3.1. Hydrochemistry
Table 1 shows the chemical data for each water sample in the nondiversion (n=11) and diversion water periods (n=17). In the nondiversion and diversion water periods, the SO42- concentrations ranged from 158 mg/L to 277.65 mg/L and from 170.49 mg/L to 631.5 mg/L, respectively. The SO42- concentrations in the diversion water period were notably higher than those in the nondiversion water period. The SO42-concentrations in NSL showed a gradual increasing trend from south to north in both periods, and the maximum value of SO42- was observed in the Nanyang sublake (NE). The highest concentrations of some major ions, such as HCO3-, Cl-, Na+ and Mg2+, were observed in the Nanyang sublake (SW) (HCO3- up to 245.51 and 215.28 mg/L, Cl- up to 155.51 and 138.02 mg/L, Na+ up to 226.59 and 170.33 mg/L and Mg2+ up to 7.77 and 42.85 mg/L in the two periods). The SO42- concentrations and concentrations of other major ions, except for Na+ and K+, in the Nanyang sublake (SW) were similar to those in the Nanyang sublake (NE). The pH and EC values in the Nanyang sublake (SW) were greatest in NSL during the nondiversion water period and reached 8.5 and 1,388 μs/cm, respectively. The concentrations of SO42- and other ions in the Dushan and Zhaoyang sublakes were lower than those in the Nanyang sublake, except for NO3- and Ca2+.
The SO42- concentrations in the main inflowing rivers around NSL ranged from 149.00 mg/L to 685.49 mg/L during the nondiversion water period. The maximum SO42- concentrations appeared in the Chengguo River, the main river entering the Zhaoyang sublake. The Sihe River, Old Wanfu River, Guangfu River, Zhushui River, Zhuzhaoxin River and Old Canal are important inflowing rivers around the Nanyang sublake. The average SO42- concentration in these rivers was approximately 319.34 mg/L, with maximum concentrations being observed in the Zhuzhaoxin River (up to 590.65 mg/L). Similar to the findings in the nondiversion water period, the SO42- concentrations in the inflowing rivers around NSL ranged from 142.06 mg/L to 671.32 mg/L during the diversion water period, and the maximum value appeared in the Zhuzhaoxin River, a major inflowing river of the Nanyang sublake (NE). The Na+ content in the Zhuzhaoxin River was also maximum value among those for all the inflowing rivers, reaching 357.04 mg/L, and the Cl- content was slightly lower than that in the Old Wanfu River. The Old Wanfu River not only displayed the maximum Cl- value (320.66 mg/L) but also the highest EC (1,967 μs/cm) and HCO3- values (451.18 mg/L) among all river samples. Additionally, there was little difference in the SO42- concentrations between the Zhuzhaoxin River and the Old Wanfu River. The Old Wanfu River is a main inflowing river of the Nanyang sublake (SW). The SO42- concentration in the Old Canal was 559.69 mg/L and was slightly lower than that in the Zhuzhaoxin River. The proportions of SO42- and other major ions in the river samples around NSL exhibited trends similar to those in NSL.
Compared to those in the rivers, the SO42- concentrations in groundwater were lower during the nondiversion water period, whereas HCO3- contents were as high as 506.47 mg/L and the Ca2+ content reached a maximum value of 143.81 mg/L. In contrast to the trends in the nondiversion water period, the SO42- concentrations in groundwater clearly increased, and other major ion contents, such as HCO3-, Cl-, Na+, Mg2+ and Ca2+, displayed marked increases during the diversion water period.
These hydrochemical data can be used to determine the water type. The main water types of NSL during the nondiversion and diversion water periods were Na+-SO42--Cl-and Na+-SO42--HCO3-, respectively (Fig. 3 ). Similar to NSL, the water types of the main inflowing rivers were Na+-SO42--Cl-and Na+-SO42--HCO3-, respectively. The water type in NSL and the surrounding rivers was similar to Gonghe geothermal water (Liu, Guo, Zhang, Wang & Wang, 2017) , where the ions mainly came from gypsum dissolution; therefore, the ions in NSL and the surrounding rivers might have the same ion sources. However, the water types in groundwater was Ca2+-SO42--Cl-during both the nondiversion and diversion water periods. This finding indicates that the impact of groundwater inputs on NSL is low.