Identification of geochemical characters and the influence of river and seawater on coastal areas are of significant impact on water resources management in coastal areas globally. Thus, it requires careful investigation of the hydrochemical evolution mechanisms and assessment of the quality of shallow groundwater. The study specifically focused on the estuarine coastal zone of Qidong, China, a city located on the Yangtze estuary. Thirty shallow groundwater samples and five surface water samples were collected during a field investigation, and 25 water quality indexes were analyzed. Methods including mathematical statistics, Gibbs figure, Piper diagram, and ionic rations were used to analyze the hydrochemical characteristics and evolution mechanisms. The spatial distribution of key parameters was assessed using a GIS-based spatial gridding technique. Results showed that the shallow groundwater in this estuarine coastal zone is weakly alkaline. The major hydrochemical parameters, including total dissolved solids (TDS), Na+ and Cl−, Mg2+, and SO42−, displayed similar spatial distributions; the distributions of Ca2+ and Mg2+ were irregular; the distributions of SO42− and HCO3− shared a similar trend, increasing gradually from the southern to northern regions of the study zone; and the values of NO3− and NH4+ were generally low in the study area. The hydrochemistry of the groundwater consists of HCO3-CaMg type and HCO3-Na, with HCO3·Cl-Na type and Cl-Na being the dominant constituents toward the coastal strip. The coastal aquifers are subjected to the ongoing influence of seawater intrusion, ion exchange processes, freshwater infiltration, and mineral (carbonate and silicate) dissolution, which impact groundwater evolution. Most of the groundwater samples are unsuitable for drinking purposes, but more than 90% of samples have irrigation suitability, based on the WHO standards and the classifications of sodium absorption ratio (SAR), soluble-sodium percentage (SSP).

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The optimal condition of polysilicon ferric chloride (PFSC) flocculant was established by thinking of the factors , such as Si/Fe molar ratios,the reaction temperature and the pH value of the solution. When the molar ratio of Si and Fe = 1, pH value = 0.6, curing temperature at 30 ~40 °C, poly ferric chloride dosage of 0.75mg/L, the rate of turbidity removal of seawater is up to 96.5%. Experimental results show that poly ferric chloride seawater turbidity removal effect is obvious.