YU SHI-YONG¹, LI MING² ,YANG SHAONAN³

（1.Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, P. R. China

2.Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029,China;

3. Investigation and Surveying Academy of Hydrogeology and Engineering geology of LiaoNing Province Dalian 116037,China）

 

Abstract: The severe shortage of the water resources has been shackled the economic development in coastal areas. Another queer phenomenon, which a mass of surface and groundwater resources flew into the sea in vain, exists at the same time. Many Engineer cases at home and abroad testify the underground reservoir to be an effective engineer method to prevent the land subsidence and seawater intrusion. The paper takes the Longhe underground reservoir’s construction as a example discussed the following issues, such as the hydrogeology theory, the technological chain, the constitute, the analysis of regulation capacity, the recharging technique and the mechanism analysis of in harnessing the drought, waterlogging and saline-alkali soil. Through the construction of the Longhe underground reservoir, the following conclusions can be drawn: the construction of the underground reservoir can not only enlarge the water supply capacity to release the water crisis but also prevent the the land subsidence and seawater intrusion, the benefit of the construction is prominent. The main principle to resolve the water resources crisis in coastal areas is combined with the increase of the water sources and the saving of the utilization; the most important principle is to save the water, in the meanwhile, keep the management and the protection in pace. The influence to the entironment in coastal beach and the continental shelf areas cannot be ignored before the design and construct of the underground reservoir.

Keywords: underground reservoir; coastal areas; land subsidence; seawater intrusion; water resources; sustainable development and utilization

1 Status of the Water Resources in Coastal Region

The coastal region have been the frontier of the economic development in China. The coastal region have vast of the coastal plain and the bayou delta, the land area have national total 14% of the land area, but the population have national total 40% of the population in China. China about 65% industrial total productions, 55% Gross National Product and 70% of big and medium city distribute equally in the coastal region. Since1980's, economic growth rate in coastal region has been keep above 10%, some the region reach to 22%. The coastal region is an important district in the political economic techno-culture domain, is also the strategic center of gravity of the China national economy and social development ^[1].

China is a nation that is short of water, the total water resources amount is about 2,800,000,000,000 m³ annually; every person is under the 2400 m³ which only is 1/4 of the world average level. Because the fast industrialization in China water pollution become worse. the fresh water resources is missing just gradually serious, special in the coastal region, the water resources amount of every person is low in the 500 ms 3, among them Tienjin, Shanghai, Dalian , Qingdao and Lianyungang etc. the city is even in the 200 m³ following^[2]. in the meantime, the research discover the total amount water resources in coastal region is 44.59×10⁹m³ annually, but the average supply water quantity only is a 8.476×10⁹  m³, utilization efficiency only is 19%.It is a very urgent problem in resolve water resources missing problem in coastal region ^[3].

For satisfying to increase continuously of use the water demand, the coastal region has to mine groundwater in great quantities. But a series of the geology environment problem because of the exploitation of the groundwater also appeared: Because mine groundwater too much which not only cause the ground subside but also cause seawater intrusion such as Ningbo city, Nantong city and Zhanjiang city all takes place big area subside, Dalian city, Qingdao City all takes place seawater intrusion ^[4]. The ground water level descends and the severity shortage of rainfall aggravates the problem. These coastal cities have the high speed economic development and the high speed population increase. The sea water intrusion already brings serious menace for economical construction and the society development ^[5].

The water resource which can be used is short at the coastal region but in the meantime, a great deal of earth's surface and underground runoff flow into ocean but didn't get full exploitation annually. People seemed to also forget a fact in the meantime, 2/3 of the fresh water that is on the earths is saved in underground reservoir. Because there are such reservoirs we can distill a great deal of water from these reservoirs then, but also when the surface water quantity is abundant, the reservoir can be re-filled with again ^[6].

Therefore, the application studies of the underground reservoir in water resources in coastal region not only can keep on the water resources sustainable exploitation but also has the very important science meaning and economic value.

2 Adjust Mechanism and the Superiority of the Underground Reservoir in Water Resources Management

2.1 Basic concepts of the underground reservoirs

The underground reservoir is to make use of the natural storage water space inside the earth crust which is a kind of groundwater development engineering of the storage water resources. Natural storage water space is the aquifer, include the crevice, hole, fissure and water-eroded cavein in the hard rock and relax deposit. a lot of storage water structure such as alluvial and proluvial fan, alluvial plain, proluvial pyramid, karst storage construct ,fracture storage water construct and some compound types and the transition type come forth by the outside and inside motive geology function within the earth crust  in natural aquifer. Whether these storage water structure is filled or not they have enormous storage water space which is named  natural underground reservoir, such as lower Liaohe river plain is a integrity nature underground reservoir which three boundaries surrounded by mountains ,the other boundary is seaside ^[7].

2.2 basic principle of retaining water

Make use of the underground reservoir's retain water is to build well in the underground reservoir to gain water resources in great quantities, make the underground space become empty, descend an earth's surface to lead through the artificial replenishment way again into the underground, renew the amount of water of the depletion. We expect to use underground storage quantity in the dry season or low flow year, expects the underground water resources to recover again in the ample flow season or high flow year. This is similar to the function of the earth's surface reservoir which rise to retain water, regulate to make use of drainage water resources ^[8].

2.3 The foundation condition for retain water

There are three condition are prerequisite before the underground reservoir can operate well.

(1)Underground reservoir capacity

There are some aquifers in plain area; front mountain and valley surrounded by mountains in coastal areas .The ample aquifers have great storage water space.

(2)A great deal of surplus water flow over the underground reservoir is necessary. There are many river flow over the earth surface in coastal area which can satisfying thee condition

(3)The channel between the transmit river and the underground storage water layer is necessary. Some area have natural channel, the other have to build artificial channel such as seep channel ^[8].

2.4 Superiority of the underground reservoir retain water

Compared to the earth's surface reservoir the underground reservoir can not only adjust and retain the drainage water resources similar to the earth's surface reservoir but also the unify the drainage water resources to realize the uniform layout and adjust. Natural underground reservoir is predominant engineering in water resources adjustment. It has big capacity, good water quality and without silted up and safe threaten. It is hard to be polluted. The underground reservoir can transfer the annual asymmetric replenishment quantity to the balanced supply water quantity of the city by regulating the surface water and groundwater resource ^[9].

In addition, the underground reservoirs still have some special advantage compared to the earth's surface reservoirs:

(1)The underground reservoir has the dual function that can intercept the earth's surface flow store the water resources.

(2) It can raise the valid utilization efficiency in retaining water. At A lot of surface evaporation can be avoid because the application of the underground reservoir in semiarid and arid region, the retain water utilization efficiency of retaining water can approach or attain 100% ^[8].

(3)The underground reservoir stores water in natural aquifer which forms the water and rock system. The system can keep the water resources of deep circulation well and realize the water resources sustainable development.

(4)The underground reservoir damming engineering not only can cuts the groundwater flow path but also impede sea water intrusion ^[7].

(5) The aquifer contained in the underground reservoir lead the earth's surface water into the underground to replenish the groundwater resources by various engineering means(natural infiltration ,wells, dike and ditch etc.) , carry out the water resources adjustment.

(6)The underground reservoir has the ability of energy storage , the recommend circulation mode include” the winter filling and summer drilling" which can makes use of energy and water resources circulation.

(7) Unlike the surface to occupy a great deal of land resources. This is in especially useful in the area that can not build the earth's surface reservoir.

(8) Little invests and quick effect compared to the earth's surface reservoir is another advantage.

(9) The underground reservoir can adjust the groundwater level which can prevent the Water logging and saline alkali land [8].

Some Euro-American nations have some successful experience in developing underground reservoir, such as American New York long island region and the north of Netherlands. Some regions adopt the way exploiting the water near the river bank which is actually a method of underground reservoir. But all can not from a total water resources management in view of the effective total water resources utilization on the whole drainage resources. It is just a start within this aspect in China. There are a handful of underground reservoir developments practice in Beijing, Shandong, Hubei, Liaoning etc that have already received the good water resources performance and social economic performance.

There are a lot of big rivers and ample transit surface water in China; many regions are beneficial to use the condition to build the underground reservoir engineering. If the ability develops exploitation well that will have important function in improving our country water supply at present condition [8].

3 System Engineering Analysis of the Longhe Underground Reservoir

3.1 Engineering general situation

3.1.1 Physical geography condition

The Longhe river region locates in the north suburb of lvshunkou district. The character of the topography is high in north east, and west and low in south and middle. The relief belongs to low mountain and hill. The total area is 39.5 km².The longhe river flow in the area which is a seasonal river .Its length is 9.4 km.

3.1.2 Hydro geological condition

The groundwater kind is bedrock fissured water of Sinian System and pore water of Quaternary period. There is not continuous separate layer .The hydraulic connection is close between the two. The research keystone is pore water of Quaternary period in the paper. The climate belongs to the mildness zone, wetness and half wetness. The amount of precipitation is 582.8 mm and average evaporation capacity adds up to 1662 mm.

1. Distributing, thickness and lithology of pore aquifer in Quaternary period

The groundwater belongs to the river valley pore water. The shape of the river valley bottom is ︺ fashion .The width of river valley is 600 m. The gravel clay and light loam distributing in the valley slope form the relative water-resisting layer. The alluvial-proluvial and marine strata is in the middle of the two layers which is water-saturated and the thickness is 5.2m-9.0m.The gravel strata is main aquifer in the zone of saturation which has fine permeability. The permeability coefficient is 10-50m/d.Most of the bedrock of valley bottom is slat, local section has quartz rock. The entire condition is benefic to the water storage.

2. Replenishment, runoff and drainage condition of the groundwater

The research area belongs to the coastal hill valley area. The groundwater flows from the two sides of valley to the cleuch, from the upper reach to the lower reach. At last it drainage into the sea. The atmospheric precipitation is main replenishment source. At present, the lower river is affected by the tidal action.   

The drainage of groundwater includes the runoff, evaporation and man-made exploitation. The river valley is the only channel that the upper stream drainage into the sea.

3.1.3 Water quality

1. One water sample is collected in dam site under contemplation to carry out the brief analysis. Many of index exceed the national drinking water standard.

Tab .1  Parallel table between the over proof items and the national drinking water standard (unit: mg／L）

 

 

 

3.2 Constitute of the engineering system

The layout map of the engineering is showed in Fig.1

1. Underground water separate dam

According to the site hydrogeology exploitation data, the axes of the underground water separate dam distribute from west to east. Five segments are in turns ①(A-B)②(B-C)③(C-D)④(D-E)⑤(E-F). The total length is 544.6m, the average width is 0.4m, the average depth is 8.78m. The three pile (including the gas pile, water pile and cement slurry pile) method is adopted. After the former engineering test the construction technique parameter of separate dam are confirmed. The total bores are 423,the aperture is 150mm, space between the two bores is 1.5m,thedepth into the bedrock is less than 1m.

2. Block and storage engineering

The Block and storage engineering dam consist of rubber dam filled with water on the river channel. The length of dam is 60m, the height of dam is 1.3m.The main function of the dam is to prevent the tidal from invade into the underground reservoir area. It has to low the height of the rubber dam to ensure the river channel smooth in flood season.

3､Artificial infiltration engineering

(1) The infiltration channels distribute in the upper river bed, the length of single channel is 50m, the space is 30-35m.The total infiltration channel.

(2)The catchment covered way are designed two groups, two lines of every group one is vertical to the river bed the other is oblique to river bed, the separation angel is 65-75 degrees, the length of 50-70m,the depth is 2.19-4.36m.

3.3Benefit analysis

Since 2000 the underground water reservoirs have been finished the water quality become a little fine. According to the water sample test near to the front ,the content of Cl^- change from the 240-1700mg/L before construction to 70-500 mg/L completion of construction. The storage quantity becomes large. The exploitation amount add up to 127.75×10⁴ m³, the exploitation amount add up to 7000 m³/d in ample flow season. The water table ascends obviously. The water head add up to 1.0m.The separate dam operates well at present.

The groundwater table keeps the high value and there is a lot of water flow over the rubber dam in many years dry. All these confirm the groundwater engineering is effective. Some advantage such as:

(1)The water supply ability increases 137.1×10⁴ m³.

(2) The soil moisture status and entironment gain improvement; the sea intrusion is hold back; the groundwater quality is improved continuously.

(3) The paradise has been built in the reservoir area which amuses many people.

4 Discussion

(1)From the Longhe groundwater reservoir engineering we can draw a conclusion that the groundwater reservoir engineering commonly is consist of recharge source, surface block, pollution drainage, underground water separate dam, water lift, water supply, salt water drainage and monitoring management system. Among the eight system parts the underground water separate dam is most important part ^[10].The fill engineering design and pollution prevent engineering must be marked out according the scientific law. Otherwise it leads to the shortage of water sources and water quality decline ^[11].

(2) The capacity of the groundwater adjusts the water resource is based on the current economical and technique condition. At the certain period the recharge water source, storage space and water lift engineering technology is limit so that the capacity of adjustment has relatively high-point ^[12]. 

(3)Many success practices at home and abroad confirm the artificial recharge water technique is feasible. The recharge method includes surface recharge and underground recharge. The surface recharge often use the river channel, ditch and pool which is feasible in the depth of burial from several meters to less than 20 meters^[13].The underground recharge often drill large open well in the river bed and the well pile is filled with the sand and gravel. The method is feasible in the depth of burial more than 20 meters especially the tiny confined aquifer and the fine grain distributing in the shallow strata site ^[14]. The key of recharge in coastal area is the selection of fine water sources.

(4)The protection of groundwater must be reinforced to prevent from the water sources drying up, groundwater polluting and make the groundwater reservoir can operate at a long time ^[15].

(5)The surface water and groundwater is two forms existed in the plain areas. The precipitation is common recharge sources for them. The forestation work should be reinforced especially in the coastal area^[16].In practical work the method that the surface reservoir and groundwater reservoir should be adjust as a entire body which not only storage water but also decrease the evaporation^[17].

(6)By adjusting the groundwater table we can prevent and resolve alkaline soil ^[18].The detail method is to confirm the proper groundwater critical depth.

5 Conclusions

(1)The water resources is shortage seriously, the groundwater is mined to excess badly. The groundwater construction is a feasible way to overcome the above problem which not only alleviate the water supply pressure but also prevent sea intrusion ^[11].

(2) The establishment of underground reservoir can adjust the framework of water utilization. The fine water can be used to drinking and food production. The common water can be used to agriculture and industry production. All these not only can resolve the water utilization in coastal area but also improve the entironment quality ^[19]. The possible method includes the economic measure and water classification utilization.

(3)Because the aquifer in the groundwater reservoir has weak pollution prevent ability the pollution cleanup work is very important before the groundwater reservoir work go into operation which can provide the precondition for the water resource adjust and retain ^[20]. It is a pity that the water quality is not accord with the drinking standard at a long time in Longhe groundwater reservoir because the previous period pollution does not clean up clearly.

(4)The main principle is to emphasize on raising source and cutting charge. In view of the raising sources it should enforce the entironment and water source forest construct in order to increase the flow quantity in dried season ^[21].In some degree cutting charge is more important than the raising source. The people must cherish the water resources and utilize water resource if we want to gain sustainable development of water resources consequently promote the civilization sustainable development ^[22].In our practice we should form the good habit of cherishing every drib of water.

(5) The key of the cooperated adjustment between the surface reservoir and groundwater reservoir is increase the surface block especially in mountainous areas so that utilize the underground reservoir capacity^[23].Detail engineering project should carry out proper design according to the site condition.

(6) The groundwater reservoir is a systemic engineering. Many law need to be validated and summarized in future engineering practice ^[24].

(7) The application of groundwater reservoir is just a start in China. Many theory and method need to be investigated. We advised that a series of theory and application criterion should be established as soon as possible ^[25].

(8)Another important problem is that environment influence to the coastal beach and continental shelf after the groundwater reservoir finished. The reason is that the drainage into the groundwater is cut and the surface water is decreased in a great measure.

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