Numerical Simulation Of Subsurface Flow And Groundwater Vulnerability Assessment In Songhuajiang River Valley

This research is proposed to formulate a new approach that used for the first time for quantitative groundwater vulnerability assessment (GVA) through GIS programs, based on visual MODFLOW as comparative technique; evaluate river-aquifer interaction and assess the effect of riverbed conductance on river leakage using the natural field conditions instead of hypothetical models that used in the pervious researches.Visual MODFLOW computer code was selected to simulate heads distribution, calculate the zone budgets of the area, evaluate the effect of external stresses on the groundwater head, to determine how much water would be diverted from the river as the result of pumping and to demonstrate how the groundwater model can be used as a comparative technique to formulate a new approach for the quantitative groundwater vulnerability assessment in order to optimize utilization of the resource.The model was calibrated using trail-and-error technique. The most important criteria that used to check the calibrated model are Root Mean Square error (RMS), the Mean Absolute error Normalized RMS error, and mass balance. The calibration of flow model is acceptable with average RMS of 0.7m, residual mean of - 0.045 m, average absolute mean error of 0.1 m and Normalized RMS value of 2.3%. The contour map of the simulated heads, elaborated acceptable model calibration compared to observed heads map. Sensitivity analyses showed that the model is more sensitive to hydraulic conductivity and least sensitive to specific storage and more sensitive to recharge factor compared to the hydraulic conductivity. A time length of ten years and the calculated heads of 1997 were used for model prediction. The predictive simulation showed that the continued pumping will cause relatively high changes in head distribution and increase average drawdown to 16% after 10 years.The river package of visual MODFLOW code was used to understand the interaction between Songhuajiang River and the nearby aquifer and the effect of riverbed conductance on river leakage. Induced infiltration, reduced baseflow, river losing and river gaining are the main factors used to demonstrate these phenomena. Changing of the riverbed conductance provides high change in river gaining rate compared to change in river losing rate. This difference can be attributed to aquiferanisotropy, river penetration, hydraulic balance between groundwater head and river stage.A new approach of GVA that formulated to relatively quantify the groundwater depletion risk was evaluated with visual MODFLOW results to determine the accuracy of the vulnerability maps. There are very close acceptable match of the groundwater vulnerability index map results with visual MODFLOW simulation results. The GVA method classifies 27% of the study area as having low vulnerability to depletion risk, 46% as having medium vulnerability to depletion, whereas 27% as having very high vulnerability to depletion. The low vulnerability areas were conformed to high productive zones whereas the high vulnerability areas were coincided with heavy pumping well fields those indicated from visual MODFLOW results respectively.Generally this research successfully formulated a new approach of quantitative GVA, confirmed that visual MODFLOW can be used successfully as a comparative technique for quantitative groundwater vulnerability and evaluated the effect of riverbed conductance on river leakage.