Residence Time of Groundwater and its Application to Aquifer Characterization in Jeju Island, Korea

The environmental tracers of ³H, ³He and CFC-12(chlorofluorocarbon-12) were employed to determine the residence time of groundwater sampled from three springs and twenty municipal wells of a basaltic aquifer and underlying sedimentary aquifers in Jeju volcanic island, Korea. Mantle He significantly affected apparent ³H/³He ages in half of the samples. Apparent CFC-12 ages show time lag up to 17 years relative to ³H in mountainous area, which is attributed to the thick unsaturated zone. ³H/³He systematics are in relatively good agreement with the reconstructed ³H input history in the island though some samples show higher initial ³H value than ³H input because of residence in the unsaturated zone or selective ³H diffusion from the portion of bomb ³H. The mean residence time of groundwater is estimated in the range of 6 to 15 years in basaltic aquifer and U formation, and greater than 60 years for the wells completed in Seoguipo formation in western coastal area by the application of lumped parameter models. The remarkable distinction of residence time in Seoguipo formation can be attributed to the low permeable layers distributed widely enough to limit leakage from the overlying basaltic aquifer. Some groundwater samples including springs show characteristics of low degree of mixing, which indicates groundwater flow in basaltic aquifer is not significantly affected by mixing between highly permeable layers such as interflow zones. Nitrate concentrations are on the background level in the wells of Seoguipo formation in western coastal area though nitrate contamination is conspicuous in coastal area throughout the island. This is in good agreement with the results of environmental tracer analysis.