雲林濱海地區地下水之海淡水交界面調查與應用規劃

Approaches to Observe Salt-Fresh Water Interface and Apply to Design Remediation System in Coastal Area

吳育生(Yu-Sheng Wu) ； 周晏勤(Yen-Chin Chou) ； 洪珮瑜(Pei-Yu Hung) ； 周瑋珊(Wei-Shan Chou)

《土壤及地下水污染整治》 5卷1期 (2018/01) Pp. 37-54

https://doi.org/10.6499/JSGR.201801_5(1).0004

摘要

濱海地區的地下水，常因海水與淡水密度的差異，形成一個楔形的海淡水交界面（salt-fresh water interface），交界面的位態主要受到地下水水位高低與海淡水的密度差所控制，並因為潮汐及豐、枯水期地下水水位高低的關係而成為一延散帶（dispersion zone）。延散帶中幾乎不具趨勢水流（potential flow），只有密度流，地下水流速極慢，地下水如果有污染，極易在此處造成污染團的累積。受到潮汐漲退的影響，交界面在濱海潮間帶的出露處出流面（outflow face）附近，在低潮位時，方具有較大的趨勢水流，具有較大的地下水流速，如果是溶解性的污染物，特別是比水輕、難溶於水的物質，地下水滲流才會較易將污染物帶入海域水質。海淡水交界面及延散帶的位態，是濱海地區地下水污染場址現地整治井之井位、井深、開篩位置等規劃設計之重要參考資訊，對於污染之整治成效，具有關鍵性之影響。海淡水交界面及延散帶位態的調查，可以透過一整排與海岸線垂直斷面之監測井不同深度之水質鹽化資料或是地電阻法來加以調查。前者花費成本較高，後者屬於地球物理的方法之一，易受環境背景之干擾，特別是在汲沙填海造地的新生地。本文利用一種簡易的調查方法，現地選取濱海地區一處適當位置之監測井，於井中適當之不同深度裝置數部連續式地下水溫深鹽度計（conductivity, temperature, and depth sensor, CTD sensor），藉由觀測不同深度之地下水水壓與地下水導電度，受到潮汐漲退之變化，可以得知該井位之海淡水交界面及延散帶之位置及深度，未來再進一步結合該井位附近水文地質、地下結構物特性之區域地下水水位等值圖，推估該井位附近地下水之海淡水交界面及延散帶之空間位態。最後並探討如果產生不同類型之污染物質，採用不同之整治方法，所需設置之整治井應如何規劃設計。

關鍵字

海淡水交界面 ； 延散帶 ； 趨勢水流 ； 出流面 ； 地下水溫深鹽度計

並列摘要

The differences in the density of saltwater and freshwater result in a wedge of salt-fresh water interface in coastal areas and offshore islands. Location and its shape of interface are affected by density differences between freshwater and saltwater and the groundwater-level. Moreover, from the variation of daily tides and seasonal changes in precipitation, it forms a dispersion zone; as a result, there is almost no potential flow, but just only a density current. Groundwater flow speed is typically slow; therefore the cumulative effect of small amounts of contaminants may occur in coastal area. The salt-fresh water interface is at the outflow face in the intertidal zones that affected by tides. In the low tide, the zone has greater potential flow and greater groundwater velocity. Therefore, the solubility of pollutants, or the light non-aqueous phase liquid (LNAPL), will flow easily into the marine environment by groundwater seepage. The location of salt-fresh water interface and shape of dispersion zone are important information and have been applied to design groundwater remediation wells, depth, and screen in coastal area. Besides, such information is also recognized as an important element to assess remedial technologies performance. To obtain the location and shape of salt-fresh water interface in coastal area, it can be done by installing saltwater monitoring wells along the coastline to observe a long-term salinity profile or conduct electrical resistivity tomography (ERT) methods. The first method takes higher cost; conversely, the latter method that belongs to geophysical techniques is an effective and economic way to explore the salt-fresh water interface. But ERT is a sensitive technology, affected by many environmental factors, especially in the reclamation area. This study conducted a simple method to estimate the salt-fresh water interface by installing several conductivity, temperature, and depth (CTD) sensors in observation wells in coastal areas to observe a long-term salinity profile and groundwater pressure. With tidal changes, we can observe the depth of the salt-fresh water interface and the location of dispersion zone. Investigation results in combination with groundwater surface contour map that can further estimate the location and shape of the salt-fresh water interface and the dispersion zone and then can apply to design an appropriate remediation system for solve different type of pollutants.

並列關鍵字

salt-fresh water interface ； dispersion zone ； potential flow ； outflow face ； CTD sensor