5. Conclusions
In the present study, to clarify the problem of increasing sulfate
levels in the SNWD-ERP, the sources of sulfate in NSL, an important
storage hub of the SNWD-ERP, were traced through hydrochemical research
and by sulfur and oxygen isotope analysis. We obtained documented
hydrochemical evidence of sulfate changes in the two periods, and the
changes were linked to seasonal overturn, evaporite dissolution and
inputs from the rivers that flow into NSL.
During the nondiversion water period (wet season), the
SO42- concentrations were
significantly lower in NSL, except for that in the Nanyang sublake, than
in the diversion period. The maximum
SO42- contribution in the Nanyang
sublake came from the inputs of inflowing rivers, especially that from
the Old Wanfu River, providing up to 32.56% of the total contribution.
In addition, the SO42- contribution
from evaporite dissolution accounted for 25.60% of the total level. In
the diversion water period (dry season), the largest source of the high
SO42- concentrations in NSL came from
evaporite dissolution, accounting for 52.18% of the total contribution.
However, the SO42- proportion from
inflowing river inputs into NSL could not be ignored, especially for the
inputs from the Zhuzhaoxin River, Old Canal and Wanfu River, with
SO42- contributions of 8.10%, 7.54%
and 7.12%, respectively. Notably, in the NYSL, the
SO42- contributions of these rivers
were higher than those from evaporite dissolution.
Thus, to guarantee satisfactory diversion water quality, it is essential
to initiate treatment methods to decrease the
SO42- content, such as by using
activated carbon or charcoal adsorption, before water is diverted from
NSL to transfer pipelines. In addition, the
SO42- contributions from sewage in
inflowing rivers are a major sulfate source, such especially for the
Chengguo and Panlong Rivers. Therefore, it is necessary to formulate
strict discharge standards to reduce the direct discharge of sewage into
the rivers that flow into NSL.