以雪山隧道導坑水之地球化學參數瞭解地表─地下水之交互作用(II)

Understanding the response of groundwater to rainfall helps to understand groundwater transport mechanism. However, previous studies use relatively short observations. In this study, rainfall amount, rainfall hydrogen/oxygen isotope, groundwater (in the Hsue-Shan tunnel) discharge, groundwater hydrogen/oxygen isotope, groundwater level above the tunnel, were measured to investigate the response indices between rainfall and groundwater. It is found that there was evident seasonality in the rainfall hydrogen/oxygen isotope but not in the groundwater. However, the variation of hydrogen/oxygen isotope in the groundwater is within that in the rainfall. First, Φ index method with data of rainfall amount and stream discharge rate was applied, and it revealed that ~21.2%-34.3% of annual rainfall became groundwater. Second, Φ index method with data of rainfall amount and rainfall isotope was applied, and it revealed that ~84.2%-89.5% of annual rainfall became groundwater. It is also found that there was 2.5 hrs-94.3 hrs lagtime between the star hour of a rainfall event and rise of groundwater level. However, there was 6 days- 21 days between a rainfall event and rise of groundwater discharge in the tunnel. Moreover, rainfall isotope cycled at a 1-year frequency but groundwater isotope in the tunnel cycled at a frequency of 2-year. It was estimated that travel time of the groundwater discharge was ~180 days- 427 days. Different infiltration percentage and different response time might be explained by the concept of velocity and celerity. Understanding whether the observed infiltration percentage/response time is controlled by velocity or celerity will help us to clarify groundwater transport mechanism and facilitate water resource management.