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

آب و خاک

Article 1, Volume 32, Issue 1 - Serial Number 57, March 2018, Pages 13-27   

Document Type: Research Article

DOI: 10.22067/jsw.v32i1.68090

Authors

Hojjat Ahmadi^* ; Mohammad Hemmati; Mehdi Motallebian

Urmia University

Abstract

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.

Keywords

Costal Aquifer; Saltwater Intrusion; Saltwater wedge; CTRAN/W; SEAWAT

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