Time-series analysis of three years of groundwater level data (Seoul, South Korea) to characterize urban groundwater recharge

A time-series analysis was performed on 3 years of daily groundwater level data from 34 monitoring wells in the bedrock aquifer of Seoul to understand the characteristics of groundwater recharge in an urban setting and thereby provide a policy background for sustainable groundwater management. The cross-correlation function was used to estimate the lag time of groundwater level change after a rainfall event. Based on the lag time, monitored wells are classified into two groups. The delay was very short (1-7 days) in Group I wells (n = 12), whereas Group II wells (n = 22) showed a longer delay (17-58 days). Such variable response was considered to reflect differences in the transmission of hydraulic pressure induced by recharging rainwater through the unsaturated zone. The estimated delay time showed significant correlation with the elevation, depth to groundwater level, and thickness of soil (or alluvium). The shorter delay Group I wells generally had shallower groundwater levels and were preferentially located toward mountainous areas. Group I wells also tended to be located in areas with thinner soil (or alluvium) cover. However, ground cover characteristics (e.g. pavement) around wells had no correlation with the delay time. The characteristics of groundwater recharge in this urban setting therefore appear to be controlled by natural conditions such as topography and soil thickness, rather than by land use. The study suggests that natural conditions should be more carefully considered in the designation of Groundwater Preservation Areas.