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(CO32(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).