The three-dimensional structure, physical and chemical properties and environmental indicative significance of ground substrate in a city of North Jiangsu Plain

This study, conducted in a city in northern Jiangsu Province, located in central-eastern China, systematically analyzed the pH, electrical conductivity, bulk density, and geochemical characteristics of the topsoil (0–0.5 m) through field surveys, drilling, sampling, and laboratory testing. Based on extensive borehole data (total drilling depth > 4.4 km), it revealed the vertical structure and sedimentary evolution patterns of the ground substrate at depths of 0–20 m. The results indicate: (1) The topsoil generally exhibits a neutral to alkaline pH (mean 7.69) with highly variable electrical conductivity (mean 241 µS/cm, coefficient of variation 71.42%), showing a “high in eastern coastal areas, low in northwestern regions” salinity differentiation pattern, primarily controlled by marine input and parent material. (2) Soil bulk density (mean 1.41 g/cm3) shows significant variation influenced by land use and matrix formation, with arable land having the lowest density and residual alluvial areas the highest. (3) Geochemical elements demonstrate distinct geomorphic zonation: hilly areas are enriched in SiO2 and Na2O, while plains and coastal zones show enrichment in Fe2O3, CaO, MgO, and organic matter. Principal component analysis suggested that elemental distribution is mainly controlled by geological background (variance contribution rate 39.99%) and human activities (16.30%). (4) Vertically, the study area can be divided into two major structural units: the Luxi uplift belt (dominated by “Soil/Bedrock” configuration) and the Sulu subsidence belt (characterized by complex “Loam-sand-clay” interbedded sedimentary cycles). Notably, the continuous clay layers widely distributed at depths of 2–5 m in the alluvial plain serve as critical water barriers controlling regional water and salt migration. The integrated model of “three-dimensional structure-physical and chemical properties-sedimentary evolution” of ground substrate in the study area was established, which provided a systematic geological basis for the optimization of regional land resources, the prevention and control of soil salinization and the management of ecological risks.