This paper is the result of hydrogeological and environmental geological engineering.

Objective To understand the groundwater flow system in the middle and lower reaches of the Yangtze River plain and analyze its hydrochemical characteristics and evolution mechanism.

Methods Based on the basic characteristics of hydrochemistry in Dangtu area of Maanshan City, the hydrochemistry evolution of shallow groundwater in this area was analyzed by using multivariate statistical analysis, hydrochemistry maps, ion ratio and reverse hydrogeochemical simulation.

Results The results show that: (1) The groundwater in the study area is mainly alkaline water with low salinity. The cations in the groundwater are mainly Ca²⁺ and Mg²⁺, and the anions are mainly HCO₃^- and SO₄^2-. (2)According to Shukarev classification, the hydrochemical types of groundwater in the study area can be divided into seven categories, among which the hydrochemical types of loose rock pore water-bearing rock group and clastic rock pore fissure water-bearing rock group are mainly HCO₃^-Ca type, HCO₃^-Ca · Na type, HCO₃ · Cl-Ca · Na type and HCO₃^-Ca · Mg type. Chemical types of fractured water-bearing rock groups in bedrock are mainly HCO₃ · SO₄-Ca · Mg type and SO₄ · HCO₃^-Ca · Mg type. (3) The over-standard rate of shallow groundwater samples in the study area is 46%, and the overall water quality is poor. The components with higher over- standard rate are Mn, COD_Mn, nitrate (calculated as N), Fe, As, ammonia nitrogen (calculated as N), etc. (4) The chemical composition of groundwater in the study area is mainly controlled by rock weathering. Besides, there is the positive cation alternate adsorption of Na- Ca. (5) The results of reverse hydrogeochemical simulation quantitatively verify that water- rock interaction plays a leading role in the formation and evolution of shallow groundwater components in this area.

Conclusions Groundwater in the study area is mainly low salinity and alkaline, which can be divided into loose rock pore water, clastic rock pore fissure water and bedrock fissure water. The main ion ratio and reverse hydrogeochemical simulation revealed that the chemical composition of shallow groundwater in this area was mainly formed after the groundwater leached calcite, dolomite and other carbonate minerals, quartz, feldspar and other silicate minerals, kaolinite and other clay minerals, rock salt and gypsum reached supersaturation.