台北盆地水文地質架構及地層下陷之探討

Abstract

台北盆地為台灣的政經中心，土地利用自地面逐漸朝地下發展，是故，對於水文地質環境的瞭解日顯重要。本研究蒐集歷年台北盆地之岩芯鑽探與觀測井水位記錄，藉由分析及對比二者，將基隆河一帶及其北方劃分為透水性較低之區域，與盆地中心的主要含水層水力連通性較低。水位歷線記錄顯示，以水位高程0公尺為界，盆地內各觀測井之水位記錄明顯呈現兩組變化模式。觀測井濾水管分別位於松山層之薄砂層、景美層及五股層中，其中淺層觀測井多位於松山層，深層觀測井則分別位在景美層及五股層內。淺層含水層水位變化主要受降雨影響，接近自由含水層的性質；深層含水層水位變化則受人為因素影響極為顯著，降雨對其影響甚小，此深度範圍之各井水位變化模式相似度極高且分布範圍廣，同時受潮汐及工業用水影響顯著，水位記錄可見不同時間單位下的週期性變化。本研究認為台北盆地地下水分層可分為兩個主要含水層，而景美層及五股層於水文地質分層中可劃分為同一含水層。 台北盆地自1950年代起大量抽取地下水，導致水位下降及地層下陷，1970年代因管制抽水，地下水位逐年回升，地層下陷亦減緩或停止，近年來之水準高程檢測發現貫穿盆地中心主幹線水準點高程變化量在±1公分(誤差範圍)之內，甚至出現地面回升的現象，然而2004年後地下工程反覆大量抽水，亦造成部分地區下陷再次發生，地表高程下降1至2公分，其沉陷中心與捷運蘆洲線、新莊線及內湖線相關。本研究認為台北盆地地表高程上下變動來自含水層之有效應力變化，土體呈現彈性變動的應變模式，與地下水位變動有直接且明顯關聯性。現今捷運工程仍持續進行，未來台北盆地之地下水位變動及其與地層下陷影響之關係，值得持續追蹤與探討。

The political and economic center of Taiwan is located in the Taipei basin, and many of the subsurface excavations are done now. In this study, we collected the borehole core data and groundwater level record of the Taipei basin from 1976 to present, and use the elevation data to find the relation between groundwater level and land subsidence. By analyzing the groundwater level record, we found 2 water level changing patterns, which can be identified by the 0m water level to two parts, the upper-part and the lower-part. The upper-part groundwater level is relative to the rain fall, but is a little bit different in every location. The lower-part groundwater level is from -40m to 0m, but human activities are highly relative with water level change. Comparing the depth of wells with core data, well screens are located in thin sand layer of Songshan formation, or Jingmei formation, or Wugu formation. The sallow wells are almost in the Songshan formation, where the deep wells are in the Jingmei formation or Wugu formation.We think there are 2 hydrogeological units in Taipei basin, which are Songshan formation and the combination of lower Songshan, Jingmei and Wugu formation. During the last decade, periodical pumping and recharging cause the water level to fluctuant. Short-term water level variation becomes the primary factors for land elevation change. We found out the reason for subsidence and uplift is the elastic rebound. Although there is time lag, the contrast between the groundwater level and the land elevation is easy to perceive.