| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 93 === According to the domestic demand and the suggestions by local experts, three sets of groundwater models, MODFLOW+MT3DMS, AT123D and FEMWATER, were selected and used to simulate pollution conditions. The groundwater models should be able to comprehend hydrogeological characteristics and pollutant data for parameter inputs. During the process of simplifying the actual complex conditions reasonably, the advantages and restrictions of models can be assessed. The sensitivity analysis of parameters was used to assess the suitability and accuracy of these models. Considering the control standards for groundwater as a baseline, the research delimited the contaminated area of the control site to help the following remediation work on the site.
For comparison, MODFLOW can be only used to simulate the groundwater flow, and other models should be used to simulate the pollutant transport. MT3DMS can only be applied to simulate the transport process, which the transport mechanism is advection-dispersion and have higher accuracy on the simulation for oil pollution. AT123D assumes that the groundwater is stable and horizontal, so that it can be used to simulate the transport of the pollutants in a simple hydrogeological condition, inducing a rough estimation for the polluted area. FEMWATER can simulate various density-drawn flow conditions and has higher accuracy on the irregular boundary, but most of advanced chemical and biological dynamics is not available, and it is relatively complicated to operate.
In this research, the area of the control site is 1400 m 900 m and the main pollutant is “benzene”. At this control site, hydraulic conductivity is 2.55 m/d, effective porosity 0.3, bulk density 1700 kg/m3, longitudinal dispersivity 100 m, and mass-loading rate 0.088 kg/d (or source concentration 8800 mg/l). All of the simulation results had a better fit to the true condition when simulation period was four years. The position of the source was speculated to be in the gas station and the most serious pollution occurred in the northern area of the gas station.
The simulation by MODFLOW+MT3DMS indicated that pollutants transported from the north to south. Whereas AT123D indicated that pollutants spread eastward. FEMWATER indicated that pollutants spread northward. The main reason causing the differences of transport direction is that MODFLOW +MT3DMS can establish heterogeneous hydraulic conductivity, but FEMWATER and AT123D can only establish hydraulic conductivity homogeneously, especially AT123D only with the ability of simulating steady state and horizontal flow. When the Type II Groundwater Control Standard for benzene, 0.05 mg/l, is used as the baseline, the polluted area simulated by FEMWATER is the smallest, compared with the results simulated by MODFLOW+MT3DMS and AT123D.
The sensitivity analyses by three models show that bulk density does not affect the output. Hydraulic conductivity is the most important sensitive parameter for MODFLOW. Effective porosity influences tremendously on the simulation of MODFLOW+MT3DMS, and both Hydraulic conductivity and longitudinal dispersivity affect more the simulation results of FEMWATER. The output results by AT123D also indicate that effective porosity is a tremendous influential parameter. The output results of three sets of models all vary with the change of mass-loading rate (sink/source concentration) proportionally.
By comparing the simulation results with the observed values, and examining the relative errors and accumulating errors, it is concluded that MODFLOW+MT3DMS gives the closest simulation results to the real field. Considering the applicability, limiting conditions, simulation results of contaminated area and friendly operation, MODFLOW+MT3DMS is the most suitable simulation tool in this research.
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