Estimating recharge in fractured aquifers of a temperate humid to semiarid volcanic island (Jeju, Korea) from water table fluctuations, and Cl, CFC-12 and 3H chemistry

Groundwater table fluctuations (WTF), chloride mass balance (CMB), apparent groundwater chlorofluorocarbon (CFC-12) ages and tritium (3H) mean residence times were used to assess recharge rates on Jeju Island (Korea), where groundwater is the main source of potable water. Given the limitations of various techniques and the respective data, the methods yield median values of 687 mm/yr (WTF), 429 mm/yr (CMB), 423 mm/yr (CFC-12) and 394 mm/yr (3H), which are lower than the multi-annual (1993–2002) average value calculated using the soil water budget (SWB) method (911 mm/yr). These underestimates are mainly due to most groundwater samples used for the analyses being located in the more arid lowland areas of the island. All methods yield highest recharge rates in the southern and eastern districts of Jeju implying a strong control of rainfall on the spatial recharge distribution. The spatial variability of recharge at the catchment scale is caused by spatially and temporally variable rainfall and evapotranspiration as well as the wide range in effective porosity and specific yield values of the aquifer lithologies. The WTF method yields reliable results in the coastal regions where low hydraulic gradients prevail. The CMB technique underestimates the recharge values of the SWB in all geographic districts probably as a result of anthropogenic Cl additions to groundwater and unaccounted for atmospheric Cl dry deposition. Median recharge estimates from the CFC-12 and 3H data show the lowest spatial correlation with those from the SWB mainly as a result of vertical anisotropies and uncertainties in the determination of effective aquifer thicknesses particularly in the perched, high level areas. The here applied methods are useful for local calibration and validation of SWB estimates in lower parabasal regions where thin unsaturated zones prevail; however, due to their inability to predict recharge in the more remote upslope areas from which, as of yet, no groundwater data could be obtained, they cannot be applied to predict average recharge values representative for the whole island.