台灣中部山區地下水及河流之地球化學示蹤劑研究

Geochemical tracers for the groundwater and streams in mountainous regions of central Taiwan

莊謹綸(Jin-Lun Chuang)

指導教授 ： 楊燦堯

國立臺灣大學/理學院/地質科學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.00758

摘要

據估計台灣全區天然地下水資源，在總蘊含量 17,332 百萬立方公尺內，有 72% 是屬於在山區蘊含的部分，而在平地區則只有不到三成的蘊含量，所以台灣山區地下水為豐富水資源蘊藏區。本研究希望利用惰性氣體（氦氣、氡氣）以及鍶同位素作為台灣中部山區地下水的示蹤方法，並輔以其他水化學之分析（陰陽離子、氫氧同位素），探討台灣中部山區地下水含水層特性、深淺井裂隙連通性以及地下水與河水的混合關係，以協助山區地下水循環模式之建立，提供水資源利用及管理之重要資訊。
本研究共取十一處「台灣山區地下水資源研究整體計畫－第一期」所建置之地下水位觀測井地下水樣本以及二十處河水樣本，採樣範圍涵蓋了大甲溪流域、立霧溪流域、美崙溪流域以及花蓮溪流域。研究結果顯示，台灣中部山區地下水與溪水皆由天水補注而來，地下水與溪水水質型態大致可分為兩類，以 Ca(HCO3)2 型及 NaHCO3 類型為主，地下水與河水受控於碳酸鹽溶解和矽酸鹽風化的岩石交互作用。地下水含水層特性主控於區域性的水文地質條件，如果水文地質條件相似，就可能產生相同的結果，並依據地球化學特性的不同，可分為四種含水層。分別為（一）降雨或地表水入滲補注快速的含水層、（二）降雨或地表水入滲補注快速，但受現地高鈾含量岩屑影響的含水層、（三）因含水岩層破碎少、含泥量高造成裂隙堵塞連通性差或有較多阻水層（砂頁互層、厚層硬泥岩等），使得降雨入滲補注速率緩慢，地下水在地底下滯留時間較長、（四）地下水層的裂隙及破碎帶（例如：斷層）連通至深部含水層。而陸源氦-4濃度、水中溶解氡氣濃度、氚-氦定年有較為顯著特性幫助區分地下水含水層
水體來源混合的部分，立霧溪流域支流匯入影響少，利用不同的地球化學示蹤劑（氡氣、鍶同位素）計算比例相近，LW-03 站位於2013年兩量較多時有約 9.72~12.7%的地下水供應河流基流，而2014年兩量較少時則有約 43.5~45.2%的地下水供應量；大甲溪流域因在大雨後五日採樣及支流匯入的影響，導致不同的地球化學示蹤劑計算比例相差甚遠，利用氡氣估算的地下水補注河水結果偏低，而用鍶同位素估算值與前人基流指數分析結果較相近。在 DJ-3 站利用氡氣計算出 CHW-10（八仙山）地下水補注比例約 3.00%，鍶同位素則為 58.1%，在 DJ-8 利用氡氣計算出 CHW-07（和平）地下水補注比例約 0.30%，鍶同位素則為 54.7%。
本研究顯示惰性氣體適合用來研究山區地下水含水層的分層特性，此外，水中溶解氡氣及鍶同位素在支流少的流域中為探討地下水與河水關聯性的良好示蹤劑，可以廣泛運用於臺灣其他流域，並提供重要資訊，幫助建立台灣山區地下水體的循環模式。

關鍵字

地球化學示蹤劑 ； 山區地下水 ； 惰性氣體 ； 鍶同位素

並列摘要

Total amount of natural groundwater resources in Taiwan has been estimated as 17,332 Mm3 of which ca. 72% is distributed in mountainous region, and less than 30% distributed in plain area. Therefore, mountainous region is a potential area which contains abundant water resources in Taiwan. We expect to distinguish the characters of different groundwater aquifers and the mixing relationship between groundwater and stream which could help to build hydrological circulating model of groundwater in mountainous region of Taiwan and offer important information for managing water resources.
In this study, 11 groundwater samples are collected from the monitoring wells of the CGS funded project (An integrated investigation of groundwater resources in Taiwan mountainous area: Phase I) and, 20 river samples in Dajia Basin, Liwu Basin, Meilun Basin and Hualien Basin are also collected. In addition to the major ions and hydrogen/oxygen isotopes of water samples, we applied noble gases (helium, radon) and strontium isotopic ratio as tracers of groundwater flow and sources in study area.
Most of the hydrogen and oxygen isotopic values fall on the local meteoric water line of Taiwan. It revealed that groundwater source in studied area is mainly from the precipitation. Besides, the geochemical data show that the groundwater and stream water samples in this study are mostly belonging to the category of Ca(HCO3)2 type and NaHCO3 type. It appears that the water compositions are mainly controlled by both processes of carbonate dissolution and silicate weathering. Furthermore, local hydro-geological condition plays an important role to differentiate the geochemical characters of groundwater in different aquifers. If the hydro-geological conditions are similar, different groundwater aquifers would show similar geochemical characters. According to the geochemical characters, we can differentiate four types of groundwater aquifer: (1) recharging by rainfall and runoff infiltration quickly, (2) recharging quickly and effecting by radiogenic regolith in situ (3) recharging slowly with more aquitards, and (4) deeper groundwater migrating along fractures or fault in studied areas. Furthermore, we can suggest that the results of terrigenic helium-4 concentration, dissolved radon concentration, and tritium-helium dating are useful tools to distinguish geochemical characters of groundwater from different aquifers.
The results show that the influx of tributaries did not play an important role for the main stram in Liwu Basin. We obtain similar results of groundwater discharging percent into stream by different geochemical tracers (radon and strontium isotopic ratio) estimation. We received a relative low groundwater discharge percent of about 9.72~12.7% to stream base flow at Liwu river (LW-03 site), due to high amount of rainfall occurred in 2013. Nevertheless, we got a much higher groundwater discharge percent of about 43.5~45.2% into stream in 2014, which may be due to less rainfall during this year. Furthermore, the influx of tributaries may be more important for Dajia Basin, because we obtained different fractions of groundwater discharging to stream estimating by radon and strontium isotopic ratios. There was about 3.00% and 58.1% groundwater discharged to Dajia river (DJ-3 site) by radon and strontium isotopic ratios respectively. At DJ-8 site, there was 0.30% and 54.7% groundwater contributed Dajia river by radon and strontium isotopic ratios respectively.
Above of all, this study reveals that the noble gases are considered as sensitive tracers for differentiating the characteristics of groundwater aquifer in mountainous region of Taiwan due to their unique geochemical characteristics. Besides, dissolved radon and strontium isotopic ratio are very useful tracers for understanding the relationship between groundwater and stream in the basins if there are less tributaries. The tracers could be used for others basin in Taiwan, and offer essential information, helping to build hydrological circulating model of groundwater in mountainous region of Taiwan.

並列關鍵字

Geochemical tracers ； groundwater in mountainous regions ； noble gas ； strontium isotopic ratio

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