臺北盆地地下水利用可行性之分析評估及管理

Abstract

臺北盆地位於臺灣北部，其範圍為海拔標高20公尺以下，面積約240平方公里。自民國六十年起臺北盆地劃為地下水管制區，限制抽取地下水，地下水位已有明顯上升趨勢，且至今呈一正常平穩狀態。為妥善運用地下水，將地下水位控制在一合理範圍下，本研究將利用MODFLOW建立臺北盆地水文地質模式，計算水平衡收支表及安全出水量，並分析地下水質及水位，以評估臺北盆地地下水利用可行性，提出合適之地下水管理方案。 模式中主要水文地質分層分別為松山層、阻水層與景美層。本研究以景美層之主要23口觀測井水位資料作為模式輸入之依據，模式率定驗證時間為民國86-92年，每月一觀測水位共七十二時期。模式率定及驗證地下水位之均方根差分別為0.67及0.43公尺。水收支平衡之模擬結果顯示，景美層地下水儲水變化量為年增加1,700萬噸；以希爾法推估之安全出水量為5,400萬噸；以地表下7.5公尺之零水位法計算之安全出水量為1.26億噸。 臺北盆地地下水水質應用因子分析法結果顯示其主要因子為鹽分殘留因子、鹼土類非碳酸鹽類因子及有機質因子，並綜合派博水質圖法分析結果得知，臺北盆地除北市東區及盆地西南部一帶水質較差，不可供民生用水使用，其餘地區水質尚稱良好。臺北盆地近年除工程抽水外並無其他抽水之用，故本研究採取較寬鬆之地下水位管理方案，可得地下水核發水權為每年8,200萬噸，而考慮地下水管制區域限制抽水，現在臺北盆地可抽用地下水每年約為2,500萬噸。本研究建議應持續觀測盆地內地下水位及水質，逐年檢討地下水安全出水量，以利評估規劃後續地下水可使用量，並逐步開放盆地內地下水使用，提高地下水資源利用效率。

Taipei basin is located in the northern Taiwan. The area below elevation 20m is about 240km2. Groundwater had been overexploitation in 1950s, resulted in the groundwater level dropping sharply. The government delineated the Taipei basin as groundwater restricted area to prevent further withdrawing in 1970. Since then, the groundwater level in Taipei basin had risen gradually, and now is bounced to the normal condition. The objective of the study was to develop a hydrogeologic model of Taipei basin using MODFLOW. The developed model is aimed to calculate water budget and safe yield. Moreover, factor analysis and Piper diagram were applied to analyze groundwater quality and to evaluate the accessibility of groundwater resources utilization in the Taipei basin. Three hydrogeologic layers present in the Taipei basin groundwater model: Sungshan aquifer, aquitard, and Chingmei aquifer. The Chingmei aquifer is the main aquifer in the Taipei basin. The model simulated groundwater level is calibrated and validated against the measured groundwater level with the root mean square error (RMSE) of 0.67m and 0.43m, respectively. The simulated water budget indicates that the groundwater annual storage increases about 17millions tons in Chingmei aquifer. The groundwater safe yield of the Taipei baisn estimated by Hill method is about 54millions tons per year, whereas the safe yield estimated by the zero water level method is about 126 million tons per year. Three main factors of groundwater quality in the Taipei basin are determined by the factor analysis and they are salinity-residual factor, non-carbonate hardness factor and organic matter factor. Results from factor analysis and Piper diagram show that the groundwater quality in eastern Taipei city and southwest basin is not suitable for domestic use, other area is in good condition. Based on the safe yield estimated by the Hill method and zero water level method, the permissible amount of groundwater withdrawn in the Taipei basin is recommended about 82 millions tons per year. However, groundwater is still prohibited from common use, except for engineering drainage in most of the basin. To comply with the restriction order, the permissible yield is reduced to 25 millions tons per year. In order to effectively use and manage groundwater resources in the Taipei basin, the restriction order should be revised. Furthermore, the groundwater level and quality should be monitored continuously and revaluated regularly.