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(III)

Understanding the response of groundwater to rainfall events will help us to realize groundwater transmission, however, long-term and continuous field observations of rainfall and groundwater data were very rare. This study analyzed (1) rainfall amount data at Pinglin Weather Station and rainfall isotope data at Pinglin Elementary School (2) tunnel groundwater discharge and groundwater isotope data collected in Hsueh-Shan tunnel, and (3) well groundwater level data from the monitoring well nearby Pinglin No. 1 shaft. These data will be used to explore the interaction between surface and groundwater in study area. Our results showed that the isotopic composition of rainfall in Hsueh-Shan tunnel area has significant seasonality, and the isotopes of tunnel groundwater samples collected at distinct depths have spatial differences. The isotope of tunnel groundwater at each sampling site all fell within the range of rainfall isotopes, reflecting that groundwater is a mixture of rainfall. Then, we used end-member mixing analysis, hydrologic Φ-index and isotopic Φ-index to evaluate the proportion of rainfall recharge to groundwater. The results of the two Φ-indexes showed that proportion of rainfall infiltrating to groundwater are 21.2% to 35% and 85.2% to 91%, respectively. Comparing the relative contributions of rainfall to groundwater in dry and wet seasons among the three methods, the results of end-member mixing analysis and isotopic Φ-index are most similar to the seasonal rainfall distribution in Pinglin. To further explore the response of groundwater to rainfall events, this study analyzed the relationship between rainfall data, well groundwater data, and tunnel groundwater data. First, the correlation between rainfall and well groundwater level, rainfall and tunnel groundwater discharge were analyzed. The results showed that the delay time of well groundwater level to the rainfall events is about 1.5 to 118.8 hours, and tunnel groundwater discharge is about 6 to 33 days. The change in well groundwater level is highly positively correlated with rainfall amount. Second, we used periodic regression analysis to fit the periodic trends of δ18O in rainfall and groundwater. The fitting results showed that well and tunnel groundwater δ18O cycle at a frequency of 1.9 years and 1.9 to 3.4 years, respectively, which is different from the one-year cycle of rainfall. Based on this calculation, the mean residence time of well and tunnel groundwater in study area is about 526 to 993 days and 448 to 1691 days, respectively. According to the results above, it can be observed that the response characteristics of groundwater to rainfall events in Hsueh-Shan tunnel area are quite complicated. The time scales of hydrograph response (well groundwater level, tunnel groundwater discharge) and tracer response (water isotope) are completely different, suggesting that groundwater transmission includes two concepts, i.e. celerity and velocity. The research results expand our understanding of surface-groundwater interaction in Hsueh-Shan tunnel area, and provide a preliminary imagination of groundwater transmission mechanism in study area, which can be used as a reference for groundwater modelling in future.