*3.3. Determination of "Five Maps"*

Five maps were generated for the Yu-Shen mining area based on the relative weights (impacts) presented in Table 1, including: overburden thickness contour, stratigraphic structure map, water-rich zoning map of aquifers, aquiclude thickness contour, and coal thickness contour. These maps were used as the input basic parameters for WCCM.

#### (1) Overburden thickness contour

The overburden strata of Yu-Shen mining area are dominantly sandstone, mudstone, and sandy mudstone. The overburden thickness ranges generally from 0 m to 650 m. As a result of intense weathering and erosion, the overburden gradually decreases from northwest to southeast as shown in Figure 3. The roof strata of the coal seams are primarily composed of medium-hard rocks (compressive strength: 20–40 MPa), such as mudstone with an average UCS (uniaxial compressive strength) of 31.4 MPa and sandstone having an average UCS of 28.9 MPa.

**Figure 3.** Overburden thickness contour (Xi'an geodetic coordinate system 1980; in km).

The bedrock is weathered and fractured in the first 30 m depth from surface, and the degree of weathering gradually decreases with the increase of depth. The bedrock from 6 m to 10 m depth is highly weathered and secondary structural planes and fracture network was developed. This bedrock layer therefore bears more water than bedrock at high depth. Water yield property of the weathered bedrock varies significantly. It is generally low, except for localized medium levels.

#### (2) Stratigraphic structure map

The spatial relationship between coal seams and aquifers (or aquicludes) in the Yu-Shen mining area has the following characteristics: The groundwater is found in close association with the underlying coal, and the bedrock is thick and often covered with a thin layer of sandy soil. The distance between coal seam and aquifer varies significantly. The thickness of bedrock above coal seam is negatively correlated with aquifer thickness. A decrease in bedrock thickness indicates increase in aquifer thickness and water yield. An increase in bedrock thickness is associated with decreases in aquifer thickness and lower water yield. The increase in bedrock thickness occurs mostly near boundaries between zones of different water yields.

The stratigraphic structure of overburden in the Yu-Shen mining area was categorized into five types: sand-soil-bedrock (I), sand-bedrock (II), bedrock (III), soil-bedrock (IV), and burnt rock (V) as shown in Figure 4. The overburden of sand-soil-bedrock is composed of sand, impermeable soil and bedrock. This stratigraphic structure is widely distributed in this mining area, covering more than 80% of its territory. The sand-bedrock overburden is very thin. The high-yield unconsolidated aquifer in this structure lies directly above the coal-bearing strata, and there is no continuous clay aquiclude separating them. The bedrock overburden consists of bedrock only which is exposed to the earth's surface. There is no need to consider water conservation here due to the extremely low water bearing capacity. Soil-bedrock overburden is composed of soil and bedrock. As it bears no water, water conservation is not accounted for in this structure. Burnt rock is the product of rock alteration resulting from the spontaneous combustion of coal seams 2−<sup>2</sup> and 3<sup>−</sup>1. Burnt rock overburden thickness ranges from 30 m to 50m, and its degree of metamorphism gradually decreases as the increase of vertical distance from the spontaneous combustion seams.

**Figure 4.** Stratigraphic structure map (Xi'an geodetic coordinate system 1980; in km).

#### (3) Water-rich zoning map of aquifers

The major types of aquifers in the Yu-Shen mining area, from top to bottom, include: the unconsolidated porous phreatic aquifer, Salawusu Formation aquifer, burnt rock phreatic aquifer, and fractured (porous) bedrock confined aquifer, as shown in Figure 5. The fractured (porous) bedrock confined aquifer has extremely low water content due to limited distribution and relatively high permeability, the burnt rock phreatic aquifer cannot form a water-storing structure. However, it is primarily recharged by groundwater from the water-bearing Salawusu Formation. Therefore, the Salawusu Formation is the major aquifer in this area which needs to be protected during mining.

Holocene (Q4) porous phreatic aquifers include Holocene eolian sand layer (Q4 eol) and Holocene alluvial deposit (Q4 al). Holocene eolian sand layer (Q4 eol) is widely distributed in Yu-Shen mining area, with an average thickness of 5 m, and is mainly composed of fine sand and silt, this layer is permeable and bears no water. Holocene alluvial deposit (Q4 al) is mainly found in large river valleys and fluvial terraces, with limited distribution and having small thicknesses. This layer basically plays no part in local water supply. These two layers and the underlying Salawusu Formation usually form a complete aquifer.

**Figure 5.** The major types of aquifers in the Yu-Shen mining area.

The water-bearing Salawusu Formation has wide distribution in the mine area. It is normally covered with eolian sand. Its thickness ranges from 0 m to 67.3 m. The water table is generally less than 10 m underground (normally 0.9–9.3 m deep). This formation mainly consists of silt, fine sand, medium sand, and contains sandy loam. With high porosity, the Salawusu Formation is the major aquifer in the study area. Its spatial distribution, thickness, and aquifer structure are strictly governed by modern topographic features and the paleogeographic environment in which it was deposited. The hydrological conditions and water yield of this aquifer vary significantly across this area. The high water-rich part of the aquifer covers a total area of 504 km2 with a thickness of 39.6–156.1 m and is characterized with permeability coefficient of 0.055–23.6 m/d. Its medium water-rich portion is about 1911 km2 in total, having depth varies from 6.42–145.50 m and permeability coefficient varies from 0.005–10.457 m/d. The low water-rich portion is 1919 km2 in total, with the depth varies between 4.4–93.8 m, and permeability coefficient varies between 0.012–6.073 m/d. The water-rich zoning map of aquifers is shown in Figure 6.

**Figure 6.** Water-rich zoning map of aquifers (Xi'an geodetic coordinate system 1980; in km).

#### (4) Aquiclude thickness contour

The main aquicludes in this mining area include the loess in the Lishi Formation and red soil in the Pliocene Baode Formation, as shown in Figure 7. The Lishi Formation is a loess formation composed of grayish-yellow sandy loam and silty clay, with 45.7% sand, 39.5% silt, and 14.8% clay. Its upper part is eolian loess (Q3m) with well-developed joints and pores. The Lishi loess is well graded, and its coefficient of non-uniformity is 2.0, and the coefficient of curvature is 1.5. The loess's moisture content varies between 11.9–17.3%, plastic limit between 16.9–18.7%, liquid limit between 25.9–31.8%, and plasticity index between 7.9–13.1. These parameters indicate that the loess is in a hard to hard plastic state. The thickness of the Lishi Formation varies with an average of 23.0 m. It is in discordant contact with the underlying layer.

**Figure 7.** Aquiclude thickness contour map (Xi'an geodetic coordinate system 1980; in km).

The Baode Formation is a red soil formation consisting of brown-red clay and silty clay. The mineralogical composition is chlorite as the dominant mineral of this formation, while kaolinite and illite present in small quantities, having medium plasticity index (from 9.5 to 12.1), medium liquid limit, and low liquidity index, this red soil is a hard plastic clay soil. The thickness of this formation is 30 m on average, which is controlled by the ancient base levels of erosion in this area. It is relatively thick along the water-shed and then decreases gradually towards the valleys on each side.

#### (5) Coal thickness contour

The Jurassic Yan'an Formation is the coal-bearing formation in this area. It contains five main coal seams referred to as 1<sup>−</sup>2, 2−2, 3−1, 4−<sup>2</sup> and 5<sup>−</sup>2. These seams are generally flat with small dip angles of 1◦ to 3◦ but their thicknesses vary significantly. The maximum seam thickness is greater than 12 m, as shown in Figure 8. Seam 1−<sup>2</sup> occurs in the upper part of the 5th section of the Yan'an Formation. Seams 2−<sup>2</sup> and 3−<sup>1</sup> are at the top of the 4th and 3rd sections, respectively. Seam 4−<sup>2</sup> is in the mid-upper part of the 2nd section of this formation and 5−<sup>2</sup> is at the top of the first section.

**Figure 8.** Coal thickness contour (Xi'an geodetic coordinate system 1980; in km).
