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.