Natural desalination of marine-affected subsurface sediments and its impact on pH along the Western Bohai Sea coast

In time defined by environmental transformation and anthropogenic pressure, understanding the natural geochemical evolution of coastal zones has never been more urgent. The Western Bohai Sea Coast (WBSC) offers a natural archive of significant land-sea evolution, comprising extensive subsurface deposits formed under marine influence during the progradation of the mid-to-late Holocene Yellow River delta. Over time, natural processes such as meteoric water infiltration and groundwater movement have altered the geochemical characteristics of these sediments. This study uses Electrical conductivity (EC) and pH measurements, combined with environmental indicators such as foraminiferal analysis, to explore the desalination of marine-affected subsurface sediments and its impact on pH along the WBSC. Under long-term percolation of meteoric water and groundwater, a spatially continuous low EC zone, or “desalination corridor”, has developed in marine-affected sediments. It narrows seaward from a broad range of active leaching to a restricted depth around the coastal groundwater table, and eventually to in the upper intertidal zone. Along with desalination, marine-affected sediments commonly develop high pH values, with the underlying mechanisms varying depending on distance from the coastline. In the central areas, the alkalinity increase is likely driven by the leaching of the Ca 2+ in the sediments, whereas near the coast, the pronounced alkalization around the groundwater table is probably induced by geochemical reactions under high-Na + , low-flux groundwater conditions. These findings highlight a critical environmental dynamic between desalination and pH increase, with implications for soil health, agricultural viability, and ecosystem functioning in coastal regions. • Natural desalination processes were studied in Holocene Yellow River delta marine-affected sediments. • Desalination links Electrical conductivity (EC) decrease with elevated pH. • A seaward-narrowing desalination corridor extends from extensive inland leaching zones to restricted depths near the coastal groundwater table and the upper intertidal range. • Mechanisms of pH increase differ between inland and coastal settings.