Sulfate source apportionment reveals dominant sulfide oxidation and increasing anthropogenic influence along the...

The Yangtze River ranks second globally in salt flux and plays a pivotal role in material transport to the oceans. As a key constituent of salt flux, sulfate (SO₄^2-) requires clear source apportionment and process characterization. This study integrates hydrochemistry, stable isotopes, and a Bayesian isotope mixing model (MixSIAR) to resolve the spatial distribution, sources, transformation pathways, and drivers of SO₄^2- across the basin. Observed SO₄^2- concentrations range from 8.44 to 277.59 mg L^-1, with region-averaged means of upstream (80.83 mg L^-1) > downstream (41.00 mg L^-1) > midstream (30.59 mg L^-1). Basin hydrochemistry reflects the combined control of rock weathering and anthropogenic activities. Isotopic signatures identify five potential SO₄^2- sources: atmospheric deposition, evaporite dissolution, sulfide oxidation, fertilizers, and sewage. MixSIAR estimates indicate that sulfide oxidation was the dominant SO₄^2- source in the dry-season samples, with the highest contribution in the upstream region (90.1 ± 4.9%). Toward the midstream and downstream, the relative contribution of sewage increased, indicating a dry-season longitudinal shift from predominantly natural processes toward stronger anthropogenic inputs. These results provide dry-season constraints on SO₄^2- sources and transformation processes in the Yangtze River and improve understanding of sulfur migration in large river systems.