Geochemical and isotopic (oxygen, hydrogen, sulfur) indicators constraining the origin and contribution ratios of dissolved sulfate in shallow aquifers from the lower Yellow River floodplain, northern China

Sulfate (SO₄²⁻) contamination of groundwater is becoming a major public health threat in the lower Yellow River floodplain of southwest Shandong Province. In this study, 72 shallow groundwater samples, 18 surface water samples and 21 industrial wastewater samples (including 4 mine drainage samples used to analyze the effects of coal mining activities) were collected to investigate the origins of sulfate in the water environment. Hydrochemical and isotopic data of SO₄²⁻ (δ³⁴S and δ¹⁸O) and H₂O (δD and δ¹⁸O) as well as stable isotope mixing models in R (SIMMR) were integrated to identify the sulfate sources and the proportions of each source in different watersheds. The results showed that chemical fertilizer is the dominant source of sulfate in surface water in most of the area, while manure and atmospheric deposition are the main sources of sulfate in groundwater. The contributions of evaporites dissolution to sulfate are most significant in surface water and groundwater in Jiaxiang and southern Yuncheng counties. In areas with intensive industrial activities, the contribution of industrial sewage to water bodies is also considerable. The groundwater in Juye and Jiaxiang Counties in the downstream of Zhuzhaoxin River is worst affected by mine drainage. The hydrogeological setting and anthropogenic activities jointly controlled the sulfate in the water. The research not only provide useful methodology and clues for similar studies in other sulfate contaminated areas, but also exemplify the vital importance of hydrogeochemical and isotopic monitoring for the sustainable management of water resources.