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نوع مقاله : مقالات پژوهشی

نویسندگان

دانشگاه ارومیه

چکیده

آبخوان‌های ساحلی منبع اصلی آب شیرین در بسیاری از مناطق دنیا می‌باشند. نفوذ آب شور بصورت یک زبانه از طرف دریا به زیر آب شیرین آبخوان باعث تخریب کیفیت این منابع می‌گردد. به علت این که مسائل تداخل آب شور بسیار پیچیده هستند و به طور کلی نمی‌توانند به صورت تحلیلی حل شوند، بنابراین روش‌های عددی ابزاری ایده‌آل برای شبیه‌سازی و پیش‌بینی نتایج می‌باشند. در این مطالعه دقت مدل‌های ریاضی CTRAN/W و SEAWAT در شبیه‌سازی و پیش بینی گوه‌ی آب شور در سه حالت الف) وضعیت جریان پایدار تحت گرادیان‌های مختلف ب) حالت گذرا تحت شرایط پیشروی گوه‌ی آب شور ج) حالت گذرا تحت شرایط پس روی گوه‌ی آب شور بررسی شد. نتایج شبیه‌سازی دو مدل با مشاهدات آزمایشگاهی گوسامی و کلمنت (8) مورد مقایسه قرار گرفت. به منظور مقایسه‌ی داده‌های واقعی و شبیه‌سازی شده از شاخص های آماری ریشه میانگین مربعات خطا(RMSE) ، معیار ناش-ساتکلیف(CE) ، ضریب همبستگی(R2)، نسبت اختلاف (r) و انحراف استاندارد عمومی (GSD) استفاده شد. نتایج نشان داد که مدل‌های CTRAN/W و SEAWAT دقت بالایی‌در شبیه‌سازی و پیش‌بینی موقیعت و حرکت گوه‌ی آب شور را با متوسط میانگین مربعات خطا به ترتیب معادل 67/0 و 58/0 سانتیمتر (کمتر از 10 درصد متوسط داده‌های واقعی) دارا می‌باشند. مدل CTRAN/W برآورد بهتری نسبت به مدل SEAWAT در محاسبه میزان دبی انتقالی از منبع آب شیرین به سمت منبع شور را نشان می‌دهد.

کلیدواژه‌ها

عنوان مقاله [English]

Assessment of Accuracy of CTRAN/W and SEAWAT Models for Prediction of Saltwater Wedge Under Intruding and Receding Conditions

نویسندگان [English]

  • Hojjat Ahmadi
  • Mohammad Hemmati
  • Mehdi Motallebian

Urmia University

چکیده [English]

Introduction: Coastal aquifers are major source of freshwater in many parts of the world. Saltwater intrusion is a serious environmental issue since 80% of the world’s population live along the coast and utilize local aquifers for their water supply.Under natural conditions, these coastal aquifers are recharged by rainfall events, and the recharged water flowing towards the ocean would prevent saltwater from encroaching into the freshwater region. However, over exploitation of coastal aquifers has resulted in reducing groundwater levels (hence reduced natural flow) and this has led to severe saltwater intrusion. Saltwater intrusion from the sea into below the freshwater of aquifer impairs the quality of these resources. Cause ofthe complexity of saltwater intrusion issues and generally they cannot be solved analytically, so numerical methods can be useful tools for simulation and prediction of salt water intrusion.

Materials and Methods: CTRAN/W is a finite element software product that can be used to model the movement of contaminants through porous materials such as soil and rock. The comprehensive formulation of CTRAN/W makes it possible to analyze problems varying from simple particle tracking in response to the movement of water, to complex processes involving diffusion, dispersion, adsorption, radioactive decay and density dependencies. SEAWAT is a three-dimensional variable density groundwater flow and transport model developed by the USGS based on MODFLOW and MT3DMS. SEAWAT is based on MODFLOW and MT3DMS. SEAWAT includes two additional packages: Variable-Density Flow (VDF) and Viscosity (VSC).In this study, the precision of CTRAN / W and SEAWAT models to simulation and prediction of saltwater wedge were investigated in three states: a) steady state salt-wedge data observed underdifferenthydraulic gradient conditions; b) transient salt-wedge data observed underintruding-wedge conditions; and c) transient salt-wedge data observed under receding-wedge conditions. Both models were initially calibrated and then the models were performed for the above conditions. The simulation results of the two models with the experimental results of Goswami and Clement (2007) have been compared. For comparing the measured data and simulated data, statistical indicators were used: root-mean-square error (RMSE), a measure of Nash-Sutcliffe (CE), the Correlation Coefficient (R^2), the ratio of difference (r) and the General Standard Deviation (GSD).
Results and Discussion: In this study, the precision of CTRAN / W and SEAWAT models to predict saltwater wedge wasinvestigated. At first step, both models were calibrated and the best values for longitudinal and transverse dispersion were obtained 0.5 and 0.05, respectively.Then simulation was performed with both models for all three modes(a- steady state salt-wedge data observed underdifferenthydraulic gradient conditions; b- transient salt-wedge data observed underintruding-wedge conditions; and c- transient salt-wedge data observed under receding-wedge conditions). The results showed thatCTRAN/W and SEAWAT models have high precision for simulation of position and movement of saltwater wedge in steady state with average of root mean square error (RMSE) equal to 1.05 and 1 cm, respectively and Both models have a higher estimate than the actual value for a steady state. As well as for transient state under the underintruding-wedge conditionsCTRAN/W and SEAWAT models have high precisionwith average of root mean square error (RMSE) equal to 0.65 and 0.44 cm, respectively and other statistical indicators were acceptable. The results of prediction of position and movement of saltwater wedgeunder receding-wedge conditionswith average of root mean square error (RMSE) equal to 0.54 and 0.56 cm, respectively provided acceptable estimates of both models. Finally, in order to determine the accuracy of the models in estimating the flow rate from the source of fresh water to the source of salt water, a comparison was made between the results of the models and the laboratory data, which showed that The CTRAN/W revealed appropriate estimation of amount of transferring discharge from freshwater reservoir to saltwater reservoir in compared with SEAWAT model. In general, according to statistical indicators, the results of both models were acceptable

Conclusion: The results showed thatCTRAN/W and SEAWAT models have high precision for simulation and prediction of position and movement of saltwater wedge with average of root mean square error equal to 0.67 and 0.58 cm (less than 10% of the average of measured data), respectively. The CTRAN/W revealed appropriate estimation of amount of transferring discharge from freshwater reservoir to saltwater reservoir in compared with SEAWAT model. In general, according to statistical indicators, the results of both models were acceptable.

کلیدواژه‌ها [English]

  • Costal Aquifer
  • Saltwater Intrusion
  • Saltwater wedge
  • CTRAN/W
  • SEAWAT
1- Abdelaty, I.M., Abd-Elhamid, H.F., Fahmy, M.R., Abdelaal, G.M.,(2014). Investigation of some potential parameters and its impacts on saltwater intrusion in Nile Delta Aquifer. J.Engineering Sciences Assiut University Faculty of Engineering. No. 4:931-955.
2- Bear, J. (1979), “Hydraulics of ground water, McGraw-Hill,” New York City.
3- Custodio, Ch. and Broggman, G. A., 1987, Groundwater problems in coastal aquifers, Studies and Reports in Hydrology, UNESCO, Paris, Vol. 45: 1-576.
4- C. W. Fetter Jr. 1972. Position of the saline water interface beneath oceanic islands. Water Resources research 8,no.5:1307-1315.
5- Dagan, G. and Bear, J. 1968. J. Hydrol. Res., v. 6:15-44.
6- Frind, E.O. 1982. Simulation of long-term transient density-dependent transport in groundwater. Advances in Water Resources 5, no. 6:73-88.
7- Goswami RR, Clement TP. Laboratory-scale investigation of saltwater intrusion dynamics. Water Resour Res 2007;43:W04418. http://dx.doi.org/ 10.1029/2006WR00515.
8- GLOVER, R.E. 1964. Ground water movement. U.S. Bureau of Reclamation Engineering Monograph 31:31-34
9- Galeati, G., G.Gambolati, and S.P.Neuman.1992.coupled and partially coupled eulerian-lageangian model of freshwater-seawater mixing. Water Resources research 28,no.1:149-165.
10- Huykorn, P.S., P.F. Anderson, J.W. Mercer, and H.O. White Jr. 1987. Saltwater intrusion in aquifers: Development and testing of a three-dimensional finite-element model. Water Resources research 23,no.2: 293-312.
11- Henry, H. R. (1960), Salt intrusion into coastal aquifers, Ph.D. thesis, Columbia University, New York.
12- Henry, H.R. 1964. Effects of dispersion on salt encroachment in coastal aquifers. Seawater in Coastal Aquifers. USGS Water-Supply Paper 1613-C: 70-84.
13- Illangasekare, T., et al. (2006), Impacts of the 2004 tsunami on groundwater resources in Sri Lanka, Water Resources., 42, W05201, doi:10.1029/ 2006WR004876.
14- Johannsen, K.; Kinzelbach, W.; Oswald, S.; Wittum, G., (2002). The salt-pool benchmark problem – numerical simulation of saltwater upconing in a porous medium. Adv. Water Resour., 25(3): 335-348
15- Konz, M.; Younes, A.; Ackerer, P.; Fahs, M.; Huggenberger, P. Zechner, E., (2009). Variable-density flow in heterogeneous porous media – Laboratory experiments and numerical simulations. J. Contam. Hydrol., 108 (3-4):168-175
16- Kolar R. L, Kibbey T.C.G , Szpilka C, Atkinson J.H. 2009. Process-oriented tests for validation of baroclinic shallow water models: The lock-exchange problem. Ocean Modelling 28(1-3):137-152 ·
17- Luyun, R.J.; Momii, K.; Nakagawa, K., (2009). Laboratory-scale saltwater behavior due to subsurface cutoff wall. J. Hydrol., 377(3-4): 227-236
18- Mualem, Y., and J. Bear (1974), The shape of the interface in steady flow in a stratified aquifer, Water Resoures., 10(6):1207– 1215.
19- Noorabadi S, . Nazemi A.H, Sadraddini A.A., Delirhasannia R. 2017. Laboratory investigation of water extraction effects on saltwater wedge displacement. Global J. Environ. Sci. Manage., 3(1): 21-32, Winter 2017. DOI: 10.22034/gjesm.2017.03.01.003
20- Oswald, S.E.; Kinzelbach, W. (2004). Three-dimensional physical benchmark experiments to test variable-density flow models. J. Hydrol., 290(1-2):22-42
21- Rezaee Pazhand, H. 2001. Application of probability and statistics in water resources. Sokhan Gastar publication, 456p (In Persian).
22- Strack ODL. A single-potential solution for regional interface problems in coastal aquifers. Water Resour Res 1976;12:1165–74.
23- U.S. Geological Survey (USGS) (2000), Groundwater resources for the future—Atlantic Coastal Zone, Fact Sheet 085-00, Reston, VA.
24- Werner AD, Bakker M, Post VAE, Vandenbohede A, Lu C, Ataie-AshtianiB, Sim-monsCT, Barry DA. Seawater intrusion processes, investigation and management: Recent advances and future challenges. AdvWater Resources. 2013;51: 3–26.
25- VanLopik J.H, Hartog N, Zaadnoordijk W.J, Cirkel D.G, Raoof A.2015. Salinization in a stratified aquifer induced by heat transfer from well casings. Advances inWater Resources 86 (2015) 32–45.
26- Xue, Y., Xie, C. and Wu, J. (1995). A three dimensional model for seawater intrusion in China. Water Resour.Res., No. 4:903-912.
27- Zhang, Q., R. E. Volker, and D. A. Lockington (2001), Influence of seaward boundary condition on contaminant transport in unconfined coastal aquifers, J. Contam. Hydrol., 49(3– 4):201– 215.
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