Source identification and coastal export of sulfate in a megacity river: Insights from sulfur isotopes and inverse...

Excessive sulfate (SO₄^2-) flow into marginal seas through megacity rivers, posing a significant threat to the ecological environment of marginal seas. Tracing and quantifying the source of SO₄^2- are essential for maintaining the health and stability of marine ecosystems. This study reported SO₄^2- concentrations and δ³⁴S_SO4 in the Yongding River, a typical megacity river discharging into the sea in northern China. The SO₄^2- concentrations averaged 147 mg·L^-1, with δ³⁴S_SO4 values ranging from +5.85‰ to +14.99‰. Based on hydrochemical indicators and ion ratios, six potential sources were identified (agriculture, sewage, industry, precipitation, evaporite, and sulfide oxidation). A Monte Carlo inverse model integrating δ³⁴S_SO4 and ionic ratios was applied to quantify source contributions. Results show that natural weathering (61%) dominates upstream, while anthropogenic inputs (75%) increase downstream, with sewage and industry sources contributing 41% and 33%, respectively. Sulfide oxidation intensifies near reservoirs, suggesting enhanced oxidation driven by water regulation. Total SO₄^2- flux increased longitudinally, with an estimated annual export of 2.31 × 10^-4 Tmol yr^-1 to the Bohai Sea. Over 70% of SO₄^2- flux originated from anthropogenic sources, highlighting the critical role of human activities in disrupting sulfur cycling in marginal sea. A hydrology-load-redox (H-L-R) framework was proposed to emphasize the importance of multi-factor coordination in pollution control and land-sea integrated watershed management. This study demonstrates that inverse model holds significant promise for application in complex human-dominated systems, offering a viable approach to strengthen pollutant tracing and risk assessment in marginal marine environments.