4.2.2. Main Ionic Components

Given the statistical table of characteristic values of As, As(III), As(V), Fe, Fe2+, Fe3+ , HCO<sup>3</sup> <sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> in shallow high-As groundwater in the Hetao Basin (Table 2), the average content of As in the shallow groundwater with high-As in the Hetao Basin reached 93.46 µg/L, and the maximal content in the HT Plain was reported to be 116.93 µg/L, nearly 2.5 times of that in the SBYR Plain; these values for the SHH Plain and the HB Plain were similar, reaching 76.20 µg/L and 80.65 µg/L, respectively. In addition, As(III) was reported to be the major form of As, and the average concentration of As took up 85.2% of the total As. The minimal value was 76.1% in the SHH Plain, and the values in the other three hydrogeological units were nearly unchanged, ranging from 84.0% to 86.1%, with the maximal value in the HT Plain. The ratios of As(III) to As(V) in the 4 plains are all very large, reaching 19.27 in the highest As area in the SHH Plain (Table 3), indicating that the groundwater in the entire Hetao Basin is basically in a reducing environment. Impacted by the toxicity of As(III) and such a high content of As(III) in the shallow high-As groundwater in the Hetao Basin, it is not difficult to understand that there are large-scale arsenic poisoning areas in the Hetao Basin and the safety of the drinking water of over one million people is affected [28,29].


**Table 2.** Statistical table of characteristic values of the main ions in high-As shallow groundwater in the Hetao Basin.

**Table 3.** As(III)/As(IV) ratios in shallow groundwater from Hetao Basin.


The content of total Fe in the Hetao Basin was higher, with an average concentration of 2.77 mg/L, and the maximum value of 5.84 mg/L was identified in the SHH Plain. The average values of the other 3 hydrogeological units between were basically the same, at 2.33–2.63 mg/L (Figure 5). The average concentration of Fe2+ was 1.28 mg/L, accounting for 46% of the total Fe. The average concentration of Fe3+ was 1.49 mg/L, and the ratio of the average concentration of Fe2+ approached 1:1. The ratio of the average concentrations of Fe2+/Fe3+ in the 4 hydrogeological units approached 1:1 across the entire Hetao Basin.

**Figure 5.** Contour map of total Fe content in Hetao Basin.

The average concentration of SO<sup>4</sup> <sup>2</sup><sup>−</sup> in the Hetao Basin reached 310 mg/L, and the relative concentration was relatively low. The maximal value was 2455 mg/L in the SBYR Plain. The average concentration of SO<sup>4</sup> <sup>2</sup><sup>−</sup> in the 4 hydrogeological units was 264~360 mg/L. Given the SO<sup>4</sup> <sup>2</sup><sup>−</sup> ion concentration partition of shallow high-As groundwater (Figure 6), the distribution area with concentrations of less than 100 mg/L was 4953 km<sup>2</sup> , and the distribution area with concentrations of 100~200 mg/L was 4493 km<sup>2</sup> . There was almost no SO<sup>4</sup> <sup>2</sup><sup>−</sup> in the groundwater in the north of the HT Plain and in the eastern part of the HB Plain, which demonstrated the effect of desulfurization.

**Figure 6.** Zoning map of SO<sup>4</sup> <sup>2</sup><sup>−</sup> concentration in Hetao Basin.

The average concentration of HCO<sup>3</sup> − of the Hetao Basin was 570 mg/L, and the relative concentration was relatively high. In four hydrogeological units, the average concentration of HCO<sup>3</sup> − of the HB Plain reached 704 mg/L, and the values of the other 3 hydrogeological units were relatively close, at 521~547 mg/L. The HCO<sup>3</sup> − concentration in the HB Plain tended to rise from the pre-piedmont recharge area to the central part of the plain (Figure 7). The highest concentration reached 2123 mg/L, which demonstrated the effect of organic carbon oxidation.

**Figure 7.** Contour map of HB Plain HCO<sup>3</sup> − concentration.
