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.