Hydrological controls on the sources of dissolved sulfate in the Heihe River, a large inland river in the arid northwestern China, inferred from S and O isotopes

The Heihe River is one of the largest and most intensely exploited inland rivers of arid northwestern China, which is currently threatened by water shortage and ecological problems driven by climate change and human activity. To constrain SO 4 2 - sources to the river and evaluate the influence of hydrological processes on SO 4 2 - sources and transport in the large river basin, chemical and isotopic measurements of surface and groundwater samples were carried out in the Heihe River basin. The river water had SO 4 2 - concentrations in a range of 117–316 mg/L with an average of 199 mg/L, characterized by δ34SSO4 values ranging from +2.5‰ to +5.4‰ and δ18OSO4 values ranging from −1.8‰ to +10.2‰. The shallow groundwater had SO 4 2 - concentrations in a range of 54–1222 mg/L with an average of 383 mg/L, characterized by δ34SSO4 values ranging from 0‰ to +7.4‰ and δ18OSO4 values ranging from −2.6‰ to +10.9‰. A significant correlation was found between δ34SSO4 and δ18OSO4 values of the river water. A gradual downstream shift from lower δ34SSO4 and δ18OSO4 values to higher δ34SSO4 and δ18OSO4 values was observed for both the riverine and groundwater SO 4 2 - , which corresponds to a change in the SO 4 2 - sources, governed by the geology, from oxidation of sulfides in the upstream part to dissolution of evaporite and/or soil sulfate in the downstream part of the Heihe River basin. Sulfate input dominated by sulfide oxidation via mountain groundwater discharge at the Yingluo Valley is a major contributor to river SO 4 2 - in the upper reaches. Admixture of groundwater SO 4 2 - enhanced by intensive agricultural activities is responsible for increasing river SO 4 2 - concentration in the middle reaches. Evapotranspiration effects in the lower reaches result in indirect mixture of groundwater SO 4 2 - via dissolution of soil sulfate that has cycled through the soil organic matter pool. This study implies that mixing of groundwater SO 4 2 - with surface water is the dominant process affecting SO 4 2 - sources and S cycling of the Heihe River, which is controlled by the regional hydrological processes affected by natural and anthropogenic factors.