利用水的穩定同位素探討雪山隧道地區雨水與地下水之關聯

瞭解地下水對降雨事件的反應有助於瞭解地下水之傳輸，然而，較少利用長時間且連續現地觀測之雨水及地下水資料評估兩者之間的關係。本研究分析(1) 1990至2020年坪林測站歷史雨量資料、2012年5月至2020年5月坪林國小的雨水同位素資料和(2)雪山隧道導坑內2006年7月至2018年6月測量之地下水流量、2012年10月至2019年5月蒐集的地下水同位素資料以及(3)2018年10月至2020年5月於坪林一號豎井附近地下水井之地下水位及其同位素資料，以探討本區雨水－地下水之關連。研究結果顯示雨水的同位素值分布具明顯季節差異，在導坑及井等不同深度採集的地下水樣本其同位素有明顯空間差異，而所有採樣的地下水同位素值均落在區域天水線上，反映出地下水是雨水的混合。根據前述成果，本研究利用端源混合分析、水文Φ指數及同位素Φ指數評估雨水補注地下水的情形，兩種Φ指數的分析結果顯示分別有31.5% ~ 63.5%和85.2% ~ 91%的年雨量入滲。比較上述三種入滲推估方法中乾季及濕季月份雨水對地下水的相對貢獻，以端源混合分析（乾季19.1% ~ 34.1%、濕季65.9% ~ 80.9%）及同位素Φ指數（乾季20.2% ~ 37.2%、濕季62.8% ~ 79.8%）的分析結果和坪林全年雨量分布（乾季31%、濕季69%）較相似，而根據泰平及坪林氣象站雨量及坪林流量站計算的水文Φ指數則顯示相對平均且不同的貢獻度分布（乾季45.9% ~ 58.7%，濕季41.3% ~ 54.1%）。為進一步探討地下水對降雨事件的反應，首先分析了雨量和井地下水位、雨量和導坑流量的相關性，結果顯示井地下水位和導坑流量對降雨事件反應的延遲時間分別為1.5 ~ 118.8小時和6 ~ 33天，且井地下水位的變化量也和降雨量呈高度正相關。其次，運用週期性回歸分析分別對降雨和地下水中δ18O的季節性趨勢進行建模，擬合結果顯示井地下水δ18O的變化週期約1.9年，導坑地下水為1.9至3.4年，雨水δ18O變化週期則為1年，再根據地下水中δ18O振幅減少的程度計算井地下水的平均停留時間約405至993天；導坑地下水則為342至1691天。根據上述研究結果，可以觀察到雪山隧道地區地下水對降雨事件的反應特徵相當複雜，透過流量反應（井地下水位、導坑流量）及示蹤劑反應（同位素）所估算之反應時間的尺度相當不同，說明地下水傳輸包含波速(Celerity)與速度(Velocity)兩種概念。研究結果拓展我們對雪山隧道地區地表與地下水關連性的理解，並提供對研究區地下水補注機制的初步想像，期望可作為未來水資源管理的參考。

關鍵字

穩定水同位素； velocity ； celerity ；停留時間；地下水傳輸

並列摘要

Understanding the response of groundwater to rainfall events helps to realize groundwater transport process. However, long-term and continuous on-site observation of hydrological data is rarely used to assess the relationship between rainfall and groundwater. This study collected historical rainfall amount data derived from Pinglin Meteorological Station from 1990 to 2020 and isotope data collected in Pinglin Elementary School from May 2012 to May 2020, as well as the deeper groundwater isotope (from October 2012 to May 2019) and discharge (from July 2006 to June 2018) collected from Hsueh-Shan Tunnel. Besides, the shallow groundwater isotope and groundwater level data from the monitoring well near by Pinglin No. 1 shaft from October 2018 to May 2020 were also analyzed to explore the relationship between rainfall and groundwater in Pinglin. The results demonstrated that the isotope of rainfall had obvious seasonal differences; and the isotope of groundwater collected at distinct depths (i.e. pilot tunnel and well) also had significant difference. All isotopic data of groundwater at each sampling plot approximately fit the local meteoric water line, reflecting that groundwater is the mixture of rainfall. This study applied the end member mixing analysis, hydrological Φ index, and isotopic Φ index to evaluate the amount of rainfall recharging to groundwater. The results demonstrated that there were 31.5% ~ 63.5% and 85.2% ~ 91% of annual rainfall recharging to groundwater, computed by hydrological Φ index and isotopic Φ index, respectively. Comparing with the contribution of rainfall recharge into groundwater in the dry season and wet season, the end member mixing analysis (19.1% ~ 34.1% for the dry season, 65.9% ~ 80.9% for the wet season) and isotopic Φ index (20.2% ~ 37.2% for the dry season and 62.8% ~ 79.8% for the wet season) were similar with the annual rainfall distribution in Pinglin, and hydrological Φ index calculated based on rainfall amount data of Taiping and Pinglin stations showed a relatively average and different distribution of contribution (45.9% ~ 58.7% for the dry season and 41.3% ~ 54.1% for the wet season). To explore the groundwater response to rainfall events, we calculated the correlation between time series of rainfall, groundwater level, and pilot tunnel discharge. The correlation results indicated that the time-lag response of groundwater level and pilot tunnel discharge to rainfall was about 1.5 ~ 118.8 hours and 6 ~ 33 days, respectively; and the water table changes were also highly correlated with rainfall amounts. In addition, the periodic regression analysis with sinusoidal wave is used to fit the periodic trends of δ18O in rainfall and groundwater, indicating that the change period of δ18O in the well, pilot tunnel, and rainfall was about 1.9 years, 1.9 ~ 3.4 years and 1 year, respectively. Based on the damping of the amplitude of δ18O in groundwater, the mean residence time of the well and pilot tunnel was about 405 ~ 993 days and 342 ~ 1691 days, respectively. It can summarize that the groundwater response to rainfall events in the Hsueh-Shan tunnel was quite complex. The estimated response time of hydrologic responses (groundwater level, pilot tunnel discharge) and tracer response (isotope) were quite different, indicating that groundwater transmission includes two concepts: Celerity and Velocity. The research results expand our understanding of the relationship between surface flow and groundwater in the Hsueh-Shan tunnel area, and provide the preliminary imagination of the groundwater recharge mechanism in the study area, which can be used as a reference for future water resources management.