JWSS - Journal of Water and Soil Science

Deterioration of groundwater resources in coastal regions due to the progression of saline water in aquifers in these regions is currently one of the important issues in providing water needs in these areas. In coastal regions, saline water enters the aquifer from below in shape of wedge. Due to the difference in the density between fresh and salty water, an interface zone forms between two fluids. In order to better understanding the importance of this issue, experiments and numerical investigations of density-depended flow and transport through a tank filled with a variety of sand, are great help in achieving this. In this research, the real sand tank was simulated using SUTRA model. This simulation includes configuration, discretization, property assignment and boundary conditions determination. Finally, the transverse macro-dispersivity coefficient was estimated for different scenarios of the solute transport in this tank. The purpose of this research is to analyze of the solute dispersion, in mixing salt and fresh water, and the effect of seepage velocity, concentration of pollutant source and heterogeneity of porous media on the flow dispersivity property. In this research, after studying the effect of different boundary conditions in SUTRA model on the development of the salt water plume, simulation of the model of heterogeneous sand tank and comparing its results with laboratory model and homogeneous model were performed. As a practical result of this research, the diagram of changes in the coefficient of transverse dispersivity against the source concentration and seepage flow velocity was plotted. In numerical simulation of heterogeneous Porous media, for all concentrations, with the exception of the concentration C₀= 35000, with increasing flow velocity, the values of the transverse dispersivity coefficient AT calculated by SUTRA decreased. Also AT for all seepage velocities, with the exception of seepage velocity u=4 m/day, increased with increasing source concentration. Also, the values obtained AT from the SUTRA model were more than the values of AT obtained from experiments. In numerical simulation of the homogeneous porous media, for all velocities, as the concentration source C₀ increases, the transvers macro dispersivity coefficient AT increases. According to the applied results, suitable solutions can be found to improve the quality of groundwater and prevent the mixing of fresh and saltwater resources.

Nowadays, with the increasing population in Iran, especially in arid and semi-arid areas, as a result of the growing importance of the quality of water resources, including groundwater, field experiments and many simulations have been conducted for the development of groundwater contamination through powerful and up- to- date software. However, in most cases, there is a tangible difference between the measured data in laboratories and the data produced with software; this is why the scientific validation and verification of the research results could be declined. In this study, in order to justify and correct these data, the calibration principle was used to minimize the error of testing and modeling. The purpose of this study was to validate and verify the SUTRA model for different scenarios of the solute transport in a sand tank with heterogeneous hydraulic conductivity to evaluate transverse dispersivity. In this study, coding was initially performed for the configuration and calibration of the SUTRA numerical model to simulate different scenarios of the solute transport in a heterogeneous sand Tank in the Hydraulic Laboratory of the University of Kassel, Germany, until acceptable values were obtained. Then the results were compared with the experimental model. In order to validate and verify the data obtained from the simulation with the SUTRA model, the relevant concentration profiles were compared with the results of the experimental model. The results of the numerical and laboratory models revealed the density effects by sinking the geometric center of the mixing zone for the low concentrations of salt, C₀ = 250 ppm. The results also showed that the width of the mixing zone between salt and fresh water depended on the amount of longitudinal dispersivity, especially the transverse dispersivity. By analyzing the results of simulation and experiment, it was observed that with increasing the velocity, reducing the amount of sinking and raising the input concentration, the time needed to achieve the steady dispersion was decreased.