Hydrochemical evolution of groundwater in the Bailang River area of the muddy coastal zone, northern Shandong, China

The coastal area of Laizhou Bay is one of the typical regions affected by sea (saline) water intrusion in China, and the intrusion has resulted in the formation of a complex groundwater system under the influence of multiple transgression and regression events. The hydrochemical characteristics and evolution of the groundwater system in the Bailang River basin, a tributary of the Laizhou Bay, was investigated, the hydrochemical characteristics and distribution patterns of groundwater from inland to the sea were comprehensively analyzed, the hydrochemical evolution process within the Bailang River basin was explored. Results indicate that inland saline water is formed primarily by the mixing of fresh water and brine, while seaward saline water is predominantly formed by the mixing of fresh water and seawater. During the brine formation process, the fresh water mixing, water-rock interaction, and cation exchange jointly altered the hydrochemical composition of brine. In addition, the mineral saturation index show that calcite and dolomite are near or at saturation level, while halite remains in an unsaturated state. From land to sea, different cation exchange processes occur in the groundwater system. In shallow freshwater and brackish water areas, Ca²⁺ is exchanged with Na⁺ adsorbed by aquifer media, while in other regions, the cation exchange processes is opposite.