*5.2. Correlation Analysis between Arsenic and Main Oxidation-Reducing Ions*

Pearson correlation analysis was applied to reveal the relationship between arsenic and the main oxidation-reducing ions in this study [50–54]. As revealed by the results of the Pearson correlation analysis of arsenic with major ions (e.g., As, Fe, Fe2+, Fe3+, HCO<sup>3</sup> −, SO<sup>4</sup> <sup>2</sup>−, NO<sup>2</sup> <sup>−</sup> and NO<sup>3</sup> −) for all shallow groundwater samples (*n* = 974) (significance level at *p* < 0.01, Table 4), no significant correlation was reported between arsenic and the ions, and the correlation coefficient was primarily less than 0.4. As indicated by the results of the separate correlation analysis regarding the groundwater samples of the four hydrogeological units (Tables 5–8), an insignificant correlation was reported between arsenic and other ions; only in the SHH Plain was a correlation coefficient of 0.6 found between arsenic and Fe, and the correlation coefficient between arsenic and NO<sup>2</sup> − was 0.5, which was more significant. No significant correlation was identified between the arsenic and the iron contents in the groundwater, probably due to the formation of insoluble iron sulfides by Fe2+ from the reduction of iron oxides and by S2<sup>−</sup> from the reduction of SO<sup>4</sup> <sup>2</sup>−. Iron sulfides have been extensively distributed in many high-As aquifers worldwide [55]. No significant correlation was identified between arsenic and HCO<sup>3</sup> − concentration in the groundwater, whereas high-As groundwater samples generally contained HCO<sup>3</sup> − in high concentrations. In this study area, the average concentration of HCO<sup>3</sup> − was relatively high, up to 570 mg/L, which might also be related to the process of microbial activity. Microorganisms can reduce iron's release of arsenic and oxidize the organic matter in the aquifer to produce a large amount of HCO<sup>3</sup> −. In addition, it might be related to high concentrations of HCO<sup>3</sup> − and the competitive adsorption of arsenate and arsenite on the surface of iron oxide.


**Table 4.** Pearson correlation analysis between As and the main redox ions in the Hetao Plain (*n* = 974).

\*\* Significantly correlated at the level of 0.01 (bilateral); \* Significantly correlated at the level of 0.05 (bilateral).

**Table 5.** Pearson correlation analysis between As and the main redox ions in the Houtao Plain (*n* = 190).


\*\* Significantly correlated at the level of 0.01 (bilateral); \* Significantly correlated at the level of 0.05 (bilateral).

**Table 6.** Pearson correlation analysis between As and the main redox ions in the Sanhuhe Plain (*n* = 190).


\*\* Significantly correlated at the level of 0.01 (bilateral); \* Significantly correlated at the level of 0.05 (bilateral).


**Table 7.** Pearson correlation analysis between As and the main redox ions in the Hubao Plain (*n* = 278).

\*\* Significantly correlated at the level of 0.01 (bilateral); \* Significantly correlated at the level of 0.05 (bilateral).

**Table 8.** Pearson correlation analysis between As and the main redox ions in the Southern Plain of the Yellow River (*n* = 190).


\*\* Significantly correlated at the level of 0.01 (bilateral); \* Significantly correlated at the level of 0.05 (bilateral).
