以TMVOC模擬空氣注入法

Abstract

國內自開放民營加油站設立後，加油站的數量大量增加。近年來土壤與地下水污染的控制與整治場址中，加油站所佔的比例亦逐漸提升，加油站污染途徑中，以油槽洩漏和輸油管洩漏機率相對大。現地地下水污染整治技術中，空氣注入法(Air Sparging, AS)屬於相當常見之整治方式。其原理為在地下水注入空氣，氣體與污染物接觸後使其揮發，此技術經常搭配氣體抽除法(Soil Vapor Extraction, SVE)使用。然而現今許多複雜地質條件使常見整治技術不易發揮，以細顆粒土壤來說，污染物在土層中不易傳輸而造成整治困難。本研究以上下分層的土壤作為對象，地表淺層為細顆粒土壤，深層為粗顆粒土壤。以加油站油品洩漏出發，使用數值模擬軟體TMVOC在上述地質條件下進行空氣注入法整治模擬。以四個操作變因進行討論，分別為空氣注入量的影響、氣體抽除量的影響、SVE井與洩漏點距離之影響以及不同AS井佈井方式，另有均質地質作為對照。研究結果顯示空氣注入量的提升有助於水相苯的整治，以水相苯的整治上，空氣注入量的影響性大於氣體抽除量的影響性，但是空氣注入量或與氣體抽除量的上升，對於非飽和層的整治無明顯幫助。SVE井越靠近洩漏點位置，由於加大了污染物蒸汽流動的壓力差與梯度，對於整治水相苯或石油碳氫化物(Total Petroleum Hydrocarbon, TPH)皆有明顯幫助。所有案例整治初期均發生污染範圍擴大的現象，需要留意風險，所有非均質土層案例在預定時間內皆沒有成功整治非飽和層。

The number of gas stations in Taiwan increased dramatically after the government lifted the ban on private ownership. However, the gas stations account for a large proportion of the soil and groundwater pollution sites. Leakage from storage tanks and pipelines accounts for the two major sources from which petroleum releases into the subsurface. Air sparging is one of the most commonly adopted in-situ remediation technology to clean up polluted groundwater in gas station sites. Its mechanism is to inject air into underground water where the dissolved constituents partition to vapor phase as they come into contact with the injected air. However, many pollution remediation technologies are difficult to be applied in cases of complicated soil structure. For example, it’s difficult for contaminants to transport within fine-grained soils. This research focuses on two-layered strata, with shallow layer of fined grained soil on top of groundwater table and a layer of sandy soil beneath. A numerical model software called TMVOC is used to simulate air sparging in a model site with the above mentioned geological condition. Four independent variables which controlled the effect of remediation are discussed separately. These variables include, the air injection rate, the vapor extraction rate, the distance between spill point and extraction wells, and the different AS well layout schemes. In addition, the remediation at a model site with homogeneous sandy soil was also as simulated for comparison. The results show that greater air injection rate contributes significantly to efficiency of remediation of aqueous benzene, but increasing air injection rate or vapor extraction rate are less beneficial to removing residual petroleum products in the vadose zone. As SVE well gets closer to the spilling point, the pressure gradient around the contaminants increases, which helps remediate dissolved benzene and total petroleum hydrocarbon (TPH) in practice. The plume expands in the beginning of the air sparging because dissolved constituents are unable to partition to vapor phase in a short time, which happens in all cases. It’s necessary to consider the risk of migrating of contamination during remediation. In all the cases with heterogeneous soil, the vadose zone cannot be cleaned up successfully within reasonable time.