Using sulfur and oxygen isotope values to partition riverine sulfate sources and illustrate their responses to hydrological processes

Orogenesis contributes to the elevation of highly eroded rock strata, whose interactions with the atmosphere, water, and biota result in the release of dissolved substances. The fluvial transport of dissolved sulfate (SO₄²⁻) from mountains to oceans is a critical component of the global sulfur cycle. However, anthropogenic activities have significantly increased the concentrations of sulfate and altered isotope compositions. It is imperative to delineate the impacts of anthropogenic disturbances and clear their transport mechanisms. To address this issue, spatial and temporal water samples were collected from the Qin River Basin (QRB) between 2012 and 2015. Sulfate sulfur and oxygen isotope values (δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-), alongside Bayesian isotope mixing models (BIMMs) were employed to identify pathways of anthropogenic inputs and quantify their contributions. The average mainstream SO₄²⁻ concentration, δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- values in the upper reaches (n = 18), middle (n = 9), and lower reaches (n = 44) were 1.09 mmol/L, 1.5 ‰ and 5.6 ‰; 1.34 mmol/L, 6.8 ‰ and 6.9 ‰; 2.31 mmol/L, 8.5 ‰ and 7.7 ‰, respectively. BIMMs results from spatial water samples indicated an increasing trend in contributions from gypsum, loess, sewage and chemical fertilizer but a decreasing trend from coal mine drainage (CMD) and pedogenic sulfate sources to riverine sulfate along the river. Results from temporal water samples at the outlet indicated that pedogenic sulfate, CMD, and loess sulfate were transport-limited, conversely, gypsum and chemical fertilizer were source-limited, and sewage has chemostatic behaviors. Despite a significant reduction in annual water discharge since 1956–2000, the average annual sulfate flux from 2013 to 2015 exceeded historical values, with approximately 45 % of riverine sulfate derived from anthropogenic input, and the flow-weighted average δ³⁴S-SO₄^2- and δ¹⁸O‒SO₄^2- values changed to 7.9 ± 1.2 ‰ and 6.4 ± 0.2 ‰. These findings illuminated the profound impacts of anthropogenic inputs on riverine sulfate flux in Qin River and offer a robust methodology for partitioning aqueous pollution sources and delineating their transport mechanisms in the complex environmental settings.