Groundwater flow field and its changing characteristics

I recharge, runoff and discharge of groundwater

The general trend of groundwater movement in various hydrogeological basins in Shule River Basin is consistent with the river flow direction. As the hydraulic conductivity of the main aquifer weakens from upstream to downstream of the river, the alternation of groundwater gradually changes from infiltration-runoff to infiltration-evaporation.

In the piedmont diluvial fan area at the foot of Yumen-Ta basin in the middle reaches, the sediments are loose and rough, and a large amount of river water seeps down. The groundwater in the seepage zone is in the state of high water mound, with good hydraulic conductivity of 3000-5000m2/d, strong horizontal runoff and hydraulic gradient of 3 ‰-7 ‰. Groundwater flows northward from the top of the alluvial fan in Changma to the fine soil belt at the edge of the fan, with weak hydraulic conductivity and gentle terrain, and groundwater overflows the surface in the form of spring water. In agricultural farming areas, a large amount of irrigation water seeps to replenish groundwater, which is buried in shallow layers. While receiving a small amount of precipitation and condensate infiltration, a large amount of evaporation and transpiration, artificial mining and spring water constitute groundwater discharge. In front of Fan, there is a groundwater watershed-Wujiatan Ma Yin Farm. The groundwater in the east flows northeast into Huahai Basin, and the groundwater in the west flows steadily to the Twin Towers.

The groundwater in the west of the rabbit gourd in front of the alluvial fan in Changma flows westward, the hydraulic gradient drops to 2.5 ‰ ~ 4 ‰, and the hydraulic conductivity is weakened. Evaporation is the main way of discharge, and most springs overflow in the form of isolated confined springs. The alluvial fan of Yulin River in the west is replenished by river water, canal water and flood undercurrent in the south valley, and the runoff in the north and northeast meets the groundwater in the west. Most of it is consumed by evaporation and transpiration in shallow water, and a small amount flows into Lucaogou in the form of spring water, passes through the northern mountain, and leaks out at Baiqibao Beach in the east of Anxi-Dunhuang Basin.

Groundwater in Huahai Basin mainly receives irrigation infiltration from rivers and irrigation areas such as Beishi River in the west, Youshi River in the south and Shankou River in the east. Groundwater flows from southwest to east with a hydraulic gradient of 2.5 ‰ ~ 3 ‰. Irrigation area is the main mining area, and the runoff in the downstream area is getting weaker and weaker, so evaporation and transpiration become its main discharge mode.

In the Shuangta Irrigation District in the east of Anxi-Dunhuang Basin, groundwater is mainly supplied by the infiltration of Shuangta Reservoir's canal system and the infiltration between fields, and flows westward. The hydraulic conductivity in the east of Anxi County is 1, 000 ~ 2,000m2/d, and the hydraulic gradient decreases by 2 ‰ ~ 3.2 ‰, while the downstream runoff gradually weakens, and the hydraulic gradient decreases by 0.8‰~ 1.5. Evaporation and transpiration in shallow groundwater area and artificial exploitation in agricultural irrigation area are the main drainage methods. The recharge of Danghe diluvial fan in the south of the basin comes from the infiltration of Danghe reservoir into the river system. The hydraulic conductivity is 3000 ~ 4000 m2/d, and the runoff is strong and flows to the fan edge. From the northeast to Danghe irrigation area, irrigation water seeps into the groundwater, and at the same time, artificial exploitation and evaporation and transpiration in the shallow groundwater area are the main drainage methods, and the groundwater runoff and the eastern groundwater runoff flow westward and gradually weaken. The alluvial fan in front of Karatashtag Mountain in the southwest of the basin receives a small amount of flood infiltration from Tugou and Dobagou in Cui Mu, and flows northwest to the downstream tail water area. The wetland in Houkeng-Wanyao Nature Reserve and the shallow groundwater areas on both sides of Shule River are dominated by evapotranspiration and drainage, with strong vertical alternation.

The groundwater contour map (Figure 4-3) compiled by using the measured data of water level in 2004 reflects the distribution of groundwater flow field.

Figure 4-3 Groundwater Flow Field in Shule River Basin

Second, the characteristics and changes of groundwater flow field

There are generally two aquifers in the oasis fine soil plain in the basin, the deeper is the confined water in the thick gravel layer of the Middle-Upper Pleistocene, and the shallower is the phreatic water in the fine soil layer. The former is formed by the diving of gravel layer covered with fine soil layer in Gobi plain of southern alluvial fan. The main source of the latter is the jacking seepage of the lower confined water. There is no good water-resisting layer between the two aquifers, which can also be regarded as a double-layer medium aquifer, and the permeability is quite different.

According to the drilling and geophysical data, the thickness of the confined water aquifer is generally 20 ~ 90m, the buried depth is generally 5 ~ 20m, the water head is generally 3 ~ 10m higher than the roof, and locally reaches/kloc-0 ~15m, which is 0.4 ~ 1.0m higher than the groundwater level and is away from the surface. The movement direction and flow field form of confined water are basically consistent with phreatic water, and the flow direction of groundwater in the basin is basically consistent with the recharge river.

The groundwater of Changma diluvial fan in Yumen-Zhen Shi basin is roughly bounded by Wujiatan, and the south is the recharge area. The isobar inclines to the south, indicating that the surface water migrates to the deep, the isobar inclines to the north in the fine soil plain, and the regional groundwater migrates to the shallow and surface. Between Wujiatan and Shule River, the isoheads are dense, which means that the groundwater at the front is blocked and discharged in a short distance, forming a spring ditch (Figure 4-4). The groundwater movement of Yulin River diluvial fan and its front edge is similar to that of Changma diluvial fan and its front edge, but on a smaller scale.

In the Shulegan Delta in the east of Anxi-Dunhuang basin, the underground hydraulic gradient gradually decreases from east to west, and the runoff slows down. It is roughly bounded by Anxi county, and the eastern part is a regional strong supply area. The water head inclines upstream, and the recharge to the west is small, and it enters the regional drainage zone. With the increase of the thickness of cohesive soil layer between aquifers, the horizon is stable, the water head in the lower layer in the west is relatively high, and the hydraulic gradient decreases, which reflects the characteristics of evaporation basin (Figure 4-5). In the southern Danghe alluvial fan area, the hydraulic gradient gradually decreases from south to north, and the stratum particles gradually become thinner until the runoff at the fan edge meets the runoff from east to west. The runoff in Tangyihu area is slow and the water head is high, forming a lake wetland, and the underground runoff flows westward. The runoff in front of Karatashtag Mountain flows from south to north, and the direction of runoff turns to northwest until the Kumtag Desert.

The northwest runoff of Huahai Basin moves along Beishi River to Ganhaizi, the alluvial fan of the southern oil river and the underground runoff at the northern foot of Guan Dan Mountain move to the deep and downstream, move to the middle and lower reaches of the basin, and the groundwater moves to the shallow and surface, and flows to Ganhaizi for collection.

Figure 4-4 Schematic diagram of groundwater isoline in Yumen-Zhen Shi Basin

Figure 4-5 Groundwater Isogram of Anxi Basin

In the past 50 years, the groundwater level in each basin fluctuated greatly, especially in the middle and upper parts of alluvial fans in Changma and Danghe, followed by irrigation areas in Changma and Danghe. However, there is no large drop funnel in the region, the shape of the flow field is basically unchanged, and the direction of runoff has not changed significantly, but there is water level fluctuation in the whole region. Affected by recharge conditions, irrigation and mining, the change of groundwater level is more prominent in Gobi area and agricultural irrigation area, but relatively gentle in fine soil barren area. Compared with diving, the dynamic change of confined water is much weaker. According to the data of observation well H6, the range of phreatic water level in this hole is 1 1.20m, while the confined water level in the same hole changes very little, only 0.75m (Figure 4-6). It can be inferred that the shape of confined water flow field and runoff direction will not change greatly except for a slight change in head gradient. In addition, in some areas where mining is concentrated, such as Danghe Irrigation District, Changma Irrigation District, Bulongji-Hedong, etc. A large number of mechanical wells have poor water sealing effect, the upper and lower aquifers have colluded, the groundwater confined water dynamics tend to be consistent, and the groundwater flow field will change more or less.

Figure 4-6 Bitmap of Confined Water and Groundwater of Observation Well H6 in Huahai Basin