臺中地區地下水之地球化學特徵

Abstract

台灣的降雨時空分布不均使得雨水無法充分儲存利用，地下水因而在緩衝及補充水資源的不足上相當重要，臺中盆地由厚礫石層組成，儲水條件良好，若能掌握該區的地下水特性將對水資源管理有莫大的助益。本研究在2015年的乾、濕兩季，於臺中盆地採集近百個水樣，包含雨水、溪水及地下水，系統分析氫氧同位素、陰陽離子、氦同位素及水中溶解氣，希望了解水體之間及水體與圍岩間的關係，並探討流體的可能來源。 氫氧穩定同位素分析結果顯示，臺中地區之水體皆落於天水線上；溪水的氫氧穩定同位素值(δ18O、δD)較當地雨水小，反映溪水源自較高海拔地區之降雨；地下水之δ18O與δD接近於溪水，故河川入滲應為主要的補注方式，且濕季溪水的貢獻量較大，經質量平衡計算，約有89%的地下水來自溼季溪水。另外，盆地南北兩部分的地下水之δ18O值也有顯著差異，其補注水源依據地理位置及氧同位素值略可分為三種，分別為大甲溪、大里溪及貓羅溪加灌溉水源，盆地中央部分的補注現象較弱，地表水與地下水組成顯著不同。 盆地內之地下水大多屬於新水(＜AD 1952)，離子性質為Ca(HCO3)2類型，氦同位素比值為1RA (RA =空氣的3He/4He比值)，溶解氣源於大氣；唯霧峰站的離子性質屬於NaHCO3類，為受壓含水層環境，氦同位素比值顯示有地殼訊號 (0.3 RA~0.5 RA)，且前人研究指出該站為臺中盆地已測的最老水體 (~27,000年)，封閉環境及極長的滯留時間讓水體得以不斷累積地殼逸氣及含水層鈾釷礦物衰變所釋放的4He，進而改變氦同位素比值。 水中溶解氡氣濃度受控於圍岩中鈾釷含量以及岩體性質。臺中盆地地下水之水氡濃度介於7~30 Bq/L，低值集中在泥層分布較多的盆地中段，推測地下水流動受阻使得氡氣不易被傳遞及累積。另外，一般情況下，溪水中的氡氣會迅速逸散至大氣而使濃度低於偵測極限(<0.2 Bq/L)，但在乾季時期，盆地中段的溪水中卻偵測到了1~3 Bq/L 的水氡濃度，反映採樣點附近有高水氡濃度之地下水流入其中。利用地下水與地表水的氡氣濃度計算後，河川中約有3-13%的水來自地下水。 此研究顯示氫氧同位素為追蹤臺中盆地水體關聯性的良好工具，而溶解氣成分則可幫助了解水體與圍岩的關係以及地下水與地表水的交互作用，為臺中盆地之地下水提供更多面向的重要資訊。

The groundwater is an important water resource in Taiwan as a result of the uneven spatial and temporal distribution of rainfall. Taichung basin in central Taiwan, possessing a conglomerate formation with high-quality groundwater, is an ideal site for natural water reservoir. Therefore, we systematically analyzed stable isotopes (hydrogen and oxygen), helium isotopes and compositions of dissolved gases of nearly a hundred water samples, including rainwater, stream water and groundwater collected from Taichung area in wet and dry seasons of the year 2015 in order to understand the relationship between water bodies and host rocks and to clarify the sources of fluids. In the δ18O vs. δD plot, all samples present a linear trend similar to local meteoric water, indicating a meteoric origin. However, river samples are relative lighter than rain samples; it demonstrates that the rivers are mainly recharged from precipitation of high-elevation areas with a lighter isotopic composition. On the other hand, groundwater is mainly recharged by river water. Because the seasonal isotopic variation of river samples is significant, we calculated relative contribution by seasons using the mass balance equation. Results show that about 89% of groundwater reflects the characteristics of precipitation in rainy season. Furthermore, there are many recharging sources including Da-Jia River, Da-Li River, Maoluo River and irrigation water to explain the variation of groundwater isotopic compositions from north to south. However, the rates of groundwater recharge from rivers at central basin might be slow because that the characteristics between rivers and groundwater are quite different. The prominent types of groundwater in the basin are Ca(HCO3)2 type and the helium isotopic ratio in dissolved gases are close to 1 RA (RA = 3He/4He ratio of air), which imply they are very fresh water (recharging water) except the sample from Wu-Feng well. Wu-Feng well exhibits NaHCO3 types of water and only has 0.3 RA~0.5 RA, reflecting crustal signals. This sample also has an older C-14 age (~27000 yrs.) than others (<200 yrs.), implying that water-bodies are confined in an isolated environment to interact with rocks and the dissolved helium is likely affected by radiogenic 4He of surrounding rocks and crustal helium flux. Aqueous radon is controlled by the uranium concentration of surrounding rock and rock textures. For Taichung basin, which is composed of alluvial gravels, there might not be significant differences between uranium concentrations; therefore, the difference in the water radon should come from other factors. The radon concentration ranges from 7-30 Bq/L in Taichung basin and it shows a lower value in the central basin where there is a mud layer distributed. Therefore, it is presumed that the mud layer will prevent radon from migration and accumulation. On the other hand, rivers usually contains undetectable radon (<0.2 Bq/L) because radon will rapidly escape to the atmosphere. However, river samples from the central part of basin have radon concentrations ranging between 1 and 3 Bq/L, reflecting that the sampling sites are in the vicinity of points of groundwater inflow. Using the difference of radon concentration between the groundwater and river water to estimate, there are approximately 3-13% of river water recharging from groundwater. This study illustrates the utility of hydrogen and oxygen isotopes to trace the groundwater source and determine the seasonal contribution ratios of river water to groundwater recharge, and demonstrates the advantage of using dissolved gas to investigate the groundwater-host rocks interaction.