4.3. Contributions from different sulfate sources
After sulfate dissolves in water, it is difficult to change its isotopic composition
(microbial reduction has little effect on sulfate in natural water); therefore, the source information is retained ( Li et al., 2011; Zhou et al., 2016) used the characteristics of the isotopic composition in natural water and successfully determined that the sulfate in the Jialing River in the rainy season is mainly derived from sulfide oxidation with a 56% contribution ratio, and in the dry season, the sulfate is mainly derived from domestic and industrial wastewater with a 49% contribution ratio. Therefore, in this paper, stable isotopes were also used to calculate the contribution ratios from each source of sulfate in NSL.
To accurately calculate the contribution rates, according to the geographic position
of each main SO42- source, NSL was divided into different areas. In the nondiversion water period, the SO42- contents in the Nanyang sublake and its inflowing rivers were high, and the concentrations in other sublakes and their inflowing rivers were low, except in the Chengguo River. Additionally, low SO42-contents have little effect on the environmental and water quality in natural waters. Therefore, the Nanyang sublake, as the key research area of NSL, was divided into different areas to calculate the contribution rates of each main SO42- source. Unlike in the nondiversion water period, NSL was divided into different areas in the diversion water period, because of the environmental influence of high SO42- contents in each sublake and the inflowing rivers during the diversion water period could not be ignored.