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

Pyrite oxidation and sedimentary sulfate dissolution are the primary components of riverine sulfate (SO42-) and are predominant in global SO42- flux into the ocean. However, the proportions of anthropogenic SO42- 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-SO42- and δ18O-SO42-) and Bayesian isotope mixing models were used to calculate the variable anthropogenic SO42- inputs and elucidate their temporal impacts on riverine SO42- 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 SO42- and fertilizer-derived SO42- inputs in the lower reaches had dramatically altered SO42- concentrations and δ34S-SO42- and δ18O-SO42- 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-SO42- and δ18O-SO42- from 10.3‰ to 9.9‰ and from 6.1‰ to 4.4‰, respectively. These outcomes confirm that anthropogenic SO42- inputs from acid mine drainage (AMD) have increased, but sewage effluents SO42- inputs have decreased.