4.1 Sulfate characteristics during the two sampling periods
The SO42- concentrations were significantly high in NSL, and they could have been influenced by seasonality. Usually, in the wet season, large amounts of precipitation dilute surface water, resulting in a decrease in some chemicals, whereas in the dry season with little rainfall, evaporation can cause salinity variations, such as for SO42- and Cl-. The maximum SO42- concentrations in NSL in the diversion water period (dry season) were more than three times greater than those in the nondiversion water period (wet season). The contents of the other major ions and pH values in the diversion water period were higher than those during the nondiversion water period (Fig. 4 ).
The SO42- concentrations in NSL were positively correlated with those in the surrounding rivers. As shown in Table 1, the SO42- concentrations in the sublakes surrounded by some inflowing rivers with high SO42- contents were significantly higher than those in other areas. During the diversion water period, the SO42- contents were highest in the Zhuzhaoxin River and in the Old Canal, the main rivers inflowing into the Nanyang sublake (NE), and accordingly, the SO42- concentrations in the Nanyang sublake (NE) were considerably higher than those in the other sublakes (Table 1 and Fig. 2 ). This result suggested that the inflowing rivers might be major contributors to the increase in SO42- concentrations in NSL. Similarly, other major ions in the sublakes of the NSL presented the same patterns as in the inflowing rivers. This result was in agreement with the results of a previous study (Valiente et al., 2017 ), suggesting that in surface waters, SO42− concentrations can be affected by external inputs (streams), beyond precipitation and evaporation processes.
The concentrations of SO42- and Cl- in surface waters can change due to rainfall and evaporation, but the relative ratios for the two sampling periods would change little (Drzewicki, Trojanowska-Olichwer, Jedrysek & Halas, 2017 ). However, as shown in Fig. 4(e) , the SO42-/Cl- ratio values in the diversion water period were generally higher than those in the nondiversion water period, implying that there could be other factors responsible for the increase in the SO42- concentrations in lakes and rivers in addition to decreased rainfall in the diversion water period, such as sewage inputs.