Controls of paleosedimentary environments and anthropogenic activities on coastal groundwater salinization: A case study of Laizhou Bay, China

Groundwater salinization presents a substantial environmental challenge for coastal regions globally, particularly along the southern shore of Laizhou Bay. The processes contributing to groundwater salinization are intricate and involve interactions between natural factors and human activities. This study examines the mechanisms underlying groundwater salinization at varying depths in the southern Laizhou Bay area, with a particular emphasis on the effects of extensive groundwater extraction. Excessive groundwater extraction has intensified salinization by altering hydraulic gradients and promoting seawater intrusion. Utilizing sediment and pore water samples from two boreholes (SG100 and WF80) alongside shallow seismic data, we identified that sedimentary strata consisting of alternating marine and terrestrial deposits, formed during multiple transgressive events since the Late Pleistocene, are a significant source of salinity. Paleoenvironmental reconstructions based on grain size analysis, combined with Cl/Br ratios and stable isotope analysis, indicated that modern seawater intrusion is a primary driver of shallow groundwater salinization. In contrast, variations in deep groundwater salinity reflect differing paleoenvironmental recharge conditions. Ancient riverbed sediments exhibit high permeability, providing an efficient pathway for the transport of water and salts from upstream freshwater bodies. This process facilitates the dilution within the aquifer, leading to the formation of low-salinity groundwater. In contrast, the low permeability of floodplain sediments significantly restricts the renewal capacity of water bodies. As a result, the saline water layer maintains a high salinity state over extended periods due to the combined effects of evaporation and limited recharge. Notably, offshore freshwater was detected within ancient river channel deposits. These findings underscore the intricate interplay between sedimentary evolution and anthropogenic activities in influencing groundwater salinization, highlighting the necessity for scientifically informed, water-type-specific management strategies.