Tracing sulfate origin and transformation in an area with multiple sources of pollution in northern China by using environmental isotopes and Bayesian isotope mixing model

Sulfate (SO₄²⁻) contamination in groundwater and surface water is an environmental problem of widespread concern. In this study, we combined stable isotope analyses of SO₄²⁻ (δ³⁴S and δ¹⁸O) and water (δ²H and δ¹⁸O) with a Bayesian mixing model (SIAR), for the first time, to identify sources and transformation of SO₄²⁻ in an area of northern China with multiple potential sources of pollution. The overall values of δ³⁴S and δ¹⁸O-SO₄²⁻ ranged from 1.3‰ to 16.3‰ and −3.8‰–8.8‰ in groundwater, and from −1.1‰ to 9.3‰ and 2.7‰–9.2‰ in surface waters, respectively. Analyses of SO₄²⁻ isotopes and water chemistry indicated that SO₄²⁻ in groundwater and surface water mainly originated from mixing of oxidation of sulfate, sewage, chemical fertilizers, dissolution of evaporite and precipitation. There was no significant correlation between δ³⁴S and δ¹⁸O and SO₄²⁻ concentration in groundwater, indicating that bacterial sulfate reduction did not affect the SO₄²⁻ isotopic composition. SIAR model showed the main sources of SO₄²⁻ in groundwater and surface water comprised oxidation of sulfide minerals and sewage. In groundwater, oxidation of sulfide minerals and sewage accounted for 37.5–44.5% and 35.5–42.7% of SO₄²⁻, respectively. In regard to surface waters, the contribution of oxidation of sulfide minerals to SO₄²⁻ was higher in the wet season (31.8 ± 9.9%) than in the intermediate (22.4 ± 7.8%) and dry (20.9 ± 8.2%) seasons, but the contribution proportion of sewage was slightly lower in the wet season (19.9 ± 8.5%) than in the intermediate (23.8 ± 8.7%) and dry (24.2 ± 8.5%) seasons. This study indicates that it is necessary for local government to improve the treatment infrastructure for domestic sewage and optimize methods of agricultural fertilization and irrigation to prevent SO₄²⁻ contamination of groundwater and surface water.