(1. China Geo University, Beijing100083; 2. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 54 1004)
Fund Project: Geological Survey Project of Ministry of Land and Resources (2003 10400043, 200310400044); Some achievements of the national key scientific and technological project (2002BA90 1A 13).
About the author: Zou (1969-), male, associate researcher, mainly engaged in the research of karst resources and environmental engineering.
Abstract: Residents in karst areas in southwest China mainly use dissolved wells, water tanks and water cellars for decentralized water supply, and their water sources come from underground rivers, karst springs and rainwater respectively. Based on a large number of investigations and practical projects, this paper summarizes the decentralized water supply modes in southwest karst areas, respectively analyzes the regional suitability of three water supply modes, discusses in detail the design ideas, design parameters and construction precautions of each mode, and gives the design schematic diagram combined with practical projects. Finally, the main problem of decentralized water supply-water quality safety is analyzed, and the corresponding measures are put forward.
Key words: karst area; Decentralized water supply; Water cellar; Water tank; Dissolve well
Water is an indispensable and precious resource for human survival and production, and it is also an active influencing factor in the ecosystem. Underground groundwater has become an important water supply source for human society's production and life because of its good water quality, wide distribution and stable water supply. Karst groundwater resources are the main water supply source in karst areas, and even the only fresh water source in many places. In Europe, more than 50% of drinking water is supplied by karst groundwater [1]. It can be seen that the status and role of karst water resources in social and economic development is very important.
Although the average annual rainfall in the karst area in southwest China is above 1 100mm, the total water resources in this area are very rich (see the table below) [2]. However, due to various reasons, there is a shortage of water resources in this area: ① The special geological environment with double-layer structure of surface and underground [3], coupled with large terrain difference, strong karst development, thin topsoil, few vegetation, strong soil permeability and low water conservation ability, lead to the lack of surface water and deep groundwater in karst areas, and the development and utilization of water resources are costly and difficult, resulting in a helpless situation of "people walking on the mountain and water flowing under their feet". (2) Rainfall has strong seasonality, and its annual distribution is extremely uneven. The rainfall for four consecutive months (May-August or June-September) accounts for 60%-70% of the annual rainfall, which is often characterized by floods in summer and droughts in winter and spring. (3) The geographical conditions in this area are special, the land is small and scattered, and the residents are scattered, so it is impossible to build large-scale water conservancy projects. It can be seen that the southwest karst mountain area is not resource-based water shortage, but seasonal water shortage and engineering water shortage [4].
Statistics of Underground River Flow in Southwest China
According to incomplete statistics, there are 2.4 million people and 2.02 million livestock in 39 karst mountainous counties in Guangxi, and the water shortage population is scattered, mostly below 500 people, especially in peak and cluster mountainous areas, which makes it more difficult to supply water on a large scale. Seasonal water shortage and engineering water shortage in southwest karst mountainous areas have become the leading factors restricting local social and economic development.
In order to solve the problem of drinking water for people and animals in southwest karst mountain areas, through years of exploration, combined with the local landform and water resources characteristics, a variety of water supply modes suitable for decentralized water supply in southwest karst mountain areas were developed, including dissolving wells, water tanks and water cellars, with water sources coming from underground rivers, karst springs and rainwater respectively. The characteristics, applicable conditions, design methods and existing problems of these three main water supply modes are discussed below.
1 water cellar water supply
Applicable conditions of 1. 1
Water cellars are common in peak-cluster mountainous areas of Yunnan-Guizhou Plateau, and are mainly used for scattered water supply for people and animals in peak-cluster mountainous areas with high terrain and extremely developed vertical karst on the surface. Hydrogeologically, this area belongs to the groundwater recharge area, and the precipitation seeps down along the vertical cracks. Surface runoff can only be formed in rainy season and rainstorm, and there is no surface karst spring. Precipitation is the only source of water resources in this area.
In recent years, large-scale rainwater collection and utilization has been carried out in karst mountainous areas in southwest China. Many places in Guangxi, Guizhou and Yunnan have built countless water cellars under the organization of the government, which effectively solved the drinking water and irrigation water for people and animals in karst mountainous areas and greatly promoted the development of life economy.
In order to collect rainwater conveniently, all water cellars are built below the ground (Figure 1), and each water cellars has a ditch on the ground to collect rainwater (eaves water and surface runoff) (Figure 2) [5]. Because of the need to store water, the volume of water cellar is relatively large, generally 20 ~ 30m3, and the larger one can reach more than 50m3.
Figure 1 family puddle
Fig. 2 Schematic diagram of rainwater collection project (small water cellar)
(According to small reports)
Design idea of 1.2
In order to ensure the water quality of the cellar, the cellar is generally designed as a sealed cylindrical or square pool with a rain-collecting platform (Figure 2). Its advantages are as follows: ① The cellar is buried in the soil, similar to a miniature underground water storage system, which is beneficial to the water quality; ② Water cellar is not easy to be damaged by human activities, and it is durable; (3) The water cellar is sealed, so that pollutants will not fall into the cellar, and microorganisms in the water are not easy to survive and reproduce because of the opaque seal; (4) Using artificial rainwater collecting platform to collect water can prevent rainwater from being polluted before entering the cellar. The key point of the design is to control the water quality, so that the water cellar can be effectively used and easily maintained. The water cellar capacity is determined according to the water shortage time, water consumption population, water consumption quota and other parameters.
Determination of related parameters of 1.3 water cellar
The volume formula of (1) water cellar is: v = n× I× t, where: v is the volume of water cellar; N is the number of water users; I is the water quota, which is 40L/ (person-day) according to the national standard and the actual value; T is the continuous water shortage time, which is determined according to the actual water shortage period. In actual design, the volume of water cellar should also consider the water consumption of livestock and poultry.
(2) Determination of specifications of water cellar. The diameter of domestic small-scale water cellars is 2 ~ 3m, and the effective water storage depth of water cellars is generally about 4m.
(3) Determination of rain collecting platform. Because drinking pits for people and animals require high water quality, artificial cement rain collecting platform should be adopted in the design, which is mainly used to collect rainwater and can also be used as a threshing ground for farmers. The formula is: v = FP (1-η). Where: f is the rainwater collection area; η is the rainfall loss coefficient caused by evaporation, concentration, etc. , generally take 0.3; P is the rainfall, which is determined according to the actual situation. For example, the measured rainfall is 1500mm, and the F value is 30m2, which is arranged in a rectangle with a specification of 6m×5m. In order to collect water more effectively, a waterline with a height of 50 mm was set at the edge of the rainwater collection plateau.
The design of water cellar needs to consider not only the water storage volume, but also the water quality. In the design and construction, integrated construction can be carried out (Figure 3) to ensure the water quality safety of the water cellar.
Fig. 3 Design section of family water cellar
1.4 construction points
The following three points should be paid attention to in the construction: ① The cellar site should be selected on a solid foundation, and the location should be higher than the house where people live, so as to facilitate water intake; (2) The manhole should be reserved for the cover plate of the cellar to overhaul and clean the cellar pool, and at the same time, it is convenient to take water in buckets when the water level in the cellar is too low; (3) Before the rainy season comes, the water cellar should be disinfected to prevent the water quality from deteriorating. Generally speaking, the temperature of pit water can be kept at 4 ~ 7℃, and most other indicators are within the allowable range of the national drinking water quality standard except for Escherichia coli.
Benefit analysis of 1.5 water cellar
According to the follow-up survey of beneficiary farmers, most farmers have changed from relying on people to pack horses for generations to using tap water, which not only improves water quality but also saves labor. According to estimates, after the construction of small water cellars, each household can save at least one backwater 120 working days each year, and these saved labors can be used for going out to work or developing production on the spot. Based on the low per capita daily income 10 yuan, the average annual income or income of each household can reach more than 1200 yuan, and the payback period of small water cellar investment is only 14 years. According to the service life of 30 years, the investment benefit ratio can reach 1∶22, and the long-term economic benefit is very considerable. If small water cellars are widely used in agricultural irrigation in mountainous areas, it can greatly improve agricultural production conditions, change the situation that mountain agriculture depends on the weather for food, and greatly increase crop yield, which is of great significance for accelerating the pace of poverty alleviation of poor farmers in mountainous areas and developing rural social economy.
2 Dissolved well water supply
2. 1 Applicable conditions
Dissolving well is mainly suitable for the bottom of peak-cluster depression and the edge of karst valley, and it is a regional groundwater runoff area. There is no surface spring exposed in this area, but an underground river runs through it. Dissolving well actually belongs to the skylight of underground river pipeline. Because of the low terrain, underground river water gushes out from the skylight (Figure 4). It can be seen that the construction conditions of dissolution wells are harsh and can only be built in suitable places.
2.2 Key points of design and construction of dissolution well
In order to take water conveniently, it is necessary to reform the natural dissolution well-to build a water well on the basis of the skylight. Because the dissolution well is exposed to the surface, it can be used directly and conveniently like a mountain pond. In order to ensure that the water quality is not polluted by human beings, many dissolved wells are built in series (Figure 5), that is, 1 main well is connected in series with 2-3 pools, and water is used separately according to the purpose to protect the water quality. The main well is used as a drinking water well, and the water flows from the main well to the downstream pool through a small slot. The first pool next to it is used as a domestic washing pool, and then the water in the back pool is used for watering and sundries cleaning.
Fig. 4 Natural solution well
Figure 5. Hunan Xintian Kuangjia Sanlian Jiejing.
The size of dissolution well is determined by its flow and total population. Because drinking water and water are separated, when the flow rate is 0.5 ~ 1L/s, a dissolution well with a volume of 2m× 1.5m×2m can meet the domestic water demand of about 1000 people.
The construction of dissolution well (main well) is also relatively simple. Firstly, the silt in the natural solution well should be thoroughly cleaned, and then the natural solution well should be expanded. The depth should be below the complete bedrock surface 1 ~ 2m, and the periphery should be enlarged according to the size of the solution well. The construction of each pool shall be carried out in accordance with ordinary pools. Typical dissolution well design is shown in Figure 6.
Natural block stones and cement are often used as building materials for dissolving well walls. Gravel and sand layers with a certain thickness are laid in layers at the bottom of the well to preliminarily filter the groundwater. Natural stones and cement are also used as building materials for the bottom and wall of the pond. Dissolving the well is mainly to prevent the leakage of the pond wall, while using the pond needs to prevent the leakage of the pond bottom and the pond wall.
Fig. 6 Schematic diagram of dissolution well design
In the process of design and construction, it is very important to set a certain range of protective belts and ditches around the dissolution well to ensure that the surrounding sewage does not penetrate into the dissolution well and that the surface overflow does not flood the dissolution well during the rainstorm. The distance of the protective belt shall not be less than 15m, and there shall be no pollutant residue within the protective distance. The ground of protective belt shall be solidified with cement to prevent sewage from infiltrating.
3 tank water supply
3. 1 Applicable conditions
Water tank water supply can be applied to the whole peak-cluster mountain area, and the first condition is that there must be spring water (most of which belong to surface karst springs), and spring water is its main water source. This kind of area generally belongs to the groundwater discharge area in Koizumi domain, and the groundwater is mainly horizontal movement.
Surface springs have regular springs, but most of them are seasonal springs, and there is only water flow in rainy season. For example, the water source of the water tank in Hechi mountain area is seasonal mountain spring water. The spring water flowing out after the rain seeps through the soil, vegetation and cracks of the mountain itself is very clear, and only when the mountain spring suddenly appears in the rainstorm season will water be ejected. Therefore, each water tank can only be filled once a year, and villagers must live on the water stored in their respective water tanks 1 year.
3.2 Design and Construction
Water tanks generally use square masonry structure (cement plastering to prevent leakage), or they can be used in the form of water tanks, which are mostly built on the surfaces of their homes; Using the natural height difference of the terrain, water is pumped into the water tank from Gao Quan mouth. Water diversion facilities include bamboo troughs (split and spliced by bamboo), wooden troughs (hollowed and spliced by whole wood), plastic water pipes and iron pipes. (Figure 7).
Where there is perennial water supply, the volume of the water tank can be appropriately smaller, generally 3 ~ 5m3 for a single household (Figure 8), and the centralized water supply tank can be determined according to the population and water quota (Figure 9).
Fig. 7 process design of water tank
Fig. 8 Lotta household water tank (suitable for 7 households)
Fig. 9 Water tanks used by more than 200 people in Emeidang Village, Xintian, Hunan Province.
Where there is only seasonal water supply, the water tank volume is relatively large. When the volume is too large, it can be divided into several water tanks with medium volume. The calculation formula of water tank volume is basically the same as that of water cellar.
3.3 Preventive measures
The construction of water tank mainly pays attention to the following points: ① Suitable construction site. The geotechnical layer of water tank foundation must be uniform and stable to avoid uneven settlement; (2) The exterior of the water tank must be reinforced to ensure that the water tank does not crack or leak; (3) Inside the water tank, the lining material must be pasted after plastering to ensure the water quality safety.
4 problems and countermeasures of decentralized water supply
4. 1 location layout
The unreasonable site selection and layout is mainly manifested in the construction of water cellar. Because the construction of rainwater harvesting project is mainly carried out by farmers themselves, and the government is only responsible for initiating and providing certain financial and technical support, there is a lack of unified, scientific and reasonable planning, which leads to improper site selection and layout, which makes the rainwater harvesting project unable to play its due benefits. Most of the water cellars were built without considering topography, geological conditions and water collection conditions. Some water cellars were not used because of poor geological conditions, and settlement and cracks appeared soon after they were built.
The location layout of water cellar construction is actually an optimization problem, and the following principles should be followed [4]:
(1) adjust measures to local conditions, make full use of the characteristics of large terrain elevation difference in karst mountainous areas, realize gravity gravity gravity water collection and gravity irrigation as much as possible, reduce investment in water collection field and save energy;
(2) The rainwater collection project should be built in a place with good engineering geological conditions, and the slope should be stable to avoid being built in a place affected by mudslides and prone to flash floods;
(3) Take up cultivated land as little as possible;
(4) The foundation should have a thick clay layer to prevent leakage. If the bottom is sand, gravel, pebbles or weathered rocks, it must be impervious.
4.2 Design and Construction
As a small water supply and drainage project, decentralized water supply facilities have their own characteristics, which are often ignored in design and construction. No attention is paid to ancillary facilities, such as there is no drainage ditch around the dissolution well, and there is no trash rack and grit chamber in front of the diversion ditch of the water cellar, which leads to poor water quality of decentralized water supply. The quality of building materials is unqualified, mainly the quality of cement. Pay attention to the proportion of cement, and avoid using unqualified cement, cement that gets wet during storage and transportation, and bagged cement that has been stored and transported for more than 5 months.
4.3 Water quality safety
According to the investigation and analysis, the water quality of decentralized water supply source is polluted to varying degrees, especially the water quality of dissolved wells and uncovered water tanks is poor, and there are often floating objects such as algae, leaves, grass cuttings and chironomid larvae, and the total number of bacteria, COD, ammonia nitrogen and nitrite nitrogen exceed the standard; The water quality of the covered water tank is qualified, and all other indexes meet the drinking water standard except the high pH value [6]. The water quality of water cellar with rainwater as water source changes greatly, which is mainly affected by local air and ground pollutants [7].
The polluted water quality of decentralized water supply often leads to many diseases such as cholera, typhoid fever and dysentery, which is harmful to the health of residents in karst areas [8]. Because the households in karst mountainous areas are too scattered, even if diseases are caused by the water quality problems of scattered water supply, it is difficult to be counted and warned. Therefore, we should pay more attention to the above problems and carry out corresponding research.
In order to improve the water quality of decentralized water supply, the following measures should be taken:
(1) It is forbidden to pile up domestic garbage near the water source, and special attention should be paid to cleaning the rain-collecting platform before it rains.
(2) The building materials should be strictly sanitary materials that can prevent pollution, and the water cellar and water tank should be made of cement materials, but they need to be lined to prevent the harmful components in the cement from dissolving out.
(3) The pit mouth of the water cellar should be covered, and a trash rack and a grit chamber should be set before the water intake. Slow filters are also needed to treat sewage. After the rainstorm and flood season, it is necessary to clean the cellar once; When the water quality is seriously deteriorated, alum or bleaching powder should be put into disinfection [9].
(4) PC pipe or galvanized steel pipe should be used for water diversion pipe of water tank, and PVC pipe cannot be used for water supply; For the slow-updating water tank (updated once every five days), it is necessary to set up a slow filter for treatment, and clean the water tank regularly to prevent bacterial growth and scale formation in the water. For small water tanks, they can be cleaned once every half month.
(5) Set a certain range of protective belts and unblocked drains around the dissolution well to ensure that the surrounding sewage does not penetrate into the dissolution well, and the surface overflow will not flood the dissolution well during the rainstorm; In order to prevent algae propagation in the dissolved well caused by sunlight, it is best to add a movable water intake cover plate on the dissolved well, clean the shaft wall of the dissolved well once a year in dry season, and replace the filter material at the bottom of the dissolved well once every 3 to 5 years.
Of course, as far as the current situation is concerned, the most important measure to ensure the quality of decentralized water supply is to boil the water before drinking it.
refer to
[1]COST 65( 1995): hydrogeological aspects of groundwater protection in karst areas, final report (COST action65). -Council of Europe, director? Science, research and development, report EUR16547en: 446p; Brussels, Luxembourg
Chen Mengxiong, Ma Fengshan. Groundwater resources and environment in China [M]. Beijing: seismological press, 2002.
Yuan Daoxian, Zhu Dehao, et al. China karst [M]. Beijing: Geological Publishing House, 1993.
Chen Zhixiang, Wang Hongtao, Chen Musheng. Present situation and countermeasures of rainwater harvesting project in karst mountain areas of southwest China [J]. Water supply and drainage in China, 2004, (9): 3 1 ~ 33.
[5] Xiao. Construction of small-scale rainwater harvesting project in southern mountainous areas [J]. Agricultural technical service, 2004, (4): 50 ~ 5 1
Ian Kang Jun, Ian Hong Yan. Research progress on water quality of rainwater harvesting project [J]. China Water Supply and Drainage, 2002, 18 (8): 23 ~ 25.
Zhang Xiaoling, Liang Huiguang. Pollution of drinking water in rainwater catchment and ways to improve water quality [J]. Journal of Gansu Agricultural University, 1998, 16 (3): 84 ~ 88.
[8] Zhan Sen ·P·K, Enxinke ·H·J, jayasinghe G et al. Domestic transmission route of pathogens: in? Indoor pollution during drinking water storage in developing countries [J]. Tropical Medicine and International Health, 2002,7 (7): 604 ~ 609
Liu Hongliang, Yu Zhentai, Ma Wei, et al. Experimental study on the treatment of cellar drinking water by simple continuous drinking water sterilizer [J]. China Industrial Hygiene, 2000, 16 (10): 927 ~ 928.