Evaluation of the hydrochemical evolution characteristics and renewable capacity of deep fresh groundwater in the Hangzhou Bay New Zone, China

The deep fresh groundwater in the Hangzhou Bay New Zone lies in a confined aquifer. The roof of the freshwater layer is 80–110-m deep, and its average thickness is 22 m. Freshwater occupies the downstream portion of the aquifer structure, while saline water occupies the upstream portion of the aquifer structure; this rare freshwater distribution characteristic causes widespread concern. Based on the hydrogeological conditions of the study area, the spatial and temporal evolution characteristics of groundwater in the Hangzhou Bay New Zone are researched by analyzing the hydrochemical characteristics and isotope tracing results of water samples. Furthermore, the formation mechanism of the fresh groundwater and its renewal capacity are evaluated. The results show that (1) the groundwater chemical type is influenced by the total dissolved solids (TDS). With an increase in the TDS of the groundwater, the main anions in the groundwater evolve from HCO₃⁻ to Cl⁻, and the major cations evolve from Ca²⁺ and Mg²⁺ to Na⁺. (2) The hydrogeological conditions, the hydrochemistry evolution characteristics, and the ²H, ¹⁸O, and ³H characteristics of the groundwater indicate that the freshwater does not receive a supplement of modern water and that no hydraulic connection exists between different layers. (3) According to ³H and ¹⁴C dating, the formation period of deep fresh groundwater and saline water in the study area was 13.0–17.9 Ka.B.P, and, 10.6–15.6 ka.B.P (the late Pleistocene), respectively. It is predicted that the fresh groundwater is the continental sedimentary water buried in the Pleistocene, as a result of sealing the well-isolated original sedimentary fresh water because of direct upstream infiltration and downstream salinization in the late transgression (mainly ice transgression in the later stage). (4) The fresh groundwater has little renewable capacity due to the absence of recharge sources.