Several serious groundwater outbursts occurred during excavation of the Hsuehshan Tunnel in northern Taiwan. In order to better understand the hydrological characteristics of the rock formations overlying the tunnel, tritium ( 3H) concentrations and radiocarbon (14C) ages of groundwater samples from the tunnel were measured. Samples from the gently dipping and less fractured sandstone/shale beds of the Makang Formation to the west have 3H content of less than 0.6 TU and conventional 14C ages of older than 8400 yrBP, suggesting pre-bomb recharge and relatively low flow rates (≤_n8 m yr−1 ). In contrast, samples from the highly fractured and steeply dipping Szeleng meta-sandstone to the east have 3H contents of 1.2~2.5 TU and conventional 14C ages of younger than 7400 yrBP, implying origin of post-1999 recharge and higher flow rates of 130~500 m yr −1 . Tritium and carbon isotopes concentrations reflect local hydrogeological characteristics of the rock formations along the Hsuehshan Tunnel and were helpful in assessing excavation methods.