Isotope evidence for temporal and spatial variations of anthropogenic sulfate input in the Yihe River during the last...

Pyrite oxidation and sedimentary sulfate dissolution are the primary components of riverine sulfate (SO₄^2-) and are predominant in global SO₄^2- flux into the ocean. However, the proportions of anthropogenic SO₄^2- inputs have been unclear, and their tempo-spatial variations due to human activities have been unknown. Thus, field work was conducted in a spatially heterogeneous human-affected area of the Yihe River Basin (YRB) during a wet year (2010) and drought years (2017/2018). Dual sulfate isotopes (δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-) and Bayesian isotope mixing models were used to calculate the variable anthropogenic SO₄^2- inputs and elucidate their temporal impacts on riverine SO₄^2- flux. The results of the mixing models indicated acid mine drainage (AMD) contributions increased from 56.1% to 83.1% of upstream sulfate and slightly decreased from 46.3% to 44.0% of midstream sulfate in 2010 and 2017/2018, respectively, in the Yihe River Basin. The higher upstream contribution was due to extensive metal-sulfide-bearing mine drainage. Sewage-derived SO₄^2- and fertilizer-derived SO₄^2- inputs in the lower reaches had dramatically altered SO₄^2- concentrations and δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- values. Due to climate change, the water flow discharge decreased by about 70% between 2010 and 2017/2018, but the riverine sulfate flux was reduced by only about 58%. The non-proportional increases in anthropogenic sulfate inputs led to decreases in the flow-weighted average values of δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- from 10.3‰ to 9.9‰ and from 6.1‰ to 4.4‰, respectively. These outcomes confirm that anthropogenic SO₄^2- inputs from acid mine drainage (AMD) have increased, but sewage effluents SO₄^2- inputs have decreased.