Geochemical Characteristics of Groundwater in Taichung Area, Central Taiwan

• Main Authors: Jui-Fen Tsai, 蔡汭芬
• Other Authors: 陳正宏
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/85847317367956380217

碩士 === 國立臺灣大學 === 地質科學研究所 === 104 === 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.