**5. Discussion**

*5.1. The Hydrogeochemical Process of As Mobilization in Aquifers*

Most of the potential sources of arsenic in the shallow groundwater of the Hetao Plain are believed to be from arsenic-bearing Quaternary strata derived from local aquifer sediments [31,32,46]. Under natural conditions, where the water-rock interaction is strong and the geochemical environment of certain aquifers is suitable for arsenic migration and accumulation, the aquifers often have higher arsenic concentrations. The migration of arsenic in the groundwater of the Hetao Basin occurs in a strong reducing environment with rich organic matter. The anaerobic environment where the surface lacustrine clay deposits is located is particularly conducive to the formation of As(III), causing As(III) to be the dominant valence state in the high-As groundwater in the study area. pH is an important factor affecting arsenic migration in aqueous systems. In the area of high-As groundwater in the Hetao Plain, the pH is also relatively high. Arsenic exists in the groundwater mainly in the form of two valence states as As(V) or As(III) anions, so it is more likely to be absorbed by positively charged substances in an aqueous medium, such as iron and manganese oxides, goethite, gibbsite and ferrihydrite. The increase of pH will reduce the adsorption of colloid and clay minerals to arsenate or arsenite in the form of anions and enhance their migration performance [47].

Numerous studies show that the finer the soil particles are, the greater amounts the arsenic adsorption are, and the higher the arsenic content is [48,49]. The silty-fine sand layer or the silty-fine sand interbeds with clay and silty clay are widely spread in the research area as identified by drilling data, and the organic matter content of the sediment is relatively high. Moreover, as the depth increases, the sediment particles become finer, and the arsenic content tends to increase [31]. The lacustrine silty clay and sandy clay in the sediment have a strong adsorption capacity for arsenic. When the arsenic element enters a depression with an alluvial or groundwater flow system, part of the arsenic is adsorbed by the clay and deposited directly, and part of the arsenic interacts with the ferric hydroxide colloid in the river or lake water to form insoluble precipitates. At the same time, the study area is rich in Fe2+ and Fe3+, and iron ions have a strong ability to fix arsenic. Once groundwater rich in iron and arsenic ions flow to the central area of the plain, the slow flow of groundwater, caused by the low hydraulic gradient, poor aquifer permeability and small recharge, will limit aquifer flushing, solute (As) transport and its removal from the system. In areas with slow groundwater movement, aqueous As concentrations are very sensitive to releases of small amounts of arsenic from the various hydrogeochemical processes described above. These reasons also explain the high-As distribution in the three main regions.
