Hydrochemical characteristics and environmental indicators of aquitards since the Late Cenozoic on Sunshine Island, Jiangsu Province, China

•A novel method is proposed to obtain the hydrochemical characteristics of strata.•This novel method can be used to identify offshore freshened groundwater.•The hydrochemical characteristics of aquitards provide environmental indicators.•Sources of aquitard salinity since the Late Cenozoic were determined. To analyze groundwater recharge and salinity sources in the Yangtze Delta region since the Late Cenozoic, a 402 m deep well, HYYK1, was drilled using a single monitoring hole through multi-layer groundwater. Soluble salt, water, and geotechnical samples were collected continuously. A novel method was developed to determine the hydrochemical characteristics using a soluble salt index combined with geotechnical indices (water content, wet density, and specific gravity). A Piper trilinear diagram and ion ratio method combined with δD, δ18O, and 14C data were used to analyze groundwater recharge and salt sources. The salinity of the upper Qp1 and its overlying aquitards was mainly NaCl and the salinity of the lower Qp1 and Neogene (N) aquitards was mainly NaHCO3. 14C dating data indicate the water of the N and lower Qp1 aquifers to be paleo-groundwater. The δ18O and δD values of porewater from the Qp3, Qp2, and Qp1 aquitards indicate that the porewater was subjected to relatively intense evaporation and that seawater mixing decreased with increasing depth. The δ18O and δD values of porewater from the N aquitard indicate that the porewater is mainly recharged by atmospheric precipitation and is subjected to intense evaporation. Calcium nodules in the Qp1 and N aquitards are the result of intense evaporation. Seawater intrusion is the major source of porewater salinity in the upper Qp1 and its overlying aquitards, with the proportion of sea-derived salt recharge decreasing with increasing depth. Porewater salts of the lower Qp1 and N aquitards are mainly from feldspathic carbonate weathering and evaporite salt dissolution. Compared with previous approaches, the method developed in this study provides a more effective way to identify offshore freshened groundwater, which can be used as desalination raw water and determine its hydrochemical characteristics and formation processes.