Tritium Concentrations and Radiocarbon Ages of Gushing Groundwater from Hsuehshan Tunnel, Northern Taiwan

Abstract

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.

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