Aquifer structure and groundwater types

Controlled by regional climate, hydrology, geology and landform, hydrogeological conditions show regional differences. According to the properties of water-bearing media, groundwater types can be divided into bedrock fissure water, clastic rock pore fissure water and loose rock pore water.

First, bedrock fissure water

Bedrock fissure water occurs in the cracks of pre-Mesozoic metamorphic rocks, sedimentary rocks and intrusive rocks, and is distributed in Qilian Mountain, Yushugou Mountain, Nanjie Mountain, Guantan Mountain, Beijie Mountain in the south and Beishan Mountain in the middle of the study area. Due to the strong tectonic movement and physical weathering in the southern bedrock mountain area, the surface layer is composed of weathered cracks, structural cracks and fault-broken rocks, forming a hydraulic connection and a unified water-bearing system. According to statistics, groundwater TDS is generally less than 0.5g/L, and groundwater runoff modulus is 0.5 ~ 2l/(s km2). Water-bearing rock groups in bedrock fractures in mountainous areas of central corridor are poor and uneven. Borehole water inflow is generally less than 500m3/d, and limestone formation is rich in water, and the water inflow can reach more than 1000m3/d; Because of the low altitude, scarce precipitation and poor water-rich bedrock in Beishan area.

Second, clastic rock pore fissure water

(A) Jurassic-Cretaceous clastic rock pore fissure water

It is distributed in the right side of Changmaba and some areas of Guan Dan Mountain and Yushugou Mountain, with sporadic outcrops in front of Beishan Mountain. These rock formations are located in the piedmont of Qilian Mountain. Because of its soft texture and high silt content, structural cracks and weathering cracks do not extend far and have poor connectivity. These three wells and ditches come from spring water in Cretaceous sandstone, with Quan Shan flow of 0.003L/s and TDS of 4.23 g/L.

(2) Pliocene-Lower Pleistocene fissure pore water

This layer constitutes the basement of each basin, mainly exposed in the low Wopu area in the east of Shule River and the eastern end of the southern section. According to the exploration data, the second farm of Ma Yin Farm in Yumen Town and the area of Shuangta-Shuanghekou are exposed. The buried depth of roof is generally 20 ~ 60m. The buried depth in other areas is relatively large, generally greater than 100m, and most of the boreholes are not exposed. Its lithology is a set of yellow, gray, orange and brick red alluvial conglomerate, glutenite, sandstone and piedmont clastic rocks, which are coarse-grained from bottom to top and cemented with calcium. Under the strong action of groundwater, gravel and sandstone will become loose.

The main aquifer in Yumen Town is buried in the section of 30 ~ 160m, and the aquifer is multi-layered from north to south. Borehole reveals that there are 4 ~ 8 aquifers in the range of 150m, with a total thickness of 40 ~ 80m and a unit water inflow of 0.3 ~ 3l/(s m). For example, in the water supply hole of Yumen Town, five aquifers are exposed within the range of 149m, with a total thickness of 75.5m, a unit water inflow of 0.59 ~1.80l/(s m), a water head of 6. 1m higher than the surface, and an aquifer permeability coefficient of 3.55 ~/kloc-0. The TDS of groundwater is less than 0.5g/L, which belongs to HCO _ 3- sulfuric acid-magnesium-calcium type water with good water quality.

The buried depth of the roof in Shuangta Reservoir and Xiaowan area is 50 ~ 60m, the water head is 0.5m higher than the surface, the unit water inflow is 0.15l/(s m), the permeability coefficient is 9m/d ... The groundwater quality is good, and the TDS is 0.48g/L, which belongs to SO4-Mg-Ca type water. This layer is buried 20 ~ 25m in Tuhulu and Bulongji areas. The aquifer thickness is only 4.9 ~ 10.9 m.

In Ma Quan Miao area in the west of Anxi, drilling revealed that the aquifer between 128.6 and 202 m was 13.4m thick, and the lithology was sandy gravel. The buried depth of water level is 3.45m, the unit water inflow is 0.5 L/(s m), and the TDS of groundwater is 9.5 g/L, which belongs to Cl-Na type water.

Although the Pliocene and Pleistocene aquifers are exposed in many places, only Yumen Town has a thick aquifer, shallow burial depth, good recharge conditions, strong water abundance and good water quality, which is of water supply significance.

Pore water of Quaternary loose rocks

Water-bearing rock formations are mainly composed of pore phreatic water, confined water aquifer and Holocene pore phreatic water aquifer of Middle-Upper Pleistocene, which are distributed in alluvial and lacustrine strata of each basin. The lithology of aquifer is alluvial gravel, gravel, sand and lake sand, clayey silt and loam. It is the main aquifer in the study area and is rich in pore water.

Quaternary is a single gravel aquifer in the south of the basin, with gravel confined aquifer in the north of the basin and the lower part of the alluvial plain, and clayey silt and clayey aquifer in the upper part.

(1) Yumen-Zhen Shi basin

The aquifer thickness is 20~300m, and the general change law is that it gradually becomes thinner from south to north, and the buried depth of water level gradually becomes shallow from south to north, and it transits from more than 20 ~ 300 m at the top of alluvial fan to less than 3m in the north of fine soil plain. Roughly bounded by the Gobi front of Changma diluvial fan and Yulin diluvial fan, the south is thick gravel layer with diving. In the northern and eastern fine soil zones, the aquifer is divided into two layers, the surface layer is 10 ~ 15m with clay sand, and the lower part is gravel, containing phreatic water and confined water (Figure 3- 1). Ziqiao-Zhen Shi fan-shaped depression, the aquifer is multi-layered cohesive soil with gravel and medium-coarse sand, the surface layer is phreatic water, and the lower part is confined water.

Figure 3- 1 hydrogeologic profile of Yumen-Zhen Shi basin

1. Single phreatic zone

The thickness of the aquifer is 100 ~ 300 m, which gradually decreases from south to north. Changma diluvial fan is the richest water area in this area, and the water inflow of a single well is more than 5000m3/d; Yulin alluvial-diluvial fan aquifer is rich in water of 3000 ~ 5000m3/d; Fan edge and west side1000 ~ 3000m3/d; The local area in the northern piedmont is less than 1000m3/d (Table 3- 1). The water quality is good, and the TDS is 0.80 ~1.17g/l.

2. Diving-confined water area

The upper phreatic layer in the fine soil plain area is a thin sand layer, with a single layer thickness less than 1m and poor water yield, generally less than1000m3/d. The confined aquifer in the north of Changma alluvial fan is a gravel layer with a thickness of 20 ~ 80m, and the water yield is1000 ~ 3000m3/from the outer edge of the alluvial fan to most sections. In the fine soil area at the northern foot of Ma Yin Mountain and the east of Yulin diluvial fan, the aquifer is sandy gravel, with a single layer thickness of 3 ~ 10m and a water-rich amount of less than 1000m3/d (Table 3- 1). The groundwater quality is good, and the TDS is 0.80 ~ 1. 17g/L, which belongs to SO4-Cl-Na-Mg type water.

(2) Anxi-Dunhuang basin

The thickness of Anxi aquifer gradually increases from the piedmont to the center of the basin, which is generally 30 ~150m. Dunhuang territory gradually became thinner from Danghe diluvial fan to Beishan piedmont. The area around Danghe alluvial fan is more than 200m, and the area in front of Beishan Mountain is less than 30 m. The aquifer thickness in the middle of the basin, including Anxi County and Dunhuang City, is stable at100 ~ 200 m..

Table 3- 1 Yumen-Statistics of Borehole Water Inflow in Zhen Shi Basin

1. Groundwater aquifer

The diving area is only distributed in the alluvial belt in front of Qieshan Mountain in the north and the middle and upper part of Danghe River alluvial fan. Aquifer lithology: the piedmont alluvial belt in the northern part of the mountain is a single gravel, and the Danghe alluvial fan is composed of pebbles, gravels and medium coarse sand from south to north. The buried depth of water level is generally 30 ~ 100m, and Danghe alluvial fan is greater than 100m ... This aquifer is rich in water, and the water inflow of a single well of Danghe alluvial fan is 1 0,000 ~ 3,000m3/d, and the water inflow of other areas is less than 1 1,000m3/d.

Table 3-2 Statistics of Borehole Water Inflow in Anxi-Dunhuang Basin

sequential

2. Diving-confined water

The vast fine soil plain is a distribution area of phreatic water and confined water with multi-layer structure (Figure 3-2). In the Anxi Delta, the surface phreatic aquifer is gravel with a thickness of 20 ~ 70m, which is distributed in a strip shape along Nancha-Sigong Farm and the northern edge of the modern Shule River. The water inflow of a single well is 1 0,000 ~ 3,000m3/d, while the surface phreatic aquifer in other areas is thin and medium fine sand, and the water inflow of a single well is less than 1 3,000m3/d. Roughly bounded by Nancha-Daliang Gobi, the water inflow of single well in the east is greater than 1000m3/d, and that of single well in the west is less than1000m3/d. The TDS of confined water is greater than 1.0g/L, which belongs to SO4-Cl-Na-Ca type water. Danghe alluvial fan and its adjacent fine soil plain and the west of Yumenguan (Ya Dan Scenic Area), the water inflow of confined water is greater than 1000m3/d, the TDS of groundwater is 1 ~ 3g/L, and the local area is 3 ~10g/L; The water inflow of other single wells in West Lake, Tangyi Lake, North Lake area of Dunhuang and plain area west of Mamitu is less than 1000m3/d, and the salinity exceeds 3 ~ 5g/L. ..

Figure 3-2 Hydrogeological Profile of Anxi-Dunhuang Basin

(3) Huahai Basin

The Quaternary unconsolidated layer is 50 ~ 200m thick, which is over 200~400m in the southern piedmont. The aquifer is 30 ~ 200 m thick and gradually becomes thinner from south to north. The lithology of the aquifer is sandy gravel in the south, medium-coarse sand and sand in the middle and medium-fine sand and fine sand in the north (Figure 3-3). It is roughly bounded by Huahai Township-Jiugang Farm, with a single diving area in the south, and the water inflow of a single well is greater than1000 m3/d; The north is a multi-layer phreatic confined water area, and the water inflow of a single well is generally 100 ~ 1000m3/d (Table 3-3), and the water inflow of a single well near Beishihe River is less than100m3/d. ..

Figure 3-3 Hydrogeological Profile of Huahai Basin

Table 3-3 Statistics of Borehole Water Inflow in Huahai Basin