*2.2. Geological Backgrounds*

The Hetao Basin can be divided into four geomorphic zones: mountains, plateaus, plains and desert. Spatially, the Hetao Basin exhibits a geologic structure of "three sags and two uplifts", and its tectonic pattern basically regulates the stratigraphic framework and sedimentary (phase) pattern of the Quaternary sedimentary sequence, as well as the spatial distribution of the sedimentary system [40]. It can fall into the Linhe sag area, Xishanzui uplift area, Sanhuhe sag area, Baotou uplift area, as well as the Hubao sag area (Figure 1).

The Quaternary strata can be split into four rock formations from top to bottom. The first rock formation (Q3–4) is composed of grayish-yellow, light gray alluvial mediumfine sand, silty-fine sand mixed with medium-coarse sand, alluvial silty-fine sand, fine sand, silty clay, and other interbeds exhibiting different thicknesses with local muddy clay

intercalation and thicknesses of 10–260 m. Overall, the second rock formation (Q<sup>2</sup> 2 ) covers black-gray and grayish-brown limnetic facies mud, muddy clay, muddy silty-fine sand and silty clay, with local multilayers of mirabilite. Such a formation is extensively and continuously distributed in the basin and pertains to a landmark formation significantly correlated with the formation of high-As groundwater with thicknesses of 30–170 m [26,27]. The third rock formation (Q<sup>2</sup> 1 ) comprises brown-yellow, light gray and gray-brown alluvial silty-fine sand, medium-fine sand, medium-coarse sand and thick layers of clay silt, as well as silty clay interbed, with local light grey muddy clay, which is the main confined aquifer with thicknesses of 20–319 m. The fourth rock formation (Q1) covers the lacustrine facies-based gray, grayish-brown and grayish-green clay silt, silty clay and silty-fine sand, and medium-fine sand interbed. The bottom is sandy gravel, which is not reached [38–40].
