Hydrogeochemistry of seepage water collected within the Youngcheon diversion tunnel,Korea: source and evolution of SO4‐rich groundwater in sedimentary terrain

In the Youngcheon Diversion Tunnel area, South Korea, 46 samples of tunnel seepage water (TSW) and borehole groundwater were collected from areas with sedimentary rocks (mainly sandstone and shale) and were examined for hydrogeochemical characteristics. The measured SO₄ concentrations range widely from 7·7 to 942·0 mg/l, and exceed the Korean Drinking Water Standard (200 mg/l) in about half the samples. The TDS (total dissolved solid) content generally is high (171–1461 mg/l) from more shale‐rich formations and also reflects varying degrees of water–rock interaction. The water is classified into three groups: Ca? SO₄ type (61% of the samples collected), Ca? SO₄? HCO₃ type (15%) and Ca? HCO₃ type (24%). The Ca? HCO₃ type water (mean concentrations=369 mg/l Ca, 148 mg/l HCO₃ and 23 mg/l SO₄) reflected the simple reaction between CO₂‐recharged water and calcite, whereas the more SO₄‐rich nature of Ca? SO₄ type water (mean concentrations=153 mg/l Ca, 66 mg/l HCO₃ and 416 mg/l SO₄) reflected the oxidation of pyrite in sedimentary rocks and fracture zones. Pyrite oxidation resulted in precipitation of amorphous iron hydroxide locally within the tunnel as well as in high concentrations of Ca (mean 153 mg/l) and Na (mean 49 mg/l) for TSW, and is associated with calcite dissolution resulting in pH buffering. The pyrite oxidation required for the formation of Ca? SO₄ type water was enhanced by the diffusion of oxygenated air through the fractures related to the tunnel's construction. The subsequent outgassing of CO₂ into the tunnel resulted in precipitation of iron‐bearing carbonate. Copyright © 2001 John Wiley & Sons, Ltd.