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

Measurements of the concentrations of dichlorodifluoromethane (CFC-12), tritium (³H), 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 ³H, and binary mixtures of the two. Model calculations using dispersion models and binary mixing based on ³H and CFC-12 concentrations demonstrate a non-atmospheric excess of SF₆ relative to CFC-12 and ³H 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.