*5.3. Correlation between Formation of High As Groundwater and Geological Environment*

Large-scale high-As groundwater deposits tend to be found in two types of environments, i.e., inland or closed basins in arid or semi-arid regions and aquifers derived from alluvium under strong reducing conditions [23]. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. The Hetao Basin, which is located in the west of Inner Mongolia, pertains to the typical temperate continental arid and semi-arid climate. The annual precipitation is 245.5 mm, and the average water evaporation is 2100 mm [38,39], which is 9 times the average rainfall.

The Hetao Basin is a Mesozoic-Cenozoic rifted basin formed in the late Jurassic, located between the Yinshan uplift and the Ordos platform. The tectonic movement of the Hetao Basin is very active, and tectonic faults have developed since the Cenozoic. The tectonic systems are dominated by high-angle normal faults and fault-bending belts. The deep east-west fault formed in the piedmont of Yinshan Mountain made the Ordos block continue to squeeze to the northwest, and the Hetao Basin between these two began to sink into depression. Since the Cenozoic, the Ordos block has been pushed from the northwest direction, and the squeeze turned into a southeastward pulling, which caused the Hetao Basin to sink significantly, forming a tectonic pattern termed as "three sags and two uplifts", in which the basin and mountains intersect each other and the depression and uplift are adjacent to each other (Figure 1). The fault depressions are favorable places for arsenic enrichment. Strong evaporation and weathering accelerate the decomposition rate and biogeochemical cycle of minerals in the bedrock weathering zone and promote the migration and enrichment of arsenic in water. Moreover, in the depression, the underground flow is stagnant, forming a structural water storage area and

a gathering place for various elements. The enclosed tectonic conditions and long-term inheritance and subsidence of the Hetao Basin has resulted in the formation of fine-grained clastic sediments dominated by inland lacustrine facies. The thickness of the Quaternary sedimentary reached 200–1500 m. Center areas of the low-lying lake basins located in piedmonts contain fine-grained sediments and high amounts of organic matter, and these reducing environments create favorable conditions for the release of arsenic from the sediments and the accumulation of arsenic in the water. The coverage of fine-grained sediments isolates the air exchange of oxygen between the aquifer and the surface, forming a closed reducing environment, prompting the release and migration of arsenic.

Mineral-water interactions can make arsenic enter groundwater from the sediment adsorption phase. However, arsenic in groundwater must accumulate to a certain concentration before it can form high-As groundwater [23]. Obviously, in the middle of a low-lying basin, especially in areas with a large thickness of lacustrine sediments, slow groundwater runoff and evaporation as the main form of discharge, it is particularly easy to develop high-As groundwater [56]. There are three relatively concentrated areas of high-As groundwater points in the Hetao Basin, located in the discharge area of groundwater according to the contours of the groundwater table (Figure 8), where there is a small groundwater depth and a hydraulic gradient of less than 0.8‰, and the groundwater flow is slow [55]. As the mentioned three areas are situated in the pluvial and alluvial-lacustrine interlace lowland and interfluvial lowland, characterized by poor water flow, strong evaporation and widespread saline-alkali land, the sediments are saturated in long periods. These factors enhance the release of arsenic from sediments into the water, causing its high concentration.

Due to the low rainfall and large evaporation, agricultural production in the Hetao Basin is primarily supported by the water from the Yellow River, and the introduction of considerable Yellow River water up-regulates the groundwater water level and forms a large area of soil salinization. Furthermore, the rising water level makes air unable to enter the reduction environment formed by the stratum. Besides, the pH value of the surface water in the arid and semi-arid climate environment is basically high, thereby creating favorable conditions for the dissolution of arsenic from the stratum.
