Hydrochemical characteristics and formation mechanisms of phreatic groundwater in Shanghai

Objective  Phreatic groundwater environment is critical to Shanghai’s establishment as an ecological city. Clarifying the characteristics and formation mechanisms of the phreatic groundwater environment in different sedimentary settings provides valuable reference for optimizing water resource management approaches and establishing soil−water environmental governance frameworks.

Methods  This study conducted sampling and analytical testing of groundwater samples from 102 phreatic groundwater monitoring wells in Shanghai area, and carried out research on the origin and evolution mechanisms of the groundwater environment based on hydrochemical methods.

Results  The results show that the pH of phreatic groundwater in the study area ranges from 6.93 to 8.23, indicating neutral to weakly alkaline, with TDS generally below 1000 mg/L. The hydrochemical characteristics of phreatic groundwater in the lacustrine plain area and the coastal plain area show minor differences, while the phreatic groundwater in the delta plain area exhibits certain variations in major ion concentrations and hydrochemical types compared to the other sedimentary areas. The hydrochemical origins of phreatic groundwater in the three sedimentary areas are fundamentally similar, primarily dominated by water−rock interactions and influenced by evaporation. Phreatic groundwater in areas along the river and coast shows no significant impact from seawater intrusion. Positive cation exchange is an important factor influencing the hydrochemical evolution of phreatic groundwater. Phreatic groundwater is generally affected by human activities.

Conclusions Based on the characteristics and formation mechanisms of the phreatic groundwater environment in the study area, this study proposes recommendations for groundwater management approaches and soil−water environmental governance frameworks for the three types of sedimentary environments. In lacustrine plain and coastal plain, soil and water environmental remediation should primarily focus on addressing point−source pollution. In delta plain, polluting industries should be avoided, and remediation efforts require a combination of point−source and area−wide approaches, with emphasis on regional−scale management.