本研究以大屯火山地區連續一年的天水、南磺溪流域、北磺溪流域共7口淺層地下水井水樣之穩定同位素（δ2H、δ18O）為基礎，估算本區域地下水的平均滯留時間（mean residence time, MRT），並結合現地量測資料(in situ)以及前人的地下水化學離子資料探討本區域的地下水平均滯留時間、δ2H、δ18O組成與地質、火山活動的關係。
為尋求合理MRT，本文利用原始資料、過濾氣候極端值、簡單移動平均（simple moving average）和加權移動平均（weighted moving average）等方式得到TB-MW-27、TB-MW-29和TB-MW-30等區域合理的估算結果，分別約為240、1083和990天。而地下水的穩定同位素組成以及水位時序變化分別顯示南北磺溪流域地下水的補注高程皆來自七星山和大屯山，然而受到不同迎風面以及不同地質材料的影響，使得南、北邊流域演化出不同的地下水特性。除此之外，根據穩定同位素的偏離值指示TB-MW-26可能不定期受到火山氣體的影響，而且TB-MW-26、TB-MW-29也有天水入滲明顯的行為，顯示TB-MW-26、TB-MW-29區域有可能發育較良好的裂隙。
The objective of this study is using stable isotopes(δ2H、δ18O) of precipitation and shallow groundwater to estimate groundwater mean residence time(MRT) and integrate in-situ data and chemical ion data of groundwater to investigate the relationship between the mean residence time of groundwater, stable isotope composition, geological features and volcanic activity in two catchments(South and North). Samples were taken at biweekly basis from 7 wells and precipitation for over one year period.
Four different methods were used to obtain a reasonable estimated MRT for wells of TB-MW-27, TB-MW-29, and TB-MW-30, with values approximately 240, 1083, and 990 days, respectively. The stable isotope composition of groundwater and the water level time series indicate that the recharge elevations of groundwater in the two catchments both originate from Mountain Qixing and Mountain Datun. However, due to the influence of different windward sides and geological materials, groundwater systems have evolved in the southern and northern regions. According to stable isotope deviation, TB-MW-26 may be periodically affected by volcanic gases. Both TB-MW-26 and TB-MW-29 show significant infiltration from precipitation, indicating well-developed fractures in the TB-MW-26 and TB-MW-29 areas. This suggests that the sampling duration, extreme rainfall events during the study period, and heterogeneity of geological materials may influence the estimation of groundwater MRT.
