**1. Introduction**

High As groundwater has been reported as one of the most serious geological environmental issues facing the international community [1,2]. Drinking contaminated water such as high-As groundwater can directly jeopardize human health [3–8], which has aroused great interest from a wide range of organizations in the international community and numerous national government agencies [9,10]. Endemic arsenic poisoning attributed to drinking high-As groundwater has frequently occurred worldwide in places such as India,

**Citation:** Cai, Z.; Liu, L.; Xu, W.; Wu, P.; Lu, C. Hydrochemical Characteristics of Arsenic in Shallow Groundwater in Various Unconsolided Sediment Aquifers: A Case Study in Hetao Basin in Inner Mongolia, China. *Water* **2022**, *14*, 669. https://doi.org/10.3390/w14040669

Academic Editor: Daniel D. Snow

Received: 28 November 2021 Accepted: 17 February 2022 Published: 21 February 2022

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Bangladesh, Vietnam and Thailand in Asia, Germany, Chile, Brazil, Argentina, Italy, and The United Kingdom in Europe, and the United States and Canada in America [11–20].

Several natural and anthropogenic sources are responsible for the contamination of As in groundwater. As occurs as a major constituent in more than 200 minerals, and the desorption and dissolution of naturally occurring As-bearing minerals are generally considered the principal source of As contamination in groundwater [21,22]. Arsenopyrite (FeAsS) is the most abundant As-containing mineral that commonly exists in anaerobic environments and can also be found in different concentrations in various rock-forming minerals like sulfide, oxide, phosphate, carbonate and silicate [23]. Moreover, other secondary As minerals (e.g., scorodite, FeAsO<sup>4</sup> 2H2O) can contain the pollutant as a main or trace component [24,25].

The Hetao Basin of Inner Mongolia in China has been found to be a highly typical area of high-As groundwater [26,27]. Due to drinking high-As groundwater, arsenic poisoning patients were reported in the early 1990s [28]. The As-affected area reached over 3000 km<sup>2</sup> . The affected population exceeded 1 million, 400 thousand people drank high-As groundwater (As > 50 µg/L), and over 2000 residents of the 776 villages were confirmed as arsenic patients [29].

On the whole, groundwater As in the Hetao Basin has been considered to occur naturally in Later Pleistocene–Holocene alluvial–lacustrine aquifers (generally ranging from 10 to 50 m) [30–32]. In the typical arsenic poisoning area of the HT Plain, studies on the arsenic morphology and trace element concentration in groundwater have basically clarified the causes of arsenic enrichment in groundwater and its effect on arsenic poisoning [33,34]. Smedley et al. investigated the migration of As and other trace elements in the aquifer of the alluvial plain of Hohhot Basin in Inner Mongolia and highlighted that the high concentrations of As in groundwater were dependent on the strong reduction environment, as reflected in the high content of soluble Fe, Mn, NH4–N and DOC in groundwater [23]. When initially studying the migration, enrichment and transformation of As in arsenicaffected areas of the Hetao Plain, Lin Nianfeng et al. highlighted that clay soil and humus soil could help enrich As and reduction environments could covert As5+ into As3+ [35]. Guo Huaming et al. performed an indoor microbial leaching experiment through the in situ collection of sediments and indigenous microorganisms from a high-As aquifer in the Hetao Plain, Inner Mongolia. As indicated by the above studies, the release and transformation processes of arsenic as impacted by indigenous microorganisms consisted of the release of As(V) in sediments, the reduction of As(V) in solution, as well as the release of As(III) in sediments. The reductive dissolution of Fe/Mn oxide minerals under the action of indigenous microorganisms was highlighted as a major cause of arsenic release in sediments [31]. Gao Cunrong et al. explored the distribution and hydrochemical characteristics exhibited by high-As groundwater in the riverfront area of the Hetao Plain and highlighted that the high-As groundwater was largely distributed in the depositional center of the Hetao Plain and in small patches locally. They also found that the arsenic content in groundwater a short distance inland varied significantly. The over-standard rate of iron content in groundwater in the high-As areas significantly exceeded the nonarsenic-rich area, and the formation of high-As groundwater in this area was considered to be highly dependent on the characteristics exhibited by the sedimentary environment and sediments [36,37].

Numerous scholars in and outside China have conducted considerable investigations and research work in the HT Plain and the HB Plain. Most of the relevant studies were local and did not comprehensively assess the regional spatial distribution and formation mechanisms of high-As groundwater. Besides, arsenic in natural water bodies shows significantly high spatial variability [23], and the conclusions of the local studies are generally difficult to verify in the region. However, the formation and evolution mechanisms of high-As groundwater should be explored from the time and space perspectives to select an appropriate typical area or a geological section.

This study focused on the entire Hetao Basin, which fell into four hydrogeological units under the different geological and environmental conditions (i.e., Houtao Plain (HT Plain), Sanhuhe Plain (SHH Plain), Hubao Plain (HB Plain), and South Bank Plain of Yellow River (SBYR Plain)). Considerable groundwater samples have been collected, especially samples from the SHH Plain and SBYR Plain, which have not yet been studied, and the distribution and hydrochemical characteristics exhibited by shallow high-As groundwater have been systematically studied.
