[Seasonal Variation and Sources Identification of Dissolved Sulfate in a Typical Karst Subterranean Stream Basin...

Karst water, which provides approximately 25% of the world's drinking water, is especially vulnerable to anthropogenic pollutants. To determine the variations between high and low flow periods and the sources of dissolved sulfate (SO₄^2-) in small karst basins, SO₄^2- concentrations, stable sulfur and oxygen isotopes (δ³⁴S-SO₄ and δ¹⁸O-SO₄), and oxygen isotopes of water (δ¹⁸O-H₂O) were investigated in surface and groundwaters, during the high and low flow seasons, within the Babu subterranean stream basin. Analysis showed that: ① the water samples that were directly impacted by acid mine drainage exhibited high SO₄^2- concentrations (≥250 mg·L^-1) and significant seasonal variation, while the seasonal variation of non-AMD-impacted water with low SO₄^2- concentrations was not significant. ② During the high flow season, the mean δ³⁴S-SO₄ and δ¹⁸O-SO₄ values of surface water were -10.5‰ and 4.7‰, respectively, and -11.5‰ and 1.3‰ during the low flow period; the mean values of δ³⁴S-SO₄ and δ¹⁸O-SO₄ in groundwater samples were -2.9‰ and 7.1‰ during the high flow period, respectively, and -3.2‰ and 6.2‰ during the low flow period. Both surface and groundwater samples exhibited higher δ³⁴S-SO₄ and δ¹⁸O-SO₄ values during the high flow period than during the low flow period. ③ The values of δ³⁴S-SO₄ in the surface and groundwater samples were relatively stable, indicating that the sources of SO₄^2- at specific sampling sites were stable.④ The main sources of SO₄^2- in surface and groundwaters were rain, sulfide, and gypsum, accounting for 13%, 40%, and 47%, respectively, of SO₄^2- in samples taken from the basin outlet during the high flow season, and 18%, 39%, and 43%, respectively, in samples obtained during the low flow season.