5.2 Formation of bitter water
Magnesium sulfate mineral is nicknamed “bitter salt” in mineralogy
(Maik, Redel, Blank, & Meyerhof, 2019). This is because water solution
with high magnesium concentration usually exhibits a taste of bitterness
(Maik et al., 2019). The brackish water in the study area is most of the
Na-Mg-Cl-SO4 type, which indicates the magnesium is the
one of major cations in water. Bitterness is another typical feature of
the brackish water in the Zuli River Catchment.
According to the above discussion, the dissolution of carbonate minerals
exists in groundwater of headwater. The Mg vs. Ca diagram (Fig. 8b)
shows groundwater samples in headwater were close to the 1:1 line, which
indicates that the occurrence of dolomite dissolution in source
groundwater. Then with the evolution of water, there is a considerable
enrichment in magnesium compared to calcium in upstream groundwater and
river water (Fig. 8b), implying that there must be an additional
source(s) of magnesium besides carbonates dissolution in the brackish
water.
In the (Ca+Mg) vs. SO4 diagram, the distribution of
groundwater samples maintains a good linear relationship (Fig. 8a),
reflecting the theoretical dissolution of sulfated minerals containing
calcium and magnesium. Although sulfate minerals bearing magnesium, such
as epsomite (MgSO4*7H2O) or hexahydrate
(MgSO4*6H2O), are not the main
composition of minerals in the Zuli River Catchment (Tsunekawa et al.,
2014; Fan et al., 2016). But their existence at depth, even in small
quantities, could explain the good linear relationship between (Ca+Mg)
and SO4 in the brackish water. Those minerals are
characterized by high solubility, which can permit a faster dissolution
process and a great impact on the magnesium concentration of water
(Márquez et al., 2017). The dissolution of these magnesium sulfate
minerals is the initial and most important source of magnesium in
brackish water and result in the brackish water to be bitterness.
According to the above discussion, the dissolution of gypsum is an
important source of salinity in brackish water, and the rise of
gypsum-derived solutes (Ca2+ and
SO42-) along with the salinization of
water. Due to these carbonate minerals has reached saturation in
brackish water, which was reported in the previous study (Liu et al.,
2019). Then with Ca2+ continuously entering the water,
incongruent dolomite dissolution and calcite precipitation are driven by
gypsum dissolution due to the common ion effect (Appelo & Postma, 1993;
Edmunds, Bath, & Miles, 1982). These processes also lead to the further
enrichment of magnesium in water, serving as another important source of
magnesium. This process is occurring in groundwater according to this
reaction:
CaMg(CO3)2(S)+H2CO3→Ca(CO3)(S)+
Mg2++2HCO3-。
The increase of the Sr/Ca ratio and the characteristics of Sr isotope in
groundwater also confirms the existence of this process. It is
interfered with Sr isotopes that strontium mainly originates from the
dissolution of gypsum, and this process usually reduces the Sr/Ca ratio
(Cartwright et al., 2007). But with the salinization of groundwater, the
Sr/Ca ratio of upstream groundwater is also increasing (Fig. 8c). This
is because when strontium, like calcium, precipitates from solution into
calcite, the strontium is more inclined to retain in solution compared
to calcium (Pingitore & Eastman, 1986). Therefore, precipitation of
carbonate minerals tends to increase the Sr/Ca ratio in the residual
water due to the rejection of strontium in preference for calcium in
carbonates (Monjerezi et al., 2011).