Application of environmental tracers to mixing, evolution, and nitrate contamination of ground water in Jeju Island, Korea Dong-Chan Koh a, * , L. Niel Plummer b , D. Kip Solomon c , Eurybiades Busenberg b , Yong-Je Kim a , Ho-Wan Chang d a Korea Institute of Geoscience and Mineral Resources, Groundwater Resources Group, 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea b US Geological Survey, 432 National Center, Reston, VA 20192, USA c Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA d School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea Received 3 June 2005; received in revised form 7 November 2005; accepted 8 November 2005 Summary Tritium/helium-3 ( 3 H/ 3 He) and chlorofluorocarbons (CFCs) were investigated as environmental tracers in ground water from Jeju Island (Republic of Korea), a basaltic volcanic island. Ground-water mixing was evaluated by comparing 3 H and CFC-12 concentrations with lumped-parameter dispersion models, which distinguished old water recharged before the 1950s with negligible 3 H and CFC-12 from younger water. Low 3 H levels in a considerable num- ber of samples cannot be explained by the mixing models, and were interpreted as binary mix- ing of old and younger water; a process also identified in alkalinity and pH of ground water. The ground-water CFC-12 age is much older in water from wells completed in confined zones of the hydro-volcanic Seogwipo Formation in coastal areas than in water from the basaltic aquifer. Major cation concentrations are much higher in young water with high nitrate than those in uncontaminated old water. Chemical evolution of ground water resulting from silicate weath- ering in basaltic rocks reaches the zeolite–smectite phase boundary. The calcite saturation state of ground water increases with the CFC-12 apparent (piston flow) age. In agricultural areas, the temporal trend of nitrate concentration in ground water is consistent with the known history of chemical fertilizer use on the island, but increase of nitrate concentration in ground water is more abrupt after the late 1970s compared with the exponential growth of nitrogen inputs.  c 2005 Elsevier B.V. All rights reserved. KEYWORDS Environmental tracers; Residence time; Ground-water mixing; Nitrate contamination; Aquifer vulnerability; Volcanic island 0022-1694/$ - see front matter  c 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.021 * Corresponding author. Tel.: +82 42 868 3079; fax: +82 42 863 9404. E-mail address: chankoh@kigam.re.kr (D.-C. Koh). Journal of Hydrology (2006) 327, 258– 275 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol