Changing Coastal Chemistry: Natural Desalination and pH Evolution in Marine-Affected Subsurface Sediments of the Western Bohai Sea

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 salinity and alkalinity of these sediments. This study explores the natural desalination pathways shaped by meteoric and groundwater infiltration, focusing on their impact on sediment alkalinity. Seven subsurface sediment cores (5 to 6 m in depth) were collected along a land-to-sea transect, targeting shallow layers situated above -1.5 m in elevation. Electrical conductivity (EC) and pH measurements, combined with foraminiferal records, revealed a seaward-narrowing desalination corridor, indicating the ongoing influence of natural desalination processes on coastal sediment chemistry. Inland marine-affected sediments exhibited very low EC values (0.2–0.4 mS·cm⁻¹), suggesting extensive desalination. In contrast, within ~15km landward of the coast, sediments showed higher and more variable EC values (0.67–3.66 mS·cm⁻¹), with a pronounced decline toward the middle layer within the 0–1 m elevation zone. High alkalinity (pH > 9) was associated with the extent of marine-affected sediments inland, while elevated pH values at coastal sites were restricted to areas of low EC. 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. Ultimately, this study not only enhances our understanding of sediment geochemistry in coastal areas but also provides an important foundation for future research aimed at mitigating the impacts of salinization in vulnerable coastal environments.