| Summary: | 碩士 === 國立中央大學 === 應用地質研究所 === 99 === Two well dipole flow tracer test have been proposed as an in situ method to determine the longitudinal dispersivity from analysis of the breakthrough curves (BTCs) in the extracted well by assuming that only the longitudinal dispersion has an effect on the BTCs. However, the tracer must undergo transverse dispersion before it enters the extracted well and transverse dispersion will certainly influence BTCs in the extracted well. The purpose of this study is to investigate the effect of the transverse dispersion on the BTCs in the extraction well during a horizontal dipole flow tracer test and to examine the applicable condition of a two well dipole flow tracer test in determining the longitudinal dispersivity. Breakthrough curves were numerically generated by using the FEMWATER. In this investigation the steady state flow field is considered, additionally the transient transport equation is used to describe the moving of tracer. Simulation results demonstrate that the transverse dispersion exerts significant effects on the BTCs in the extraction well in an aquifer with a large hydraulic conductivity anisotropy ratio and a large longitudinal dispersivity. The selection of operational parameters including distance, interval distances into well has significant influme on the applying condition of the two well dipole flow tracer test for determining the longitudinal dispersivity. It suggests that longitudinal dispersivity cannot be solely determined by analysis of the BTCs, simply because the BTCs are simultaneously affected by both the longitudinal and transverse dispersions. One should note that the two well dipole flow tracer test can only be applied to evaluate the longitudinal dispersivity in an aquifer with a small hydraulic conductivity anisotropy ratio or a small longitudinal dispersivity.
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