**6. Conclusions**

Arsenic has been extensively distributed in the shallow groundwater of the Hetao Basin, and high-As groundwater was identified in the four hydrogeological units. In particular, the average concentration of arsenic in the shallow groundwater in the Hetao Basin reached 45.58 µg/L. The spatial distribution of arsenic content in the shallow groundwater

was significantly uneven in the whole basin, and the variation coefficient ranged from 2.07 to 1.57–2.18, with high variability.

All high-As groundwater in the HT Basin was weakly alkaline, achieving a pH value of 7.15–9.26. As(III) was found to be the major form of As, and the average concentration of As took up 85% of the total As. The content of total Fe was higher, with a concentration of 2.77 mg/L on average, and the average concentration of Fe2+ took up 46% of the total Fe. The average concentration of SO<sup>4</sup> <sup>2</sup><sup>−</sup> was 310 mg, and the relative concentration was lower. It is noteworthy that there was nearly 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. The concentration of HCO<sup>3</sup> − was 460 mg/L on average, and the relative concentration was higher. 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, and the maximal concentration was 2123 mg/L, which demonstrated the effect of organic carbon oxidation.

There were three concentrated areas with high-As groundwater in the Hetao Basin where the arsenic contents of groundwater were more than 50 µg/L. According to the analysis of hydrogeological conditions, three concentrated distribution areas with high-As groundwater were all in the discharge areas of groundwater with a shallow groundwater level, a water gradient of less than 0.8‰ and slow groundwater flow.

The distribution of high-As groundwater was found to be dependent on specific geological and geographical backgrounds. The special sedimentary environment evolution of the Hetao Basin lays the prerequisite for creating high-As groundwater, and the groundwater runoff condition and hydrogeochemical process of the basin can control the formation of high-As groundwater. The high pH and organic-rich reducing environment in the Hetao Basin, the dissolution, reduction, and precipitation of various minerals and the mutual transformation of different forms of arsenic contribute to the origin of the high-arsenic groundwater. The long-term lacustrine-dominated paleogeographic environment and closed tectonic conditions, slow refreshment and stagnant groundwater, arid-semi-arid climatic conditions, as well as sediments with high arsenic content and regional arsenic-rich environments constitute the feasible conditions for high-arsenic groundwater.

**Author Contributions:** L.L. and Z.C. designed the idea and structure of the article, W.X. analyzed the data, P.W. drew the graph, and Z.C., L.L. and C.L. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (No.41672241) and the Geological Survey Project of China Geological Survey (NO.DD20190433).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
