Quantitative Assessment of Groundwater Discharges and Inflow Fluxes due to Dynamic Interactions between Groundwater and Surface Water

Abstract

Traditionally, groundwater and surface water have been managed as isolated components of the hydrological cycle. But in many cases, groundwater and surface water are hydraulically connected and continuous exchange of water and solute occurs across the interface leading to the change in water quantity and quality. Therefore, it is necessary to assess water exchange flux at the groundwater-surface water interface although the process is often complex and difficult. In this study, numerical approach was used to estimate the groundwater discharge and inflow fluxes at groundwater-surface water interface. Three different types of groundwater-surface water interaction process were considered and it includes riverbank filtration (RBF) process, submarine groundwater discharge (SGD), and seawater intrusion into the artificial underground storage cavern. The river water seepage rate into the radial collector well in riverbank filtration facility near the Nakdong River, Korea was calculated to prevent the river water depletion and to maintain sustainable water supply. The result showed that the calculated maximum river water infiltration rate was small compared to the annual river water discharge rate except during the dry season. The geologic and hydraulic features of the study area, such as the high hydraulic conductivity of the aquifer, the high recharge rate, and the geographical proximity of the filtration facility to the Nakdong River, make the region suitable for RBF. Next, this study focused on the high ratio of recirculated submarine groundwater discharge (RSGD) to total SGD in aquifer-ocean interface and investigated the effect of the time scale of sea level fluctuation on the increase in RSGD rate. The results showed that not only the amplitude but also the period of sea level fluctuation controls the total amount of SGD. A nondimensional number P* that incorporates the combined effect of sea level oscillation and aquifer properties was proposed to represent the magnitude of periodic forcing from the sea boundary into the aquifer as a mechanism for the enhancement of RSGD. Based on the simulation results, it was concluded that the differences in inland recharge rate and P* can modify the contribution of periodic changes in sea level on the increase in RSGD. The underground oil storage cavern in Yeosu, Korea undergoes the significant increase in Cl- concentration in seepage water and seawater intrusion was suspected to the source of salinity within the cavern. The extents of salinity of seepage water by seawater intrusion were estimated through monitoring and numerical modeling. The calculated mixing ratios of seawater in cavern seepage were generally less than observed mixing ratios based on the Cl- concentration in seepage water. This result suggests seawater intrusion is the main contributor of increase in Cl- in storage cavern but additional Cl- sources exist and one probable source is the saline water entered into the cavern during transfer of crude oil from ships to the storage caverns.