Xie Zhenhua¹，Cui-yu¹，Xu Miaojuan¹，Cui Yali²，Xing Guozhang¹

1 Beijing Institute of Hydrogeological & Geological Engineering, Beijing, 100037;

2 China University of Geosciences, Beijing, 100083

 

Abstract: The location, water source, methods and effects of groundwater-surface water conjunctive use in Beijing Plain are focused on in this paper. The analysis of Hydrogeological conditions indicates the upper and middle parts of the alluvial-pluvial fans of Yongding River and Chaobai River are the best suitable places for groundwater recharge because of large storage, plenteous surface water source, high infiltration capacity and good pumping facilities. The capacities of the groundwater reservoirs in the two fans are 6.24×10⁸m³ and 6×10⁸m³ respectively. Moreover, in wet year a mass of water is discharged from Miyun Reservoir and Guanting Reservoir for preventing flood, which can be used as artificial recharge water source. The modes of groundwater recharge include riverway, plain reservoir, sand pit, large open well and infiltration gallery and so on. Furthermore, the suggestions of groundwater development are put forward for South-North Water Diversion. The results of groundwater numeric modeling on two alluvial-pluvial fans indicate that groundwater is replenished through groundwater-surface water conjunctive use. It can not only increase groundwater safe yield, but can also improve assurance of municipal water supply and avoid geological and environmental problems resulted from continual groundwater drawdown.

Key words: groundwater reservoir, conjunctive use, groundwater model, Yongding River, Chaobai River

 

Dominated by southeast monsoon climate, the precipitation and surface runoff in Beijing distribute unevenly in a year, and vary greatly from year to year. Continual dry years and continual wet years appear sometimes, which result in alternant occurrences of drought and waterlog, and badly threaten municipal water supply security. Groundwater recharge can not only increase its safe yield, but can also improve assurance of municipal water supply and compensate deficit of its storage, and avoid geological and environmental problems resulted from continual groundwater drawdown. Groundwater-surface water conjunctive use will improve the efficiency of water resource utilization, and realize mutual supplement of exploitation and utilization of surface water and groundwater ^[1-4] in space and time.

Since 1970s, many experiments of groundwater artificial recharge in different infiltrative patterns have been made in UMPAF of Yongding River and Chaobai River ^[5-10]. All the experiments and researches have promoted the study of this area.

1.Hydrogeology of Groundwater Reservoirs

In the upper and middle parts of the alluvial-pluvial fans (UMPAF) of all main rivers in Beijing, the aquifers are mostly made up of monolayer or dual layers of sand and gravel, which are coarse-grained, very thick and rich in water, and have the perfect space for groundwater storage. Researches indicate that, over-exploitation of groundwater for many years has released huge space, and UMPAF of Chaobai River and YongdingRiver, which are the two biggest groundwater reservoirs in Beijing, are especially the best suitable places for artificial groundwater recharge.

1.1 Groundwater reservoir in Yongding River

The groundwater reservoir lies in UMPAF of Yongding River. West Mountain in Beijing forms the reservoir’s impervious west and northwest boundaries, which are made up of sand-shale and lava of Carboniferous-Permian and Jurassic Periods. The east boundary is along Kunming Lake, Zizhuyuan, Taoranting and Xihongmen from north to south. The south boundary is from Xihongmen to Nangangwa through Langfa, and is the natural boundary of groundwater, where the Quaternary lithology gradually becomes fine-grained, the aquifers change from monolayer to multilayer with permeability going down, and the groundwater type changes from phreatic water to confined water. The bottom of the reservoir is also impervious for being made up of Quaternary moraine mud gravel or Tertiary semi-cemented sand-gravel and mudstone. So this area has the perfect conditions to form groundwater reservoirs, see figure 1. The capacity of the groundwater reservoir in Yongding River is 6.24×10⁸m³.

1. Quaternary aquifers    2. aquitards     3. bedrocks   4. phreatic water tables

Figure 1  Hydrogeologic united cross section of dispatching in Yongding River catchment

 

1.2 Groundwater reservoir in Chaobai River

The groundwater reservoir lies in UMPAF of Chaobai River, with slope and pluvial deposits at the piedmont on the east, north and west boundaries (The lithology is farraginous and the permeability and storage capability are poor). The east and west boundaries are located at the front edges of the second terrace of Chaobai River, and have poor connectivity with the first terrace, so can be regarded as relatively impervious boundaries. The south boundary is from Xiangyang Strobe to Maxinzhuang, and it is the outflow area of groundwater, where the lithology becomes finer, flow section comes narrower, and thickness of the aquifer is smaller. In fact it is a relatively flow-obstructive barrier. North of the groundwater reservoir’s bottom is bedrock, and south of it is distributed by a more than 15m thick, stable clay sand and sandy clay layer at the depth of 90-100m underneath the ground, which forms a relatively impervious boundary, see figure 2. The two groundwater reservoirs in Chaobai River already have huge regulative space for water recharge after many years’ exploitation. The capacity is 6×10⁸m³ according to the calculation result.

1. Quaternary aquifers    2. aquitards     3. bedrocks   4. phreatic water tables

Figure 2  Hydrogeologic united cross section of dispatching in Chaobai River catchment

2 Capacity and Water Source of  Groundwater Reservoirs

2.1 Capacity

2.1.1 Riverway infiltration experiment

Yongding Riverway infiltration: Guanting Reservoir discharged water in demand of controlling flood from October 1995 to June 1997, and there was 3.6×10⁸ m³ water infiltrating and recharging groundwater in the riverway from Sanjiadian Strobe to Lugouqiao Strobe. Consequently the groundwater level in the area along this part of Yongding River increased greatly, with the maximum range of 15m, and average of about 3-5m, which greatly lightened the situation of groundwater shortage in Fengtai area.

Chaobai Riverway infiltration: Miyun Reservoir discharged 4×10⁸ m³ water from late April to late June in 1995, with 10324×10⁴ m³ water infiltrating into groundwater in the riverway. So the storage quantity of aquifers was increased by 8824×10⁴ m³, and the groundwater levels generally went up. The levels in the north area of Xiangyang Strobe rose 1.3m in average and 6m at the maximum.

Block engineering in the riverway: at present, there are two rubber-dams and Lugouqiao Flood-sharing Strobe constructed above the Yongding Riverway, and six rubber-dams and Xiangyang Strobe constructed above the Chaobai Riverway. Those hydrolic engineerings can retain certain quantity of discharged water from reservoirs and mountain flood, which can infiltrate into ground and recharge groundwater at a certain extent.

2.1.2 Sand pit infiltration in Xihuang village

The sand pit in Xihuang village lies at the top part of the alluvial-pluvial fan in Yongding River, with an area of 2.82×10⁴ m² and the maximum volume of 22.8×10⁴ m³, and its infiltrative capacity is about 10m/d. Discharged water from Miyun Reservoir was diverted to Yongding Water Channel via East-West Water Diversion Engineering from April 21st to August 1st in 1995, then transferred to the sand pit for recharge eventually. The groundwater was recharged by 1718×10⁴ m³ in total, and the infiltration capacity of the sand pit was 1.0 m³/s or so. The monitoring results of water level and quality indicated that, sand pit infiltration made important effect on controlling groundwater drawdown around, and the groundwater quality was improved after surface water infiltration.

2.1.3 Large open well experiment near Shougang

Shougang is located at the top part of the alluvial-pluvial fan in Yongding River. The aquifer there is single layer of coarse-grained sand gravel. The depth of groundwater level in this area had already been over 30mwhen making experiments in 1980. The large open well is 25m in depth and 8m in diameter. The experimental recharge rate could reach 0.5m³/s, and with the recharge time going longer, the well was stagnated and the recharge rate descended gradually.

The recharge experiments in the above three patterns have proved they have great effects and can be directly applied to engineering construction. Therefore, if surface water is abundant, we can retain and store it through dispatching engineering, recharge it into the ground and increase groundwater storage quantity, which will reduce the problems such as groundwater drawdown in large areas resulted from continual over exploitation.

2.2 Water source

2.2.1 Groundwater reservoir in Yongding River

Yongding Riverway flows from northwest to southeast across east of its UMPAF, and the Yongding Water Channel diverting water from Sanjiadian Strobe extends from east to west at the piedmont. Jingmi Water Channel flows through northeast of this area, to west Balizhuang then joins into Yongding Water Channel, and goes into Yuyuantan Lake at last. So, surface water sources that can take part in recharge at UMPAF of Yongding River include discharged flood from mountains within the area of 1520km² from Guanting to Sanjiadian, discharged water from Guanting Reservoir to Yongding River and discharged water from reservoirs along Jingmi Water Channel. Considering on the distance of water diversion, discharged flood from mountains in Yongding River and discharged water from Guanting Reservoir are more convenient to be selected as conjunctive use water source with groundwater. This part of water source is discharged to lower reaches through Sanjiadian Strobe, and the discharge quantity changes with precipitation, see table 1.

Table 1      Annual discharge quantity of Sanjiadian Strobe from 1994 to 1997             unit: 10⁴m³

2.2.2 Groundwater reservoir in Chaobai River

The main surface water sources of this area for recharge come from Miyun Reservoir and Huairou reservoir, and other four middle and small reservoirs, Huairou Reservoir can be regarded as the adjustive reservoir for Miyun. Dashuiyu and Beitaishang Reservoirs can be connected with Miyun and Huairou Reservoirs through Jingmi Water Channel, which will realize uniform control and dispatch of surface water sources. Shachang Reservoir is at the east of Chaobai River, connective with it by the trunk of Chao River. Precipitation in 1995 was comparatively abundant, so Miyun Reservoir discharged about 4×10⁸m³ water to the riverway, and the other four reservoirs discharged 0.22×10⁸m³ water( table 2).

Table 2         Discharge quantity of middle and small reservoirs from 1981-2000          unit: 10⁸m³

Year	1981	1982	1983	1984	1985	1986	1987	1988	1989	1990
Dashuiyu Reservoir	0.00	0.11	0.10	0.00	0.12	0.12	0.17	0.19	0.00	0.00
Beitaishang Reservoir	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.05	0.00	0.00
Shachang Reservoir	0.00037	0.27	0.0023	0.00	0.43	0.13	0.51	0.24	0.08	0.32
Year	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Dashuiyu Reservoir	0.18	0.06	0.0088	0.20	0.02	0.18	0.0004	0.15	0.00	0.00
Beitaishang Reservoir	0.00	0.025	0.00	0.10	0.00	0.32	0.11	0.13	0.04	0.01
Shachang Reservoir	0.16	0.18	0.03	0.39	0.21	0.06	0.00	0.02	0.00	0.00

3 Suggestions for Conjunctive Use

3.1Groundwater-Surface water conjunctive use before the water source from South-North water diversion coming into Beijing

In the case of no diverted water source outside Beijing,,the water supply must rely on its own water resource, and then the emergent well fields have to be established in recent years. The quantity of groundwater exploitation can be enlarged based on the actual demands, and the contradiction between supply and demand can be resolved by emergent water sources. But the emergent water sources can’t be exploited without limitation. It must be stopped at first when surface water is enough. If the emergent well fields and the surface water reservoirs can be connected by water plants and water transferring pipelines, then it is more helpful to the conjunctive use of these two part water resources.

3.1.1 The conjunctive use between the Huarou Emergent Well Field and the reservoirs in Miyun and Huairou

The Lianghe emergent well field in Huairou lies on the alluvial-pluvial fan of Huai River, which is a part of the alluvial-pluvial fan in Chaobai River. It is east to Huairou Reservoir, and near to Jingmi Water Channel and the water intake at Huairou Reservoir of the Ninth Water Plant. The emergent water source adopted the solid groundwater exploitation scheme with deep-shallow wells combination, and 21 pairs of wells (42 pieces) were constructed. The estimated capacity of annual emergent water supply is 1.2×10⁸m³/a.

When municipal water supply is seriously deficient, water in the emergent well field will be exploited and transferred into the existing water supply pipelines net (the Ninth Water Plant). And the project will be started up in order to supply water to urban Beijing.

3.1.2 The conjunctive use between Pinggu Emergent Well Field and Haizi Reservoir

The emergent well field is in the areas of Wangduzhuang-Magezhuang and Yukou-Zhongqiao on the alluvial-pluvial fans of Ju River and Cuo River respectively, with large storage capacity and good conditions for recharge, easy to be exploited and replenished. The surface water can directly recharge groundwater by riverways with the discharged water from Haizi Reservoir. It took the mode of united exploitation both in Quaternary deposits and bedrocks, and the estimated capacity of water supply is 27×10⁴-30×10⁴ m³/d，and about 1.0×10⁸m³/a in a year. But it’s necessary to construct transferring-distributing pipelines net for Beijingmunicipal water supply, at the same time it can reduce the exploitation intensity of groundwater, using water of Haizi Reservoir replacing the exploitation quantity of groundwater for agriculture and parts of industries in the Pinggu Plain.

3.2Groundwater-surface water conjunctive use after the water source from South-North Water Diversion coming into Beijing

After the center line of South-North Water Diversion transferring water into Beijing, it will be apart of the municipal water supply system and distributed uniformly. The distribution principles are: prior to use water from South-North Water Diversion, and considering water from Miyun Reservoir secondly; prior to use surface water and conserve groundwater, meanwhile storing water into Miyun Reservoir and the aquifers, and eventually reaching the purpose of dispatching water from South-North Water Diversion indirectly.

The planning urban area will be located at UMPAF of Yongding River and at the end of the South-North Water Diversion Engineering. The utilization of water from South-North Water Diversion can replace the exploitation of groundwater at a certain extent. And groundwater can be stored and conserved by reducing pumping rate and stopping some self-prepared wells. Especially in the western suburb, it is the strong recharge area for groundwater, and the aquifer is mainly consisted of single sand and gravel, which is coarse grained, rich in water and good for recharge. When the surface water supply increases, the effect of groundwater conservation will be more obvious in the water levels.

In summarization, groundwater-surface water conjunctive use can efficiently improve the utilization degree of water resources, and reduce needless waste. The more united dispatching is used, the more assurance of water supply we can get. It is definitely a scientific and reasonable solution to resolve the increasingly serious contradiction of water resources in Beijing.

4 Simulations of Groundwater Reservoirs for Regulative Engineering’s Effects

4.1 Regulative engineering of Yongding Water Channel - Nanhan River

Nanhan Riverway (infiltration capacity can be up to 1 m³/s), the discarded sand pits and large open wells along both sides of Nanhan Riverway and Yongding Water Channel were taken as the recharge engineering. According to the numerical modeling^[11], it could provide water resources 2.59×10⁸m³ for recharge. The recharge period lasted for three months, and the total infiltration quantity is 2.07×10⁸m³ with the ratio of 79.8%. During the late period of recharge, the groundwater levels rose 1-10m in the area, at which water source wells of the Third Water Plant are located (figure 3).

1.rising range of water level  2.residential area  3.regualtive area  4.sand pit  5.large open well

Figure 3  the rising ranges of groundwater levels resulted in infiltration of regualtive engineering of Yongding Water Channel-Nanhan river

 

4.2 Regulative engineering of Huairou Emergent Well Field

The groundwater levels in the area of the alluvial-pluvial fan in Huai River declined due to exploitation of the emergent well field. So the rubber dams were constructed on riverways of Huai, Yanqi and Sha to store water so that the alluvial-pluvial fan in Huai River could be recharged, and the discharged water from reservoirs of Beitaishang, Dashuiyu, Huairou and Miyun was also used for this reason. The modeling results indicated that the regulative engineerings could provide water resources 1.08×10⁸m³，and the total infiltration quantity was 0.88×10⁸m³ with the ratio of 81.48%. During the late period of recharge, the groundwater levels in the area of the emergent well field rose 2-6m, and the effects even extended to the area of the eighth water works, where the groundwater levels rose 2-4m near Niulan Mountain (figure 4). It effectively reduced a series of environment and geology problems resulted in groundwater levels decreasing after the emergent well field was in operation.

 

1. well field  2. emergent well field  3. study area  4. modeling area

5. rubber dam  6. rising range of water level  7. Xiangyang Strobe

Figure 4  the rising ranges of groundwater levels resulted in infiltration of the rubber dams on Huai Riverway

The estimating results of the groundwater dispatching model of the alluvial-pluvial fans in Yongding River and Chaobai River show that, the artificial recharge of groundwater has good effects, and the engineering is feasible. It has positive effects on groundwater replenishment, and also has significant social and environmental benefits. So, it is necessary to strengthen implementing studies on the groundwater-surface water united dispatching after the South-North Water Diversion Engineering is put in practice, and to try the best to implement it step by step after asking for opinions of relevant departments.

References

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