5. Conclusions
In the present study, to clarify the problem of increasing sulfate levels in the SNWD-ERP, the sources of sulfate in NSL, an important storage hub of the SNWD-ERP, were traced through hydrochemical research and by sulfur and oxygen isotope analysis. We obtained documented hydrochemical evidence of sulfate changes in the two periods, and the changes were linked to seasonal overturn, evaporite dissolution and inputs from the rivers that flow into NSL.
During the nondiversion water period (wet season), the SO42- concentrations were significantly lower in NSL, except for that in the Nanyang sublake, than in the diversion period. The maximum SO42- contribution in the Nanyang sublake came from the inputs of inflowing rivers, especially that from the Old Wanfu River, providing up to 32.56% of the total contribution. In addition, the SO42- contribution from evaporite dissolution accounted for 25.60% of the total level. In the diversion water period (dry season), the largest source of the high SO42- concentrations in NSL came from evaporite dissolution, accounting for 52.18% of the total contribution. However, the SO42- proportion from inflowing river inputs into NSL could not be ignored, especially for the inputs from the Zhuzhaoxin River, Old Canal and Wanfu River, with SO42- contributions of 8.10%, 7.54% and 7.12%, respectively. Notably, in the NYSL, the SO42- contributions of these rivers were higher than those from evaporite dissolution.
Thus, to guarantee satisfactory diversion water quality, it is essential to initiate treatment methods to decrease the SO42- content, such as by using activated carbon or charcoal adsorption, before water is diverted from NSL to transfer pipelines. In addition, the SO42- contributions from sewage in inflowing rivers are a major sulfate source, such especially for the Chengguo and Panlong Rivers. Therefore, it is necessary to formulate strict discharge standards to reduce the direct discharge of sewage into the rivers that flow into NSL.