臺灣西南部沄水自流井之水化學及溶解氣隨時間變化之研究

Abstract

根據地球化學研究指出，活動構造區在大規模地震發生之前，岩石因構造應力高度集中而產生微裂隙，釋放原封存於孔隙內的流體，或產生連通不同含水層的流體通道，造成地下流體的化學性質改變。 本研究在2014年間於鄰近中崙及關子嶺活動構造區的沄水國小旁設立溶解氣體及水化學連續監測站，分析沄水自流井內每日的鈉、鉀、鈣、鎂、碳酸氫根、氯、硫酸根等離子濃度及碳、氫、氧等同位素比值變化，即時監測甲烷、氮、氧、氬及二氧化碳等溶解氣體濃度及水中溶解氡氣，並根據其時序變化探討鄰近中崙及關子嶺等活動構造區內化學性質變化與相關地震事件之間的關聯性。 監測結果顯示，自7月12日及9月18日起溶解氣體及水化學共有兩起同步且顯著的異常變化顯現於氯離子、碳同位素比值及溶解氣體組成，但近期皆無較大規模地震發生，以地震活度方法計算全年度有感地震，發現流體異常與相關地震事件的發生時間有延遲的現象，因缺乏高壓裂隙系統之流體通道，使得沄水站不僅無法觀測到的震前異常變化且與流體實際變化相差2個月以上。沄水站井水在異常變化後具有快速回復的特性，顯示非受到外來流體混染，而是來自於裂隙圍岩釋放的孔隙流體。 水化學性質上，沄水站流體主要為地層孔隙內古海水與極少量天水性地下水混和鈉/氯比值約為0.94與海水的0.85明顯不同，表示其流體與圍岩發生離子交換反應，造成鈉離子富集及鈣、鎂離子虧損，顯示裂隙圍岩對流體性質具有至關重要的影響力。

Several studies have proposed that micro-fracturing prior to major seismic events may be responsible for changes of the groundwater chemistry. Precursory changes in groundwater chemistry are often attributed to the new fractures, the opening of pre-existing micro-fractures or the destruction of hydrologic barriers. It is believed that isolated aquifers may have chances to subsequently mix together and trapped gases in the rock matrix can escape when subjected to earthquake activities. The aim of this study is to probe into the probable mechanism of geochemical variations of groundwater that plays in an earthquake cycle bying the data collected from the Yunshui dissolved gases and groundwater monitoring station during the year of 2014. Dissolved ions Na、K、Ca、Mg、HCO3、Cl、SO4, stable isotope C、H、O, dissolved gases CH4、N2、O2、Ar、CO2 and dissolved radon were measured with the multi-monitoring system. Results show that the chloride and dissolved methane concentrations of groundwater have two anomalies around July 12 and September 18. However, no immediate corresponding earthquake has been detected at the Yunshui station. By using earthquake activity function from calculating the annual earthquakes, the anomalies were found to emerge with a time delay of more than 2 months. It could be attributed to non-existence of a high pressure fluids pathway. The anomalies have the property of fast recovery. It is suggested that the source of anomalies is the trapped fluids of the host rock. The fluid of of Yunshui Artesian Well is mainly formation water, which is a mixture of the ancient sea water and the small amount of meteoric water. The Na/Cl ratio of Yunshui groundwater is 0.94, higher than the average seawater value of 0.85. The geochemical characteristics inferred that the extra Na in groundwater probably resulted from ion exchange. It also support that the importance of the host rock in aquifer.