定流量非水相液體於土壤中滲流之研究

Abstract

由於在低飽和度狀況下，土壤的導水度甚低，因此傳統的定水頭或變水頭量測方法，不僅耗費頗長時間，且量測準確度較低。導致試驗時間長以及精確度低的主要原因之一，是因為在滲流過程中，非飽和試體中各部分的含水比會同時改變以達到新的平衡狀態，同時造成各部份水水頭不能穩定，造成無法判斷實驗是否已達穩定狀態(Steady state)。 本滲透儀試驗將利用低流量之蠕動幫浦作為滲流液體循環裝置，強制流入和流出的液體量相等，使試體提早達到穩態滲流，並利用RETC程式預測滲透係數值，瞭解非水相液體於土壤中，在不同的張力狀況下其保持行為。試驗採用的土壤為渥太華砂及寶二水庫現地土樣；所採用的非水相液體為：九五無鉛汽油與柴油。 進行滲透儀實驗時，在相同的體積含水比之下，與空氣界面張力越大則其滲透係數越小；另外試驗土壤的顆粒越小，相對的滲透係數也會跟著越小。 以RETC程式預測液體之滲透係數值，較為準確的為滲透儀保持曲線所得之結果。在同一體積含水比下，本滲透儀試驗方法所得導水度k，也較其他試驗方法來的準確。

Direct measurement of hydraulic conductivity of unsaturated soils in the laboratory is difficult because not only the experimental setup is more complicated than that for measuring the hydraulic conductivity of saturated soils but the time for reaching steady state is often very long. Furthermore, the rate of inflow and outflow may not equalize for a long period of time since the water content within the specimen keeps varying with time to adjust to the imposed hydraulic gradient.

In this study a low flow rate peristaltic pump is used to force equal inflow and outflow rate of liquid permeating through the specimens to establish steady state flow condition. Hydraulic conductivity can thus be determined as soon as the water levels in the head monitoring pipettes are stabilized, which, theoretically, should take only a short period of time. The RETC program is used to predict the hydraulic conductivity with empirical equations by utilizing SWCC data. The soils used in the study were Ottawa sand and Bow-Shan local sand. The NAPLs used were 95 gasoline and diesel.

The results of permeability tests showed that the liquids with higher surface tension of air yield lower permeability at the given volumetric water contents. On the other hand, the smaller the soil particles, the lower the permeability.

The results also show that the hydraulic conductivity predicted utilizing SWCC data obtained during permeability test is more accurate. However, at any given volumetric water content, the hydraulic conductivity predicted of permeability tests are more accurate than the other tests.