The Laizhou Bay area contains a large amount of Quaternary brine resources, which have been gradually depleted by long-term high-intensity mining. The local brine genesis is still controversial, and the dominant theory of the evaporation of brine formation limits the exploration of brine resources to the land area, while the confirmation of freezing brine formation would greatly expand the brine storage range. In this research paper, the genesis of underground brines was analyzed based on the major ion concentrations of 62 samples of underground brines at different depths at 24 stations along the south coast of Laizhou Bay. The results show that the underground brines originated from seawater; however, their chemical components were changed during the evolution process after formation. The hydrogeochemical modeling results show that the mixing of seawater and fresh groundwater also affects the chemical composition of brines. The large-scale exploitation of brine resources in recent decades has also led to significant changes in the chemical composition of the underground brine in Laizhou Bay compared with the earlier period. The special geographic environment and the development of the brine chemical industry have led to the evolution of underground brines in Laizhou Bay being affected by a variety of factors, which makes the traditional analysis of brine genesis in this region not applicable. Furthermore, although evaporation and concentration are the dominant factors in the formation of brine, there is still a lack of evidence to rule out the existence of the freezing pathway.

The excessive exploitation of groundwater is becoming a serious global issue. Different from other regions, groundwater extraction in coastal areas usually stops and moves inland after causing seawater intrusion. The abundant salt fields in the Laizhou Bay area of China provide a unique case of maintaining high-intensity underground brine mining even after seawater intrusion. The intensive exploitation of underground brine has led to significant changes in the groundwater flow field. However, there is still a lack of research on how different factors affect the groundwater level in this mining situation. In this paper, time series analysis methods were used to investigate the impact of brine water extraction, tidal fluctuations, and precipitation on the groundwater level in the Laizhou Bay area. The results indicate that brine extraction is the main factor controlling the changes in groundwater level, with the cessation and resumption of extraction resulting in a 93.4 cm increase and a 122.5 cm decrease, respectively. Different rainfall patterns can also lead to an increase in groundwater levels, especially when a heavy rainfall event can cause a 61.2 cm increase. Tidal fluctuations can cause periodic fluctuations in the groundwater level, with a variation amplitude of approximately 11% of the tide itself.