical simulation model was calibrated using field measurements. The results of the simulations suggest that the seawater intrusion has occurred in the study area since the pumping activity increased in the region. Five scenarios were simulated for future predictions: (1) Present recharge and discharge conditions are maintained, (2) Discharge increases at a rate of the municipality need as a result of increasing population, (3) Discharge decreases by 12% from the present value, (4) Discharge decreases by 25% from the present value, and (5) No discharge occurs or recharge amount equals to the present discharge amount condition supplied to the aquifer. These scenario results suggest that seawater intrusion in the Sel&ccedil / uk sub-basin would progress in the following years unless proper management measures are taken into consideration. KEYWORDS: seawater intrusion, density dependent groundwater flow, solute transport, contamination, Sel&ccedil / uk sub-basin.

En contexte insulaire volcanique, les aquifères du domaine côtier présentent de très fortes hétérogénéités liées à leur structure, à la disparité de leurs recharges et à l'influence des conditions en aval. Les travaux envisagés ici consistent à élaborer une méthodologie adaptée à différentes configurations hydrogéologiques dans l'optique de mieux imager et comprendre l'évolution du biseau salé. L'acquisition des données est effectuée sur différents sites présentant des configurations hydrogéologiques variées sur le plan de la recharge, de la structure de l'aquifère et de la géomorphologie de la limite en aval. Un réseau d'observation est mis en place sur la côte ouest de l'île de La Réunion, dans la zone littorale allant de L'Ermitage à Saint-Leu. Cette zone est particulièrement intéressante dans le sens où elle présente les types de limites en aval les plus communes aux îles volcaniques à savoir, des côtes rocheuses, des plaines d'arrière-récif et des plaines d'accumulation fluviomarine. Sur La Réunion et la Grande Comore, le milieu volcanique est investigué au moyen de méthodes géophysiques et par une analyse de séries temporelles hydrogéologiques. Les outils géophysiques employés confirment la présence d’un milieu souterrain hétérogène, avec la présence sporadique de chenaux d'écoulement souterrain préférentiel susceptibles de contrôler la dynamique des intrusions marines. Les simulations numériques effectuées par la suite corroborent ces observations. L'analyse des signaux enregistrés indique une forte influence des forçages hydro-climatiques externes sur les nappes côtières. Elle permet aussi de distinguer deux familles de comportement selon le contexte géologique des ouvrages instrumentés. Les résultats de ces travaux mettent en avant une méthodologie de gestion des nappes côtières adaptée au milieu volcanique. Ils contribuent également à l'orientation des programmes de prospection de la ressource en eau souterraine dans ces contextes très complexes. / In volcanic islands context, the aquifers of coastal areas have very strong heterogeneities related to their structure, the disparity of their recharge and the influence of the downstream boundaries (rocky coasts, detritic cones...). The works considered here consist to develop a methodology adapted to different hydrogeological configurations in the aim to better understanding the development of saltwater intrusions. Time series recording and geophysical investigations are carried out on various sites, with different hydrogeological conditions, on the west coast of Reunion Island and throughout Grande Comore Island. The groundwater observatory tested in the pilot study area of Reunion was exported to three others experimental sites situated in Grande Comore Island (Indian Ocean). The geophysical investigations reveal a very important lateral heterogeneity along the shoreline due to the existence of 3D volcanic structures such as palaeovalleys filled by recent lava flows. In coastal areas, these structures exert an important role towards the marine intrusion dynamics, like it was confirmed by numerical simulations. Time series recording has allowed the hydrodynamic characterisation of the coastal groundwater and has enhanced the understanding of low and high frequency variations of the groundwater resources quality

Cette thèse porte sur l’étude d’un problème inverse de sources pour un modèle bidimensionnel d’intrusion saline. Dans un premier temps, nous nous intéressons à la modélisation du phénomène d’intrusion saline dans un aquifère côtier non confiné. En tenant compte des hypothèses particulières, nous obtenons dans le cas stationnaire une équation elliptique de la charge hydraulique dont le second membre est constitué des sources ponctuelles. L’étude du problème direct consiste à analyser le modèle dérivé et à établir un résultat d’existence et d’unicité d’une solution. Ensuite, dans la partie problème inverse, il s’agit de l’identification de termes sources à partir des mesures locales. Nous traitons les trois questions relatives à ce problème inverse, l’identifiabilité, l’identification et la stabilité. Concernant l’identification, nous formulons le problème inverse comme un problème de contrôle avec une fonctionnelle coût qui calcule l’écart quadratique entre les mesures expérimentales et celles obtenues par la résolution du problème direct. L’optimisation de cette fonction nécessite le calcul de son gradient que nous obtenons par la méthode de sensibilités et par la méthode de l’état adjoint. Quant à la stabilité, nous établissons deux types d’estimations, logarithmiques et lipschitziennes, pour les positions et les intensités de sources dans le cas de l’équation elliptique obtenue et en considérant toujours des mesures intérieures. De plus, nous avons généralisé les résultats des estimations lipschitziennes pour l’équation elliptique de la forme –Δu+k2u=F. La dernière partie de la thèse est destinée à montrer les résultats de l’identification numérique en fonction des paramètres intervenant dans le modèle principal. / This thesis deals with the study of an inverse source problem for a two dimensional seawater intrusion model. First, we focus on the modeling of the seawater intrusion phenomenon in a costal unconfined aquifer. Then considering some specific assumptions, we obtain, in the steady state, an elliptic equation of the hydraulic head with a left hand side formed by point wise sources. The study of the direct problem aims to analyze the derived model and to establish a result of existence and uniqueness of solution. The inverse problem concerns the identification of sources from local measurements. We are interested in the study of uniqueness, identification and stability.Concerning the identification, we transform the inverse problem to a control problem with a cost functional computing the quadratic error between the experimental measures and those obtained by solving the direct problem. To optimize this function, we need to compute its gradient and this can be done by the sensibility and the adjoint methods. Moreover, regarding the stability, we establish two types of estimates, logarithmic and lipschitz, for sources positions and intensities in the case of the elliptic equation assuming interior observations. Furthermore, we have generalized the results of Lipschitz estimates for the elliptic equation –Δu+k2u=F. The last part of the thesis is intended to show the results of the numerical identification based on parameters involved in the main model.

L'objet de cette thèse est d’étudier l'existence de solution pour une classe de systèmes d'évolution fortement couplés, ainsi que la limite singulière d'une équation aux dérivées partielles d'advection-réaction-diffusion.Au chapitre 1, nous d écrivons brièvement la dérivation d'un modèle d'intrusion saline pour des aquifères confinés et non confinés. Dans ce but nous nous appuyons sur la loi de Darcy et la loi de conservation de masse en négligeant l'effet de la dimension verticale.Au chapitre 2, nous considérons un système qui généralise le modèle d'intrusion saline dans des aquifères non confinés. C'est un système non linéaire parabolique dégénéré fortement couplé. Après avoir discrétisé en temps, gelé et tronqué des coefficients et finalement régularisé les équations, nous appliquons le théorème de Lax-Milgram pour prouver l'existence et l'unicité de la solution d'un problème linéaire associé. Nous appliquons ensuite un théorème du point fixe pour démontrer l'existence d'une solution du problème non linéaire approché. Nous obtenons de plus une estimation d'entropie, qui permet en particulier de démontrer la positivité de la solution. Finalement, nous passons à la limite dans le système et dans l'entropie pour prouver l'existence de solution pour le problème initial.Au chapitre 3, nous montrons l'existence de solution pour un système qui contient en particulier le modèle d'intrusion saline dans des aquifères confinés. Ce système est semblable au système du chapitre 2, mais la pression intervient comme inconnue supplémentaire. Il se rajoute la contrainte que la somme des hauteurs inconnues est une fonction donnée et la pression est en fait un multiplicateur de Lagrange associé à cette contrainte. Nous obtenons de nouveau une inégalité d'entropie et nous effectuons également une estimation sur le gradient de la pression.Au chapitre 4, nous nous intéressons à la description d'interfaces abruptes qui se déplacent selon un mouvement donné, par exemple le mouvement par courbure moyenne. Des singularités peuvent apparaître en temps fini ce qui explique la nécessité de définir une nouvelle notion de surface. Dans ce chapitre, on introduit la notion de "varifolds", ou surfaces généralisées, qui étendent la notion de "manifolds". A ces varifolds on associe une courbure moyenne généralisée ainsi qu'une vitesse normale généralisée.Au chapitre 5, nous considérons une équation d'advection-réaction-diffusion qui intervient dans un système de chimiotaxie-croissance proposé par Mimura et Tsujikawa. L'inconnue est la densité de population qui est soumise aux effets de diffusion et de croissance et qui a tendance à migrer vers des forts gradients de la substance chimiotactique. Quand un petit paramètre tend vers zéro, la solution converge vers une fonction étagée ; l'interface diffuse associée converge vers une interface abrupte qui se déplace selon un mouvement par courbure moyenne perturbé. Nous représentons ces interfaces par des varifolds définis à partir de la fonctionnelle de Lyapunov du problème d'Allen-Cahn. Nous établissons une formule de monotonie et nous montrons une propriété d'équipartition de l'énergie. Nous prouvons de plus que le varifold est rectifiable et que la fonction de multiplicité associée est presque partout entière. / The aim of this thesis is to study the existence of a solution for a class of evolution systems which are strongly coupled, as well as the singular limit of an advection-reaction-diffusion equation.In chapter 1, we describe briefly the derivation of a seawater intrusion model in confined and unconfined aquifers. For this purpose we combine Darcy's law with a mass conservation law and we neglect the effect of the vertical dimension.In chapter 2, we consider a system that generalizes the seawater intrusion model in unconfined aquifers. It is a strongly coupled nonlinear degenerate parabolic system. After discretizing in time, freezing and truncating the coefficients and finally regularizing the equations we apply Lax-Milgram theorem to prove the existence of a unique solution for the elliptic linear associated system. Then we apply a fixed point theorem to prove the existence of a solution for the nonlinear approximated problem. We obtain in addition an entropy estimate, which allows us in particular to prove the positivity of the solution. Finally, we pass to the limit in the system and the entropy in order to prove the existence of a solution for the initial problem.In chapter 3, we prove the existence of a solution for a system that contains in particular the seawater intrusion model in confined aquifers. This system is very similar to that introduced in chapter 2, only the pressure is a new unknown and we have the constraint that the sum of the unknown heights is a given function. The pressure is the Lagrange multiplier associated to the constraint. We obtain again an entropy estimate and we establish an estimate on the gradient of the pressure.In chapter 4, we are interested in the study of sharp interfaces that moves by a certain flow, by mean curvature flow for example. Singularities may occur in finite time which explains the necessity of having a differnet notion of surfaces. In this chapter, we introduce the notion of "varifolds" or generalized surfaces that extend the notion of manifolds. To these varifolds we associate a generalized mean curvature and a generalized normal velocity.In chapter 5, we consider an advection-reaction-diffusion equation arising from a chemotaxis-growth system proposed by Mimura and Tsujikawa. The unknown is the population density which is subjected to the effects of diffusion, of growth and to the tendency of migrating toward higher gradients of the chemotactic substance. When a small parameter tends to zero, the solution converges to a step function; the associated diffuse interface converges to a sharp interface which moves by perturbed mean curvature. We represent these interfaces by varifolds defined by the Lyapunov functional of the Allen-Cahn problem. We establish a monotonicity formula and we prove a property of equipartition of energy. We prove also the rectability of the varifold and that the multiplicity function is almost everywhere integer.