4.3.2 Diversion water period
With the increased high SO42-concentrations to high levels in inflowing rivers, the distributions of the SO42- content in NSL during the diversion water period was complicated. According to the SO42- distributions, NSL was divided into three major areas, named as the Zhaoyang and Weishan sublake areas (ZWSL), the Dushan sublake area (DSSL) and the Nanyang sublake area (NYSL) (Fig. 7(b) ). To accurately calculate the contribution rates from each source, ZWSL, DSSL and NYSL were further divided into three subregions, two subregions and four subregions, respectively, in accordance
with the locations of adjacent major sulfate sources. The first three subregions were the convergence areas of the Hanzhuang Canal and the Weishan sublake, the Panlong River and the Weishan sublake, and the Chengguo River and the Zhaoyang sublake. The fourth and fifth subregions were the lake mixing areas of the Baima River and Dongyu River with the Dushan sublake, respectively. The sixth subregion was the mixing area of the Sihe River and the Nanyang sublake. The final three subregions were the mixing areas of the Old Wanfu River and the Wanfu River, the Zhuzhaoxin River and the Zhushui River, and the Old Canal and the Guangfu River. It was hypothesized that the δ34SSO4 value in each subregion reached the value in each corresponding sublake during the diversion water period.
To calculate the SO42- contribution rates, Eq. (3) and Eq. (4) were used for the first six subregions, and Eq. (1) and Eq. (2) were used for the latter three subregions. A δ34SSO4value of 11.78‰ for ZWSL was used for the first, second and third subregions, and 14.63‰ was used as the δ34SSO4 value for DSSL for the fourth and fifth subregions. Additionally, 12.86‰ was used as the δ34SSO4 value of the lake in the seventh subregion. The remaining subregions used 14.10‰ as the δ34SSO4 value of the lake (Table 1 and Fig. 7(b) ). The calculated contribution rate of each river in NYSL to the respective subregions was divided by 12, and contribution rate of each river in DSSL and ZWSL were divided by 6 and 9 respective. The contribution rates of each source to the NSL was obtained. The results are presented in Table 3 .
According to Eq. (3) and Eq. (4) , the total SO42- contribution rates of evaporite dissolution to NSL was 52.18%, with the maximum contribution rate occurred during the diversion water period. The SO42- source from inflowing river inputs cannot be neglected, especially for the Zhuzhaoxin River, Old Canal, and Wanfu River (total contribution of 22.76%). Moreover, these three rivers all flowed into the Nanyang sublake, where the SO42- contributions from inputs were largely higher than those from evaporite dissolution.
In contrast to trends in the NYSL, the SO42- sources for the DSSL and ZWSL both came mainly from evaporite dissolution, and the contribution rates of inflowing rivers provided lower amounts of sulfate. In addition, the Hanzhuang Canal is a channel for artificial water transfer from Jiangsu Province to Shandong Province during the diversion water period. As shown in Table 3 , the contribution rate from artificial water transfer to NSL was low compared with that from evaporite dissolution and with the contribution of other inflowing rivers. Notably, sulfate from diversion water was not the main source in NSL.
By comparing the contributions of SO42- sources in NSL in the nondiversion and diversion water periods, mainly concentrating on the Nanyang sublake, it was evident that the contributions had significant differences. For example, the Wanfu River was not the main SO42- source for the Nanyang sublake in the nondiversion water period, but it provided the maximum contribution during the diversion water period. Similar changes also occurred in the Old Canal. In addition, the SO42- contribution rate of the Zhuzhaoxin River increased greatly from the nondiversion to diversion water periods. However, the SO42-contribution of the Old Wanfu River decreased dramatically. The SO42- contribution from evaporite dissolution to the Nanyang sublake changed little between the nondiversion and diversion water periods.
4.4. Sources of sulfate in the inflowing rivers
The main sources of sulfate in rivers and water transfer included atmospheric deposition, evaporite dissolution, the oxidation of sulfides, and human inputs (Brenot et al., 2007; Cao et al., 2018; Killingsworth & Bao, 2015; Sun et al., 2017; Yoon et al., 2008 ). The sulfate from human inputs into the river system came mainly from domestic sewage. Therefore, the SO42-sources in the rivers around NSL were divided into four types: a) atmospheric deposition, b) evaporite dissolution, c) sulfide oxidation, and d) sewage.
From Fig. 5 , the values of δ34SSO4 and δ18OSO4 from samples such as those from the Old Canal, Guangfu River, Zhuzhaoxin River, Wanfu River and the Old Wanfu River in both periods were associated with sources of evaporite dissolution. Specifically, the Zhuzhaoxin River, the Old Canal and the Wanfu River, all entered the Nanyang sublake with high SO42- contributions. Although the SO42- from evaporite dissolution was less than 30% in the NYSL according to the calculated contribution rates (for two periods), the rivers entering the Nanyang sublake had high concentrations and contribution rates of SO42- , which came from evaporite dissolution. Therefore, it is reasonable to assume that the δ34SSO4 and δ18OSO4 values of the Nanyang sublake plot in the evaporite dissolution area in Fig. 5 .
The δ34SSO4 and δ18OSO4 values of the Panlong River, Chengguo River, Baima River and Dongyu River were in the range of the sewage sources; therefore, the SO42-sources of these rivers were mainly from artificial inputs during the diversion water period. These rivers are large rivers with wider watershed areas and are important water resources for the cities in each watershed. Domestic sewage from large cities, such as Jining and Zaozhuang, might be received by these rivers. In addition, the Zhushui River plots in the overlapping area between evaporite dissolution and sewage, as shown in Fig. 5 . The SO42- source of the Zhushui River during the nondiversion water period mainly plots in the area of evaporite dissolution. During the diversion water period, sewage became one of the primary sources, and the values of δ34SSO4 and δ18OSO4 in the Zhushui River were influenced, resulting in shifts in the values of δ34SSO4 and δ18OSO4 to values that plot in the sewage area.
The δ34SSO4 and δ18OSO4 values in the Hanzhuang Canal were in the range of atmospheric deposition and plotted near the sewage area, which could have been the result of a mixture of sewage and atmospheric deposition sources. The SO42- source of the Chengguo River in the nondiversion water period was atmospheric deposition, but the source in the diversion water period involved both sewage and atmospheric deposition. By comparing the SO42-contents of the Chengguo River in the nondiversion and diversion water periods, sewage had a greater contribution rate in the diversion water period. However, in the nondiversion water period, due to sufficient rainfall and the abundant coal resources in the Chengguo River Basin, sulfur dioxide from coal combustion might have eventually transformed into sulfate and then entered the Chengguo River through precipitation.