台灣地區掩埋場覆蓋層採用蒸發散覆蓋的可行性研究

Abstract

由於台灣地區降雨量分布不均，南北差異甚大，雖然總雨量頗高，但實際上乾溼兩季分明且中部及南部地區年降雨量較少類似半乾旱氣候。故本研究將考慮台灣地區的氣候條件，輸入北部(淡水)、中部(台中)、南部(台南)的歷史氣象資料，以蒸發散覆蓋為覆蓋層型式，控制地表植生條件，透過蒸發散的概念將入滲水排除達到減少水流通過覆蓋層底層的下滲。本研究以HELP模式及HYDRUS-1D模式進行數值模擬，來計算水流通過覆蓋層底層的下滲量。其中HELP模式為擬二維的分析方式，該模式僅考慮水文平衡沒有考慮土壤保水特性與非飽和土壤導水度的關係；而HYDRUS-1D模式雖是描述土壤的一維的水流下滲行為，但分析上考慮土壤保水特性與非飽和土壤導水度的影響。分析結果顯示，單一土層類型有低水力傳導性並且有高的貯水能力，使單一土層覆蓋增加貯水容量，提高蒸發散量以限制水流的下滲。而單一土層蒸散覆蓋的土層厚度取決於降雨量及土壤的貯水容量。毛細阻隔層類型上層為細顆粒層下層為粗顆粒層，結合了單一土層類型的特性，並藉由毛細作用力將水流限制在細顆粒層以減少下滲至粗顆粒層。

Alternative covers of landfills have been proposed and permitted on a case by case basis in the United States. Researches over the past decade indicated that evapotranspiration (ET) cover with either monolithic layer or capillary barrier have been proven to be better or equivalent to traditional RCRA Subtitle D covers. A number of researches and site demonstration projects showed that ET cover can also effectively minimize percolation and meet the requirement of the cover system in regions with wet climate. Thus ET covers may be suitable for the landfills in southern Taiwan where evapotranspiration is greater than precipitation and rainfall often occurs in very short periods. In this study, the effectiveness of ET covers with monolithic fills and capillary barrier has been investigated by computation by Visual HELP and HYDRUS-1D with historical meteorological data representing the northern (Tamsui), central (Taichung), and southern (Tainan). The results show that monolithic covers of a single layer of fine-grained soil with low hydraulic conductivity and a high storage capacity can be quite effective in deterring infiltration. The design of these properties allows monolithic covers to manipulate the water balance by increasing the storage, evaporation, and limiting percolation. The amount of precipitation and the storage capacity of the soils are the critical factors for determining the thickness of these covers. On the other hand, capillary barriers consist of a layer of fine-grained soil underlain by a layer of coarse soil are also very effective by limiting infiltration with the low relative conductivity of the coarse layer.