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The quality of groundwater resources in coastal aquifers is affected by saltwater intrusion. Over-abstraction of groundwater and seawater level rise due to climate change accelerate the intrusion process. This paper investigates the effects of aquifer bed slope and seaside slope on saltwater intrusion. The possible impacts of increasing seawater head due to sea level rise and decreasing groundwater level due to over-pumping and reduction in recharge are also investigated. A numerical model (SEAWAT) is applied to well-known Henry problem to assess the movement of the dispersion zone under different settings of bed and seaside slopes. The results showed that increasing seaside slope increased the intrusion of saltwater by 53.2% and 117% for slopes of 1:1 and 2:1, respectively. Increasing the bed slope toward the land decreased the intrusion length by 2% and 4.8%, respectively. On the other hand, increasing the bed slope toward the seaside increased the intrusion length by 3.6% and 6.4% for bed slopes of 20:1 and 10:1, respectively. The impacts of reducing the groundwater level at the land side and increasing the seawater level at the shoreline by 5% and 10% considering different slopes are studied. The intrusion length increased under both conditions. Unlike Henry problem, the current investigation considers inclined beds and sea boundaries and, hence, provides a better representation of the field conditions.

This research investigated the effect of layered heterogeneity on transient saltwater upconing in a laboratory-scale coastal aquifer. The experiments were conducted in a 2D-laboratory flow tank, and the response of the saltwater wedge to pumping was analysed in a heterogeneous aquifer system, where a low permeability layer was constructed in the middle of the aquifer. The SEAWAT code was used for validation and to perform additional simulations to explore the sensitivity of the critical pumping rate and the critical time to the main parameters characterising the low-permeability layer, which included its permeability, thickness and position. The experimental results showed that the presence of layered heterogeneity noticeably altered the shape and the intrusion length of the upconing wedge without inducing a change in the abstraction rate “triggering” saltwater upconing mechanism compared to the homogeneous case. The numerical results of the layered aquifer provided excellent agreement with the experimental data for both the transient toe length and the shape of the steady-state saltwater wedges. The sensitivity analysis revealed that the critical pumping rate and the critical time was found to decrease considerably with decreasing hydraulic conductivity and thickness of the middle layer, which evidences the higher vulnerability of such layered aquifer systems to the saltwater upconing, in comparison to idealised homogeneous systems. The results nonetheless showed that varying the position of the interlayer induced very little change on the critical pumping rate, but the critical time would tend to decrease as the low permeability layer was moved deeper away from the pumping well, particularly for smaller middle layer thickness.

The Republic of Djibouti has an area of 23,000 km2, a coastline 370 km long and a population of 820,000 inhabitants. It experiences an arid climate characterized by high daytime temperatures and low and irregular rainfall (average of 140 mm/year), resulting in continuous periods of drought. These difficult climatic conditions and the absence of perennial surface water have progressively led to an intensive exploitation of groundwater to meet increasing water demands in all sectors (drinking water, agriculture and industries). In coastal areas, seawater intrusion constitutes a significant additional risk of groundwater degradation. This study is focused on the coastal aquifer of Tadjourah which supplies water to the city of Tadjourah, currently comprising 21,000 inhabitants. The main objective of this work is to assess the current resources of this aquifer, its capacity to satisfy, or not, the projected water demands during coming years, and to analyze its vulnerability to seawater intrusion within the frame of climate change. Three RCPs (Representative Concentration Pathway) were used to simulate different climate scenarios up to 2100. The simulated rainfall series allowed to deduce the aquifer recharge up to 2100. The code Seawat was used to model seawater intrusion into the aquifer, using the recharge data deduced from the climate scenarios. The results indicate that the risk of contamination of the Tadjourah coastal aquifer by seawater intrusion is high. The long-term and sustainable exploitation of this aquifer must take into consideration the impact of climate change.

TEZ12055 Tez (Yüksek Lisans) -- Çukurova Üniversitesi, Adana, 2019. Kaynakça (s. 63-66) var. XXV 67 s. :_res. (bzs. rnk.), tablo ;_29 cm. Deniz suyu girişimi kıyı bölgelerindeki yeraltı suyu kalitesine zarar veren önemli süreçlerden biridir. Bosaso Ovası’nda yeraltı suyunun aşırı çekimi deniz suyu girişimine neden olmaktadır. Yeraltı suyu modelleme çalışması ile Bosaso Ovası’na muhtemel deniz suyu girişimi etkileri çalışma konusu olarak alınmıştır. Bu araştırmada, SEAWAT yazılımı kullanarak farklı senaryoların deniz suyu girişimi üzerindeki etkilerini araştırmak amacıyla deniz suyu girişiminin potansiyel etkisini modellemektir. Analiz sonuçları, kıyı alanı boyunca elektriksel iletkenlik (EC), toplam çözünmüş katı madde (TDS), klorür, sodyum, kalsiyum ve magnezyum konsantrasyonunun yüksekliği ile çalışma alanında deniz suyu girişiminin bulunduğunu ortaya koymaktadır. Model 2013 yılı arazi verileri ile yeraltı suyu seviye ve TDS ortalama değeri kullanılarak kalibre edilmiştir. Yeraltısuyu çekim ve iklim değişikliğinin etkilerini araştırmak için dört farklı senaryo simüle edilmiştir. Sonuçlar, deniz suyu girişiminin sahil şeridine yakın yerlerde yüksek TDS seviyeleri ile yeraltı suyu konsantrasyonlarını önemli ölçüde etkilediğini göstermiştir. Artan yeraltısuyu çekim oranı deniz suyu girişimi arayüzünün 2040 yılına kadar yaklaşık 6,2 km kıyıdan içerilere doğru hareket edeceğini göstermiştir. Ayrıca, çekim oranındaki % 50'lik bir azalmanın, tatlı yeraltı suyunu koruyarak ve daha fazla deniz suyunun sızmasını önleyerek akiferlere rahatlama getireceğini ortaya koymuştur. Seawater intrusion is process that damage groundwater quality in the coastal regions. Groundwater overexploitation in the Bosaso Plain has resulted in seawater intrusion problem. Groundwater modelling were considered to study the effects of possible seawater intrusion into the Bosaso Plain. In this study, we present a three-dimensional variable–density numerical model developed with the SEAWAT code, which is aimed to predict the extent of seawater intrusion by different groundwater pumping scenarios. Results revealed the higher concentration of electrical conductivity (EC), total dissolved solids (TDS), chloride, sodium, calcium and magnesium along the coastal area which indicated the presence of seawater intrusion in the study area. The SEAWAT model was calibrated using both the average value of heads and TDS concentration for the 2013 field data. Four scenarios were simulated to understand the effects of pumping and climate change to the extent of seawater intrusion for a period of 27 years. The model results show that seawater intrusion significantly affects the groundwater concentrations near the coastline and demonstrated high levels of TDS. The predicted results reveal that increasing pump rate seawater intrusion interface will move about 6.2 km inland by 2040. Results also revealed that a 50% reduction in the pump rate will bring relief to the aquifer by protecting fresh groundwater and preventing any further seawater intrusion. This work was supported by Research Projects Center of Cukurova University Project No: FYL-2018-11190.

The objective of this research was to examine the response of seawater intrusion and retreat to freshwater abstraction from a well in laboratory-scale coastal aquifer under transient conditions. This has been done experimentally and numerically through qualitative and quantitative analysis. The laboratory experiments were completed in a two-dimensional laboratory tank for two beads sizes, namely 1090 µm and 780 µm. The SEAWAT code was used for the numerical simulations. The experimental results showed that the vulnerability of the pumping well to salinization was higher for the low permeability aquifer, whereby the saltwater upconing process was observed at an abstraction rate 40 % smaller in the lower permeability aquifer compared to the high permeability aquifer. In the lower permeability scenario, the inland penetration of the saline plume was up to 41% larger than in the higher permeability scenario, for an equivalent pumping rate increment. In addition, the process of decay (after the abstraction had ceased) of the wedge was slower in the lower permeability aquifer, which suggests a slower retreat of the wedge. The qualitative comparison of the shape of the saline plume and the quantitative comparison of the transient toe length data between experimental and numerical results showed excellent agreement. The flow velocity field analysis revealed that the local reduction of the magnitude of the flow velocity along the upper part of the interface was a major factor contributing to the saltwater upconing mechanism.

Διερευνώνται επιμέρους θέματα διαχείρισης παράκτιων υδροφορέων που αφορούν στην διασφάλιση της αειφορίας των υπόγειων αποθεμάτων μέσω (α) της προστασία τους έναντι στην υποβάθμιση της ποιότητάς τους που μπορεί να προέλθει από τη διείσδυση του θαλασσινού νερού και από ανθρωπογενείς δραστηριότητες και (β) της διασφάλισης της κάλυψης των αναγκών σε υπόγειο νερό κατά τις περιόδους αυξημένης ζήτησης. Η διερεύνηση βασίζεται στη χρήση δύο αριθμητικών μοντέλων υπόγειας ροής που λαμβάνουν υπόψη τις διαφορές πυκνότητας μεταξύ γλυκού και αλμυρού νερού και περιλαμβάνει τα ακόλουθα: Παρουσιάζεται ο τρόπος με τον οποίο διατυπώνονται και επιλύονται οι εξισώσεις που περιγράφουν την υπόγεια ροή υπό την επιρροή διαφορών πυκνότητας σε δύο ευρέως χρησιμοποιούμενα αριθμητικά μοντέλα, που είναι το μοντέλο SEAWAT-2000 (Langevin et al., 2003) και το μοντέλο SUTRA v2.2 (Voss & Provost, 2010). Βάσει των ανωτέρω και της εμπειρίας που αποκτήθηκε από την παράλληλη εφαρμογή τους, οι δύο κώδικες αξιολογούνται συγκριτικά και διατυπώνονται κριτήρια, στα οποία μπορεί να βασιστεί η επιλογή του κατά περίπτωση προσφορότερου κώδικα. Γίνεται η ρύθμιση αριθμητικού μοντέλου, το οποίο είναι βασισμένο στον κώδικα SEAWAT-2000, για τον παράκτιο υδροφορέα του Γλαύκου π., ο οποίος βρίσκεται στα νότια της πόλης των Πατρών και αποτελεί σημαντικό υδατικό πόρο για την περιοχή. Η ρύθμιση του μοντέλου βασίζεται σε μετρήσεις της υπόγειας στάθμης που προέρχονται από ένα σχετικά πυκνό δίκτυο γεωτρήσεων παρατήρησης το οποίο όμως έχει χρονικά περιορισμένη διάρκεια λειτουργίας. Το δίκτυο κατασκευάστηκε στα πλαίσια του προγράμματος INTERREG Ελλάδα – Ιταλία, 2000 – 2006, σε συνεργασία του Εργαστηρίου Υδραυλικής Μηχανικής του Πανεπιστημίου Πατρών και της Δημοτικής Επιχείρησης Ύδρευσης και Αποχέτευσης της Πάτρας (Δ.Ε.Υ.Α.Π.) και ολοκληρώθηκε στα μέσα του 2008. Η διαδικασία ρύθμισης του μοντέλου περιλαμβάνει τα εξής: (α) Τη συστηματική οργάνωση, συνδυασμό και αξιολόγηση των διαθέσιμων πληροφοριών σε λογισμικό συστημάτων γεωγραφικών πληροφοριών. (β) Την εφαρμογή υδραυλικών και στατιστικών μεθόδων σε συνδυασμό με την τροποποίηση και εφαρμογή μεθόδων αριθμητικής προσομοίωσης αντλητικών δοκιμών για τον προσδιορισμό των υδραυλικών χαρακτηριστικών του υδροφορέα. (γ) Τη συνδυαστική ανάλυση υδρολογικών δεδομένων και χρονοσειρών της υπόγειας στάθμης για την εκτίμηση των συνιστωσών του υδρολογικού ισοζυγίου. (δ) Τον καθορισμό κατάλληλων κριτηρίων αξιολόγησης των προσομοιώσεων. (ε) Την εμπειρική ρύθμιση των παραμέτρων του μοντέλου, η οποία επειδή έγινε παράλληλα με την συλλογή των μετρήσεων της υπόγειας στάθμης πραγματοποιήθηκε για δύο περιόδους, κατ’ αρχήν για την περίοδο 2008-2010 και εν συνεχεία για την περίοδο 2010-2012 και (στ) την εκτέλεση προσομοιώσεων Monte Carlo για την πραγματοποίηση καθολικής ανάλυσης ευαισθησίας (global sensitivity analysis, βλ. Saltelli et al., 2004) και τη διερεύνηση ύπαρξης περισσοτέρων συνδυασμών των τιμών των παραμέτρων του μοντέλου που οδηγούν σε παρόμοια αποτελέσματα (equifinality thesis, Beven, 2006). Από τη ρύθμιση του μοντέλου με τη βοήθεια των μετρήσεων της υπόγειας στάθμης που συλλέχτηκαν με το δίκτυο παρατήρησης προέκυψαν πληροφορίες για τους μηχανισμούς εμπλουτισμού του υδροφορέα στις διάφορες περιοχές, κάτι που είναι σημαντικό για τη διαχείρισή του. Το ρυθμισμένο μοντέλο του παράκτιου υδροφορέα του Γλαύκου χρησιμοποιήθηκε για την εξέταση θεμάτων διαχείρισης του υδροφορέα: (α) Προσδιορίστηκαν οι ζώνες τροφοδοσίας των γεωτρήσεων της ΔΕΥΑΠ με χρήση του κώδικα MODPATH v3 (Pollock, 1994) και εκτιμήθηκε η τρωτότητα αυτών των γεωτρήσεων. Διαπιστώθηκε ότι οι γεωτρήσεις που βρίσκονται κοντά στην κοίτη του Γλαύκου, όπως συμβαίνει με πολλές από τις γεωτρήσεις της ΔΕΥΑΠ, αντλούν σχεδόν αποκλειστικά νερό που προέρχεται από τον ποταμό. Έτσι είναι δυνατόν να προσβληθούν από ρυπάνσεις του νερού του ποταμού καθώς επίσης και από ανθρωπογενείς δραστηριότητες στις παρόχθιες ζώνες. (β) Εκτιμήθηκε ο κίνδυνος διείσδυσης θαλασσινού νερού σε περίπτωση εμφάνισης περιόδων ξηρασίας, για δύο διαφορετικά σενάρια αντλήσεων: ένα σύμφωνα με το σημερινό καθεστώς χρήσης του υπόγειου νερού και ένα για την αύξηση των αντλήσεων κατά 50%. Διαπιστώθηκε ότι για ξηρασία διάρκειας τεσσάρων ετών και αύξηση των αντλούμενων ποσοτήτων τα φαινόμενα υφαλμύρισης θα είναι έντονα. Με την επάνοδο όμως των μέσων υδρολογικών συνθηκών ο υδροφορέας ανακάμπτει. (γ) Για συνθήκες ξηρασίας εξετάστηκε η αποτελεσματικότητα του τεχνητού εμπλουτισμού για τον περιορισμό της διείσδυσης του θαλασσινού νερού. Ως μέθοδος εμπλουτισμού εξετάστηκε η εποχιακή αύξηση της στάθμης του νερού στον ποταμό με τη βοήθεια φουσκωτών φραγμάτων (βλ. Κωτσοβίνος, 1999). Διαπιστώθηκε ότι με τη μέθοδο αυτή μπορεί να επιτευχθεί σημαντική αύξηση των υπόγειων αποθεμάτων. Τέλος, εξετάζεται ως μέτρο προστασίας έναντι της διείσδυσης του θαλασσινού νερού η τεχνική των υπόγειων φραγμών. Επειδή στις μελέτες της τεχνικής αυτής που έχουν παρουσιαστεί στη βιβλιογραφία (π.χ. Luyun et al., 2011) έμφαση δίνεται μόνο στην επιρροή των γεωμετρικών χαρακτηριστικών των φραγμών (απόσταση από την ακτή, βάθος φραγμού), έγινε στην παρούσα εργασία συστηματική διερεύνηση της συναρτησιακής σχέσης ανάμεσα στην αποτελεσματικότητα της μεθόδου και τα χαρακτηριστικά τόσο των φραγμών όσο και του υδροφορέα (υδραυλική αγωγιμότητα, ανισοτροπία, υδρομηχανική διασπορά, παροχή γλυκού νερού προς τη θάλασσα, αντλήσεις στην παράκτια ζώνη, υλικό κατασκευής του φραγμού). Η διερεύνηση έγινε με τη βοήθεια του αριθμητικού μοντέλου SUTRA 2.2 (Voss and Provost, 2010), για δύο τύπους υπόγειων φραγμών: τους διαφραγματικούς τοίχους και τα υπόγεια φράγματα. Δίδονται διαγράμματα και αναλυτικές σχέσεις με χρήση αδιάστατων μεταβλητών και για εύρος τιμών των μεταβλητών αυτών που μπορούν να χρησιμοποιηθούν για τη διαστασιολόγηση φραγμών σε εφαρμογές πεδίου. Βάσει των αποτελεσμάτων που προκύπτουν από τη διερεύνηση της προαναφερθείσας συναρτησιακής σχέσης και χρησιμοποιώντας και το μοντέλο του Γλαύκου, εξετάστηκε η αποτελεσματικότητα της κατασκευής ενός φραγμού περιορισμένου μήκους στον υδροφορέα. Διαπιστώθηκε ότι πέραν του περιορισμού της διείσδυσης αλμυρού νερού σε περιόδους ξηρασίας που επιτυγχάνεται, ο φραγμός επιταχύνει την υποχώρηση της αλμυρής σφήνας όταν επανέλθουν οι συνήθεις υδρολογικές συνθήκες. In the present study coastal aquifer management issues are investigated. These issues concern measures which ensure the sustainability of the coastal groundwater and particularly: (a) protective measures against the degradation of groundwater caused by saltwater intrusion and human activities and (b) measures allowing the availability of sufficient volumes of fresh groundwater during periods of high demand. The investigation is based on the application of two numerical codes, which are suitable for simulating the groundwater flow under the influence of density differences. The investigation procedure is as follows. The application of the equations of groundwater flow with density differences and transport, the limitations and the advantages are presented for two widely used numerical models, the SEAWAT-2000 code (Langevin et al., 2003) and the SUTRA v.2.2 code (Voss & Provost, 2010). Based on the analysis above and on the experience acquired through the parallel use of the two codes, basic criteria are derived that allow for the selection of the code that best suits the needs of a certain problem. The investigation focuses on the Glafkos coastal aquifer, which is located at north Peloponnese (Greece), south of the city of Patras and is an important source of freshwater for the region. For this aquifer the SEAWAT-2000 code is implemented and calibrated. The model calibration is based on groundwater level time series that were registered by a relatively dense monitoring network, whose operation time, however, is limited. The network of the monitoring wells was constructed during 2007-2008 in cooperation of the Hydraulic Engineering Laboratory (Department of Civil Engineering, University of Patras) and the Municipal Enterprise of Water Supply and Sewage of Patras (DEYAP), in the frame of the project INTERREG IIIA GREECE-ITALY. The calibration procedure is based on: (a) The application of a G.I.S. system to organize, combine, analyse and evaluate the available information. (b) The application of hydraulic and statistical methods combined with the modification and application of pumping tests simulation methods, for the estimation of the hydraulic parameters of the coastal aquifer. (c) The combined analysis of hydrological data and groundwater level time series for the estimation of the aquifers water budget components. (d) The establishment of appropriate criteria for the evaluation of simulation results. (e) The empirical calibration of the model is performed for two periods, i.e. the period 2008-2010 and the period 2010-2012. This procedure is due to the availability of the groundwater level time series which were registered parallel to the model calibration. (f) The application of the Monte-Carlo method in order to investigate the probability that different combinations of model parameters give similar or better simulation results. The model calibration led to a better understanding of the aquifers recharge mechanisms which is crucial for the development of a groundwater management policy and the implementation of a management plan. The calibrated groundwater model of the Glafkos coastal aquifer is used for the investigation and evaluation of coastal aquifer management applications: (a) The capture zones of the municipal production wells are delineated by applying the MODPATH v3 code (Pollock, 1994). It is found out that the production wells that are located close to the Glafkos River, as it is the fact for the majority of the municipal production wells, pump water that originates almost exclusively from the river. Consequently, polluted water from the river or polluting human activities close to the riverbank can affect the quality of the pumped water. (b) The saltwater intrusion risk is estimated, in case of a prolonged drought period. Two cases are investigated; the first considers the current pumping rates of groundwater while the second considers a 50% increased pumping. It is found out that a four-year drought period combined with an increase of the groundwater exploitation will lead to significant saltwater intrusion problems. (c) The applicability and effectiveness of in-channel artificial recharge with the use of rubber dams, as a countermeasure against the saltwater intrusion, which may be induced by a four-year drought period, is investigated. It is found that the method is applicable and effectively reduces the intrusion of saltwater. Further it increases the groundwater storage in the aquifer. Finally, the construction of a cutoff wall which covers only a small part of the aquifers width, is evaluated as a countermeasure to saltwater intrusion problems which may arise in Glafkos coastal aquifer. Due to the fact that existing studies on the technique of the subsurface barriers focus only on the influence of the geometrical characteristics of barriers covering the whole width of the aquifer, a systematic investigation is curried out on the functional relationship between the effectiveness of the barriers and all the parameters influencing it, i.e. the geometrical characteristics of the barriers, the aquifer parameters (the hydraulic conductivity, anisotropy, hydromechanical dispersion, groundwater flow towards the sea) and the pumping rate. The investigation is curried out by the use of the finite element code SUTRA v.2.2 and concerns two types of barriers; the subsurface dams and the cutoff walls. The results include graphs and functional relationships for the assessment of the effect of subsurface dams and cutoff walls and the design of such structures. The results are presented in terms of dimensionless variables, with ranges suitable for field applications. Based on these results, a cutoff wall of small width is designed for the Glafkos coastal aquifer. Its effectiveness is evaluated by applying the calibrated SEAWAT-model of the coastal aquifer. It is shown that the cutoff wall not only reduces the saltwater intrusion during drought periods, but also it reduces the retreat time of the saltwater front under normal hydrological conditions.

Variable-density groundwater models require extensive computational resources, particularly for simulations representing short-term hydrologic variability such as tidal fluctuations. Saltwater-intrusion models usually neglect tidal fluctuations and this may introduce errors in simulated concentrations. The effects of tides on simulated concentrations in a coastal aquifer were assessed. Three analyses are reported: in the first, simulations with and without tides were compared for three different dispersivity values. Tides do not significantly affect the transfer of a hypothetical contaminant into the ocean; however, the concentration difference between tidal and non-tidal simulations could be as much as 15%. In the second analysis, the dispersivity value for the model without tides was increased in a zone near the ocean boundary. By slightly increasing dispersivity in this zone, the maximum concentration difference between the simulations with and without tides was reduced to as low as 7%. In the last analysis, an apparent dispersivity value was calculated for each model cell using the simulated velocity variations from the model with tides. Use of apparent dispersivity values in models with a constant ocean boundary seems to provide a reasonable approach for approximating tidal effects in simulations where explicit representation of tidal fluctuations is not feasible.