Using a dual isotopic approach to trace sources and mixing of sulphate in Changjiang Estuary, China

The dual isotopic compositions of dissolved SO42- in aquatic systems are commonly used to ascertain SO42- sources and possible biogeochemical processes. In this study, the physical parameters, major anions and isotopic compositions of SO42- in water samples from Changjiang River (Nanjin) to the East Sea in Changjiang Estuary were determined. The salinity ranged from 0‰ to 32.3‰ in the estuary water samples. The Cl-,SO42- concentrations and δ18O–H2O values followed the salinity variations from freshwater to seawater, which indicated that mixing processes might be a major factor involved in the distribution of water and solutes. The contents and isotopic compositions of SO42- suggested that atmospheric deposition, evaporite dissolution and sulphide oxidation were the major sources of dissolved SO42- in the freshwater of Changjiang River. In addition, the mixing model calculated by contents and isotopic compositions of SO42- indicated that the mixing of freshwater and sea water was the major factor involved in SO42- distribution in Changjiang Estuary. However, slightly elevated δ18O–SO4 values were observed in the turbidity maximum zone, which suggested that biological processes might affect the O isotopic compositions of SO42- there.

► Changjiang Estuary plays an important role in transportation of the water and solute. ► The dual isotopic method could be used to understand sulfate biogeochemistry in estuaries. ► Mixing processes should be a major factor involved in the distribution of water and sulphate. ► Sulphate in the Changjiang River mainly derived from atmospheric deposition, evaporite dissolution and sulphide oxidation.