Sources of riverine sulfate in Yellow River and its tributaries determined by sulfur and oxygen isotopes

Determining the cycle of riverine sulfate, affected by natural weathering and anthropogenic activities, is necessary for surface water management. Mainstream and tributaries of Yellow River from Xiaolangdi Reservoir, including Qinhe River and Yiluo River, were selected. River water samples were collected during different seasons, and dual isotopes of dissolved sulfate were measured, together with hydrochemical compositions of surface water and isotopic compositions of potential sources. Based on these, sources of dissolved sulfate in Yellow River and its tributaries with the mixing ratios of different sources in Qinhe River could be identified. The results indicate that: ① dissolved sulfate in Yellow River mainly derived from Quaternary loess, and the mean sulfate content was 2.23 mmol/L, with the mean δ³⁴S_SO4and δ¹⁸O_SO4 values of +8.9‰ and +10.4‰, respectively; ② during flood season, 24% of dissolved sulfate in Qinhe River derived from rain water, 61 percent of dissolved sulfate derived from soil, and 15 percent of dissolved sulfate derived from gypsum dissolution. During temperate season, the mixing ratios of rain water, soil and gypsum were 39%, 36% and 25%, respectively. The mean sulfate concentration was 2.44 mmol/L, and mean δ³⁴S_SO4and δ¹⁸O_SO4 values were +9.8‰ and +9.7‰, respectively; and, ③ dissolved sulfate in Luohe River more derived from sewage water and that in Yihe River was mainly controlled by soil and fertilizer. The mean sulfate concentration in Yiluo River was 1.27 mmol/L, and mean δ³⁴S_SO4and δ¹⁸O_SO4values were +10.4‰ and +6.5‰, respectively. Dissolved sulfate in local river water was mainly controlled by natural weathering processes, such as dissolution of evaporites minerals and soil sulfate. At the same time, it should not be ignored that anthropogenic activities had altered the hydrochemical composition of the water Yiluo River, which should be paid more attention by local surface water managers.