Hydrochemical evolution of groundwater in muddy coastal zone of the northern Shandong Peninsula

The northern Shandong Peninsula is one of the typical areas of seawater (saline water) intrusion in China, with complex sedimentary environment and various types of groundwater quality. Influenced by multiple transgressions and regressions, the aquifer system is complicated by the fact that the stratum in study area is composed of cyclic interbedding of unconfined river delta sand bodies and shelf muddy deposits in a high-amplitude sea level change setting. In recent decades, excessive exploitation of groundwater by human activities has resulted in fresh depression cone and brine depression cone, which have changed the hydrodynamic conditions of local groundwater and influenced the formation and evolution of groundwater. Based on the sedimentary background and distribution characteristics of aquifers in the study area, we divided the groundwater into three basins and two layers, on which analysis and discussion were conducted in marine geology, environmental isotopes, and hydrochemistry. Results show that there are obvious regional differences in the evolution of shallow groundwater, Mihe River and Bailang River are mainly recharged by precipitation, and the depression cone is dominated by Cl-Ca water, reflecting groundwater salinization. Weihe River is mainly recharged by runoff, and the depression cone is dominated by HCO₃·Cl-Na water, indicating groundwater desalination. Therefore, the formation of saline and brine in deep groundwater of the study area is mainly controlled by mixing among fresh water, seawater, and brine.