4.2. Sources of sulfate in the NSL
According to the above discussion of hydrochemical and isotopic
characteristics in NSL and the main inflowing rivers, the potential
sources of sulfate in NSL were mainly related to atmospheric
precipitation, river inputs, and evaporite dissolution. Atmospheric
precipitation is an important potential source of sulfate in surface
water (Li et al., 2011 ). According to Krouse and Mayer
(2000), the values of δ34SSO4 and
δ18OSO4 from atmospheric deposition
ranged from -3‰ and 7‰ to 9‰ and 17‰ at the global level. However, the
measured values of δ34SSO4 and
δ18OSO4 from the lake samples fell
outside of these ranges. It was also noted that there was little acid
precipitation in the study area (Yu et al., 2017 ), indicating
that atmospheric deposition was not the main source of the sulfate
present in NSL.
As seen in Fig. 5 , SO42- in
NSL mainly came from sewage and evaporate dissolution, and sewage was
only carried into the lake through the inflowing rivers around NSL, as a
policy prohibits direct sewage discharge into the lake. Therefore, the
direct sources of SO42- in NSL were
the evaporite dissolution and river inputs. In addition, the isotopic
signatures were changed by microbial sulfate reduction in the anaerobic
environment of water bodies (Young et al., 2018 ), resulting in
low SO42- concentrations but high
δ34S values; hence, information on sulfate sources
might be obscured by microbial sulfate reducers (Valiente et
al., 2017 ). However, there was no significant negative correlation
between the SO42- contents and
δ34SSO4 values during the nondiversion
and diversion water periods (Fig. 6 ), and NSL is difficult to
form an anaerobic environment due to long-term high dissolved oxygen
(Tian, Pei, Hu & Xie, 2012 ). Therefore, the effect of
microbial sulfate reducers on the sulfur isotope composition can be
overlooked in NSL.