Evidence for terrigenic SF6 in groundwater from basaltic aquifers, Jeju Island, Korea: Implications for groundwater dating

摘要

Measurements of the concentrations of dichlorodifluoromethane (CFC-12), tritium (H-3), and sulfur hexafluoride (SF6) in groundwater from basaltic aquifers in Jeju Island, Korea, demonstrate a terrigenic source of SF6. Using a lumped-parameter dispersion model, groundwater was identified as young water (< 15 years), old water with negligible CFC-12 and H-3, and binary mixtures of the two. Model calculations using dispersion models and binary mixing based on H-3 and CFC-12 concentrations demonstrate a non-atmospheric excess of SF6 relative to CFC-12 and H-3 concentrations for more than half of the samples. The non-atmospheric excess SF6 may have originated from terrigenic sources in relict volcanic fluids, which could have acquired SF6 from granites and basement rocks of the island during volcanic activity. Local excess anthropogenic sources of SF6 are unlikely. The SF6 age is biased young relative to the CFC-12 age, typically up to 20 years and as high as 30 years. This age bias is more pronounced in samples of groundwater older than 15 years. The presence of terrigenic SF6 can affect the entire dating range for groundwater in mixtures that contain a fraction of old water. (c) 2007 Elsevier B.V. All rights reserved.