| Abstract:The porewater of the aquitard reflects the original solution of the soil when it was deposited, and is of great importance for paleoclimate reconstruction. In order to analyze the recharge of aquitard porewater and its salt source since the Holocene in the Yangtze estuary, samples of labile salt, geotechnical, phreatic water, and nearshore seawater were collected. The hydrochemical characteristics of the porewater of the aquitard in the study area were obtained using the soluble salt index combined with geotechnical indexes (water content, wet density, specific gravity). The recharge of porewater and its salt source were analyzed using the bipartite element method, Piper trilinear diagram, Gibbs diagram, and ion ratio method. The results show that the mineralization of porewater ranged from 1.16-32.79 g/L, with an average value of 10.68 g/L. The percentage of haline water was the highest, followed by saline water and brackish water. The porewater type was dominated by Cl-Na type (85.6%), followed by Cl-Ca·Mg, HCO3-Ca-Na, HCO3-Ca, and Cl-Ca types in descending order. The local phreatic water type is HCO3-Ca and the deep porewater type is Cl-Na, indicating that the deep porewater sea retains the environmental information when the soil was deposited. The middle and shallow porewaters are affected by the surface effects such as atmospheric precipitation recharge, human activities and evaporation, and the porewater hydrochemical data are more discrete. The porewater δ18O and δD data indicate that the porewater sample sites are influenced by the superposition of seawater mixing and evaporation, and the evaporation is stronger. The porewater seawater recharge ratio ranged from 30.2% to 87.0%, and the atmospheric precipitation recharge ratio ranged from 13.0% to 69.8%. The salt in the soil is mainly from Holocene sea intrusion (sea source) with evaporative salt rock dissolution and feldspar weathering dissolution (crustal source). Taking deep porewater as an example, the proportion of seawater recharge to porewater salinity is 37%, and the rest of salinity is mainly from crustal sources. |
