3.2.3. Cation Exchange

Cation exchange influenced the main cations concentration, and it was judged by using the binary phase diagram of (Na<sup>+</sup> <sup>−</sup> Cl−) vs. (Ca2+ + Mg2+ <sup>−</sup> SO<sup>4</sup> <sup>2</sup><sup>−</sup> <sup>−</sup> HCO<sup>3</sup> −) and Chlor–Alkali indices [4,16].

$$\text{(1)}\quad \text{(Na}^+-\text{Cl}^-\text{)}\text{ vs. (Ca}^{2+}+\text{Mg}^{2+}-\text{SO}\_4{}^{2-}-\text{HCO}\_3{}^-\text{)}$$

If cation exchange was the dominant process influencing the contents of Na<sup>+</sup> , Ca2+ and Mg2+, the relationship between the two parameters was negative linear, with a slope of <sup>−</sup>1.0. As shown in Figure 7a, there was a certain linear relationship between (Na<sup>+</sup> – Cl−) and (Ca2+ + Mg2+ <sup>−</sup> SO<sup>4</sup> <sup>2</sup><sup>−</sup> <sup>−</sup> HCO<sup>3</sup> −) in groundwater, but the correlation coefficient was low. It was indicated that cation exchange existed in the hydrogeochemical process, but it did not play a dominant role in the changes in the contents of Na<sup>+</sup> , Ca2+ and Mg2+ .
