The groundwater resources and sustainable yield of Cheju volcanic island, Korea

Hydrogeologic data of 455 water wells comprising geologic logs, water qualities, and aquifer test results are analyzed to determine hydrogeological characteristics, water quality, and sustainable yield of the groundwater resources of Cheju volcanic island. The groundwater of the island occurs in unconsolidated pyroclastic deposits and clinkers interbedded in highly jointed basaltic and andesitic rocks as high-level, basal, and parabasal groundwater under unconfined conditions. The total storage of groundwater is estimated at about 44 billion m(3). The average transmissivity and specific yield of the aquifer are at about 0.34 m(2) s(-) (1)(29300 m(2) day(-1)) and 0.12, respectively. The average annual precipitation is about 3.39 billion m(3), of which 1.49 billion m(3) - equivalent to 44.0% of the total annual precipitation - is recharged into aquifers, with 0.638 billion m(3) year(-1) of runoff and 1.26 billion m(3) year(-1) of evapotranspiration. Based on a groundwater budget analysis, the sustainable yield is estimated at about 0.62 billion m(3) year(-1), equivalent to 41.6% of annual recharge. A low-permeability marine sedimentary formation (Sehwari formation), composed of loosely cemented sandy silt, was recently found to be situated at 120 +/- 68 m below mean sea level. If the said marine sediment is distributed as a basal formation of the freshwater zone of the island, most of its groundwater will be of parabasal type. So the marine sediment is one of the most important hydrogeological boundaries and groundwater occurrences in the area.