Intensified intrusion and retention of saltwater in the Sumjin River Estuary, South Korea

The Sumjin River Estuary (SRE) is located in South Korea and is a natural riverine system without an estuarine barrage. Civil complaints have been expressed by residents surrounding the SRE, particularly with regards to the extended upstream saltwater intrusion and the diminished production of the Corbicula japonica clam. To establish a scientific background of the complaints, more than 75 surveys across the SRE were performed, including gathering of long-term CTD monitoring data in the middle reaches of the SRE during 2015–2017. Although salinity at the mooring station normally fluctuates between zero and 20 psμ with tidal variation, the salty (>15 psμ) bottom water is consistently retained in the bathymetric depression of the middle reaches of the SRE, the main habitat for the benthic C. japonica clam, particularly during the neap tide and periods of moderate freshwater discharge. The maximum saltwater intrusion distance upstream of the estuary increased from 22.6 km to 24 km during the decade between 2005 and 2015. In addition, saltwater intrusions greater than 20 km have become more frequent over the decade. The increased utilization of freshwater in the upstream river reaches has been considered a primary constraint of the intensified saltwater intrusion in the SRE. This is enhanced by freshwater abstractions from an intake located near the SRE, particularly during low precipitation periods. The saltwater retention on the river bottom and the intensified saltwater intrusion upstream imply that estuarine ecosystem management policies are needed to incorporate salinity monitoring near the river bottom, where benthic organisms occur. The abstractions at the upstream freshwater intake are also recommended to be carefully controlled, particularly during the dry season.