Abstract: Quaternary sea level fluctuation had played important role in affecting the spatial and temporal distribution of top Holocene phreatic aquifer and 5 (Ⅰ-Ⅴ) underlying late to early Pleistocene aquifers in the Yangtze estuary region. Inevitably, the evolution of groundwater chemical field of the region was closely associated with transgression and the occurrence of aquifuge in Quaternary stratigraphy. Present database was established primarily on 245 water samples obtained from these aquifers in 78 selected Quaternary hydrogeological boreholes. The analytic items included the concentrations of K ⁺, Na⁺, Ca²⁺, Mg ²⁺, NH ⁴⁺, Al³⁺, Cl^-, HCO₃^-, CO₃^2-, and SO₄^2-, and the values of PH and temperatures. Using hydrogeochemical methods, this paper deals with the nature of groundwater chemical types and chemical evolution. The results are showed as below: (1) One phreatic aquifer and five confined aquifers in study area belong to three aquifer systems, i. e. Holocene aquifer system (phreatic aquifer), mid and upper Pleistocene aquifer system (Ⅰ-Ⅲ confined aquifers), and lower Pleistocene aquifer system (Ⅳ-Ⅴ confined aquifers). (2) Spatial and temporal distributions of brackish water and salt water in sedimentary system of the study area were controlled by Quaternary transgressions. (3) There are two main types of aquifer system can be differentiated by Piper diagram, i. e. Ⅰ-Ⅲ aquifer system and Ⅳ-Ⅴ aquifers system. The former is characterized by salt water (Cl—Na·Ca and Cl—Na), and the later is dominated by fresh water (HCO₃—Na and Cl·HCO₃—Na) and brackish water (Cl—Na). (4) There are four types of groundwater chemical evolution in study area: ① dilution of salt water by fresh water recharge through atmospheric replacement taking place only in phreatic aquifer. ② exchange of sodium absorbed in aquifer medium by calcium in water. The ratios Na/(Na+Cl), used as an indicator of ion exchange, demonstrate that the degree of exchange in Ⅳ-Ⅴ aquifers (0 65 on average) is more intensifying than that of Ⅰ-Ⅲ aquifers (0 45 on average). ③ mixture of different water types through leaky system in the Quaternary aquifers system. The groundwater composition reveals that the mixture of brackish water character due to leaky system is of the transition in chemical property between salt water in upper aquifers and fresh water in lower aquifers. Through thermodynamical calculation, we proposed that the groundwater in study area is super saturated with calcite ( SI >0).

Key words: Water chemical evolution Quaternary aquifer system, Yangtze river estuarine Region