4.2. Sources of sulfate in the NSL
According to the above discussion of hydrochemical and isotopic characteristics in NSL and the main inflowing rivers, the potential sources of sulfate in NSL were mainly related to atmospheric precipitation, river inputs, and evaporite dissolution. Atmospheric precipitation is an important potential source of sulfate in surface water (Li et al., 2011 ). According to Krouse and Mayer (2000), the values of δ34SSO4 and δ18OSO4 from atmospheric deposition ranged from -3‰ and 7‰ to 9‰ and 17‰ at the global level. However, the measured values of δ34SSO4 and δ18OSO4 from the lake samples fell outside of these ranges. It was also noted that there was little acid precipitation in the study area (Yu et al., 2017 ), indicating that atmospheric deposition was not the main source of the sulfate present in NSL.
As seen in Fig. 5 , SO42- in NSL mainly came from sewage and evaporate dissolution, and sewage was only carried into the lake through the inflowing rivers around NSL, as a policy prohibits direct sewage discharge into the lake. Therefore, the direct sources of SO42- in NSL were the evaporite dissolution and river inputs. In addition, the isotopic signatures were changed by microbial sulfate reduction in the anaerobic environment of water bodies (Young et al., 2018 ), resulting in low SO42- concentrations but high δ34S values; hence, information on sulfate sources might be obscured by microbial sulfate reducers (Valiente et al., 2017 ). However, there was no significant negative correlation between the SO42- contents and δ34SSO4 values during the nondiversion and diversion water periods (Fig. 6 ), and NSL is difficult to form an anaerobic environment due to long-term high dissolved oxygen (Tian, Pei, Hu & Xie, 2012 ). Therefore, the effect of microbial sulfate reducers on the sulfur isotope composition can be overlooked in NSL.