台北盆地地下水之地球化學特徵及其隱示

Abstract

本研究首次對台北盆地地下水之溶解氣體進行全面分析，並討論溶解氣體與地質環境的關係，以及溶解氣體的可能來源。水中溶解氣體主要可分成一般溶解氣與惰性氣體溶解氣，本研究關注之一般溶解氣為氧氣、氮氣、甲烷、二氧化碳等，惰性氣體溶解氣為氦氣、氖氣及氡氣。 台北盆地地下水觀測井共三十四口，本研究於2014年5月中及9月初期間，與洗井作業員配合採樣，收集地下水樣本，以進行水化學及溶解氣體成份分析。台北盆地地下水以Ca(HCO3)2及NaHCO3類型為主，部份來自盆地北部的樣本呈現NaCl類型，顯示有可能受到海水混染。地下水、地表水及雨水的氫氧同位素組成均落於區域天水線上，顯示主要受天水來源影響。盆地南部地下水與地表水類似，且深淺水井差別不大，顯示受到溪水影響。北部的深淺井則有同位素分層現象，其補注水源來源應不同。溶解無機碳的碳同位素分析結果顯示此研究區域溶解無機碳的產生方法以微生物相關作用為主。 至於溶解氣體方面，利用罐頂空間方法進行一般溶解氣分析，結果呈現三種特徵，分別為富集氮氣、含甲烷和含二氧化碳。氦同位素分析結果顯示盆地北部有明顯上部地函訊號（RA>2，其中RA為空氣中3He/4He的比值），其中5個井口的樣本RA = 3 - 4.2，很可能是台北盆地有記錄以來的最高值。額外的3He可能來自盆地北邊的大屯火山群，而鄰近的山腳斷層為這些地函流體提供了通道。現地測量的水中氡氣值介於0.2 – 20.7 kBq/m3，對於有深淺井的採樣點，深井的水氡值一般會較淺井的為高，顯示深層含水層比淺層含水層有更高的水氡值。

This is the first comprehensive study for dissolved gases of groundwater in Taipei Basin, northern Taiwan. In addition to conventional water chemistry, the dissolved- gas compositions of groundwater from 34 observation wells have been systematically analyzed, aiming to know the relationship between dissolved gases and geological environment, and probable sources of the gases. Using the Piper plot, most of the groundwater samples can be classified as Ca(HCO3)2 and NaHCO3 types, although a few samples from the northern basin exhibit NaCl type characteristic which may reveal the mix with seawater. Isotopic compositions of hydrogen and oxygen for groundwater, surface water and meteoric water in Taipei Basin are aligned with Local Meteoric Water Line (LMWL), indicating a meteoric origin. The isotopic compositions of groundwater in the southern part of the basin have similar characteristics with surface water. However, isotopic stratifications occur in the observation wells from northern part of the basin. Accordingly, it reveals that recharge sources for groundwater samples in northern basin are different from the southern basin. The δ13C of dissolved inorganic carbon (DIC) indicates that microbial activities are dominant in the studied area. As for dissolved gases, three major components, CH4, N2 and CO2 are identified. The groundwater samples from northern part of the basin exhibit unexpectedly high helium isotopic ratios (RA>2, where RA is the 3He/4He ratio of air). Samples from five observation wells have RA = 3 - 4.2, probably the highest 3He/4He value ever reported in groundwater samples from this basin. The high RA ratios represent signals from mantle and the source of excess 3He may come from Tatun volcanic group (TVG), north to the Taipei Basin, with the nearby active Shanchiao Fault providing a pathway for such mantle fluids. Dissolved radon concentrations are in the range of 0.2 – 20.7 kBq/m3 and the deeper well usually exhibits a higher radon value than the shallow one from the same site which indicates that deep aquifer has a higher radon level than shallower one’s.