雙井循環流場追蹤劑試驗常用來推估現地縱向延散係數，其基本假設為觀測井的濃度穿透曲線只受縱向延散影響。然而當溶質進入雙井循環流場而到達抽水井的過程中同時受到縱向及側向延散效應，進而影響觀測井的濃度穿透曲線。本研究目的為探討側向延散對雙井循環流場追蹤劑試驗濃度穿透曲線的影響，並探討在何種情況下可忽略側向延散，唯一決定縱向延散度。研究中利用FEMWATER建立一穩態雙井循環流場，在改變縱、側向延散度比與水力傳導係數異向比進行追蹤劑試驗。結果顯示雙井循環流場追蹤劑試驗濃度穿透曲線在垂直雙井連線方向大於雙井連線方向的水力傳導係數，當縱向延散較小時側向延散遞增會造成峰值濃度下降但峰值濃度到達時間不變；縱向延散較大時側向延散遞增，峰值濃度下降且溶質會受側向延散的影響而跨出原本的流線進入雙井間距離較短速度較快的流線造成峰值濃度的提前到達；當井距加大時，側向延散的影響也會越明顯。當雙井連線方向大於垂直雙井連線方向的水力傳導係數，在相同的縱向延散下側向延散的變化不影響其濃度穿透曲線。雙井循環流場追蹤劑試驗之濃度穿透曲線同時受到縱向及側向延散的影響，在垂直雙井連線方向大於雙井連線方向的水力傳導係數及縱向延散度較大有顯著的影響，故無法忽略側向延散；當雙井連線方向大於垂直雙井連線方向的水力傳導係數，且井距較小時，縱向延散才可唯一決定。 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.
