Morphology of Brine‐Seawater Interface and Spatial Distribution of Submarine Groundwater Discharge Windows in the Muddy Coast.

The brine‐seawater interface (BSI) is a unique type of groundwater‐seawater interface (GSI) characterized by the higher density of underground brine compared to seawater. This study focuses on characterizing the bay‐scale BSI morphology and identifying submarine‐groundwater discharge windows using a comprehensive in‐situ geophysical detection on the south bank of Laizhou Bay. Our findings reveal that the BSI forms an extensive mixing zone (15–20 km) without distinct contours between waters of varying salinities. The discharge windows for underground brine are located in nearshore areas with fine sand distribution and offshore pockmark areas. Hydraulic and salinity gradients drive the underground brine discharge through these windows. The aquitard window is the primary area for shallow and deep brine exchange, likely evolved from paleochannels, ancient tidal creeks, or ancient underwater barriers. These findings provide crucial modeling support for analyzing environmental evolution mechanisms and theoretical basis for planning the underground brine mining in similar coastal regions. Plain Language Summary: This study investigates a unique interface where underground brine meets seawater, known as the brine‐seawater interface (BSI). Unlike regular groundwater‐seawater interfaces, the BSI features brine that is denser than seawater. The research focused on characterizing the BSI's morphology and identifying locations where brine discharges into the sea along the south bank of Laizhou Bay. By using advanced detection methods and analyzing samples, the study found that the BSI forms a broad mixing zone of about 15–20 km. There are no clear boundaries between water of different salinities within this zone. The study also identified specific areas near the shore with fine sand and offshore pockmarks where brine discharges occur. These areas are recognized by the presence of suspended fine sediment on the seabed. The movement of brine through these discharge areas is driven by differences in water pressure and salinity. The study suggests that ancient channels and barriers underwater play a significant role in these discharge processes. Understanding these dynamics can help us manage coastal environments and predict how changes in sea and land interactions may affect them. Key Points: The bay‐scale brine‐seawater interface (BSI) is a long mixing zone with underground water salinity ranging from high to low from land to seaThe nearshore seabed fine sand area and offshore pockmarked area are the main windows for underground brine dischargeThe BSI morphology and submarine groundwater discharge/exchange windows can be identified by comprehensive geophysical detection results [ABSTRACT FROM AUTHOR]